ࡱ> ^_]ܥhc e)  .TJNRRRRRRRRTTTTTTX3TRRTRRRRTRRNRRRRRRR1RRRRHrvoje Turkulin, Jrgen Sell Durability of wooden facades Summary Wood in exterior conditions undergoes a series of chemical and physical changes that impair its aesthetic appeal and durability. The cladding of the houses is a particularly challenging application means because of the high aesthetic, thermal, other physical and technical demands over a long period of decades, sometimes centuries in use. In order to keep the functional properties of wood unchanged in a long service life, a variety of protective measures may be employed. Physical protection is marked as the most important way to keep the harmful effects of ultraviolet light and precipitation away. Design detail of wood products in facades, as well as their assembly means, are considered from the viewpoint of their durability. Chemical protection should generally be avoided, i.e. restricted to a small number of cases when it simply can not be avoided, but surface finishing plays a major aesthetic and technical role in designing and maintaining the wooden facades. Key words: wooden facades, wood protection, design details, wood preservation, wood finishing. Introduction Favourable aesthetic and technical durability of wood facades are very important bases of a quality use of wood in building. The durability encompasses the aspects of technical functionality i.e. meeting the demands for a good protection against the elements, ingress of rain and air, and it also includes good thermal and acoustic properties. Aesthetic durability may be subjectively determined, but its unchanged continuity plays an important role in architectural wood design. The projected quality may be achieved by ensuring the best durability with the smallest possible costs, which may arise in designing, building or maintaining the building. The wood should be protected by three systems of elimination of unfavourable changes in exterior applications. These are physical protection (prevention of the harmful actions on wood), proper structural protection (correct design of technical details of the products which prevent adverse changes in exposed wood) and chemical preservation (generally biocide protection of exterior wood). This is a concept of six major points: choice of an adequate building material, suitable architectural form of the facade, proper technical design of wooden parts of the facade, protective finishing and preservation, and required maintenance in service life. Choice of adequate wood species and wood-based materials Timbers differ very significantly regarding their suitability for application in building. This can not be emphasized in direct UV irradiation of the sunlight and in precipitation, when all the species change the colour and erode at the surface. During first several months of exposure a specific discoloration of particular species will be individually marked, but after a year in exterior conditions most of the wood species turns to similarly grey, plain colour. From the aesthetic point of view, the differences could be hardly noticed. The erosion of the unprotected wood surface is also similar for a majority of species utilised in building and amounts to 6 - 8 mm in a century. From this standpoint there is no sound reason for hesitating to substitute naturally durable and expensive wood species, such as larch or oak, with cheaper and easily accessed home grown species such as fir and spruce. The differences among the species become very important when it comes to the differences in hygroscopic properties and their resistance to biodeterioration. There are special requirements for wood species applied in severely exposed building portions or for those parts which are frequently wetted, which have large proportions of gaps, end-grain surfaces and other details vulnerable to wetting and moistening. These requirements are as follows: The species should be naturally resistant to the biodeterioration, i.e. to the attacks of fungi and insects; These timbers should exhibit small capillary water uptake; The species should have naturally good dimensional stability, i.e. show small movement due to hygroscopic swelling and shrinking, which is particularly important at complex products such as windows. Table 1 contains the relevant information about the most often utilised (or recommended for use) wood species for building in Croatia and in Central Europe. Only those species with small liquid uptake and those with good (or at least mediocre) permeability should be used in exterior applications and for facades. These timbers should also possess good, or at least mediocre, dimensional stability. Natural durability plays here a minor role, since all the parts of the facade (and particularly the cladding) should be well protected by design features. These elements should be well ventilated, their thickness should be small, and their end grain should be sealed. This will enable quick drying after precipitation, so they will not be subject to fungal attack. Curiously, under those criteria the larch wood does not rank significantly better than spruce, nor does it differ much from pine heartwood. Special attention should be payed to the application of wood-based boards, which are recently widely re-introduced for large weatherboarding areas. There are great differences between the properties of wooden boards according to their intended use. Some of them are specially designed for exterior use, such as cement-bonded boards, whilst the other wood-based products are completely unsuitable for use in conditions of increased humidity and other weathering hazards. Boards for cladding should be of the Weather and Boil Proof group, i.e. they should be made with water and temperature resistant (D4 after EN 204 test) adhesives, such as those of polymeric condensation type. Even durable wood-based boards demand fulfilling of special design requirements and physical protection features in order that the cladding retains its long lasting function and favourable aesthetic appeal. The wood species used for board production must also be suitable for exterior applications, because some boards (e.g. birch or Douglas fir plywood) are prone to deep cracking in the grain direction of the outer veneer. Table 2 reveals the suitability of particular type of boards for exterior applications and the conditions under which they may be used for cladding. Design of the form and shape of the facade The most efficient and often the simplest method of wood protection on facades is the physical protection, i.e. the series of measures which simply prevent the contact of wood with deleterious conditions in exterior use. The most efficient detail of a structural design is the introduction of wide eaves and overhangs, which should be at least 50 cm wide over heavily stressed facades (in west and south expositions). In our tradition even wider overhangs of 70 90 cm are used. Traditional building in continental Croatia, based on experience, has come to the practice of putting eaves on all roof planes, above lower portions of the building, over stairways, porches and protruding parts of the building, so that rain with wind can not wet the walls. Any overhang is better than none, but the narrow eaves are of very little help in taller buildings. Partly sheltered upper portions of the walls, which are exposed to sunlight only, and the lower, fully exposed areas, weather at different rain and show nasty discoloration. Wood exposed to sun turns uniformly dark, if the climate is dryer (as in alpine conditions shown on figure 4). The colour generally turns to brown, particularly that of softwoods, as seen on figure 7. Grey coloration is a characteristic of wood exposed to both sun and rain and is dominant feature of the exposure of hardwoods (oak wood on figure 3) and weathering of wet wood (figure 6). Irregular physical protection of particular parts of the facade by eaves, sills or window shutters, leads to unequal and undesired discoloration (figure 5). Windows should be withdrawn from the facade inwards as much as it is possible in order that the panes are less directly exposed to rain, but also that the window sills are less wetted. Installation of windows into a rebate from inside gives a good physical protection since the exposed frame area is significantly reduced. Central European practice (in Bavaria, Switzerland and in Austria) brick houses have windows set into the stone rebate, while in countries of traditional wooden buildings (as in Finland, for example on figure 8) the windows are positioned inward and protected by wide wooden trimming. Windows oriented towards west should be smaller to prevent overheating during summer periods. This is favourable from the functional standpoint, especially for bedrooms. The best physical protection in extremely exposed situations is the application of aluminium-clad wooden windows, which became increasingly popular during the last decade. Apart from direct wetting, wood may be moistened by splash water from the ground and from the green vegetation on the horizontal areas near the facade. The smallest elevation of wood cladding from the ground should ammount 40 cm, but this rule is generally neglected, and wood quickly decays near the overgrown soil. Traditional wood building practice respected this feature of physical protection, so that the lower portions of wooden houses were stone-build (as on figure 4) or was the whole building elevated from the ground by positioning on the cornerstones. Modern examples are shown on figures 7, 8 and 10. Structural protection by proper detailing Detailing of the parts of the facade may significantly contribute to the reduction of undesired effects of water and sun on durability of wood parts, and they are generally summoned as protection from the contact of water with wood. Basic rules are as follows: Water entry into wood should be prevented, and water may stay within wood portions no longer than two weeks. All end grain surfaces must be covered or sealed because axial permeability and water uptake of wood is fourty times larger than latteral uptake. Edges of wood boards must also be protected. Grooves, holes and openings on the surface should be avoided. Open gaps, joints, checks and cracks are places where water may penetrate by capilary movement and collect deeper in the wood. Glued joints are generally not resistant to sun and rain and should not be exposed in exterior surfaces, albeit it is sometimes tollerated that they are fully covered by opaque high-build coating. All horizontal or very little sloping surfaces should be avoided or covered with metal, ensuring that the metal sheath is ventilated from beneath. Alternative is to cover the endangered surfaces with cheaper, easily replaceable wood material. It is recommended that all the horizontal surfaces are sloped at 13 to 15. Large, significantly inclined and dark painted surfaces should not be exposed to sun. The temperatures of wood due to summer insollation my reach seventy degrees, and as a consequence rapid drying and shrinking my produce checking which may contribute to water uptake. The solid wood members of larger dimensions and cross-sections are particularly prone to this damage Wood should be protected from precipitation, but also from splash water that recoils from exterior horizontal surfaces. Wood should be raised from those surfaces and/or protected with metal sheets. Gaps on exterior surfaces must be wider than 5 mm in order to prevent capilary uptake of water. Horizontal gaps must be made sloping outwards, where the rim of the upper board acts as a drip. The joints of facade boards should be covered with laths or designed as overlapping boards or tongue and grooved boards. Wider gaps are best sealed with mastic. Allowing for the movement of the shrinking and swelling wood is essential. The gaps must be wider than the greatest swelling dimension of the joined members. Overlapping must not be done so that one edge is fixing the other, and tongue and grooved joints must allow for movement of the joints. The most common faade style is timber cladding by solid or wood-based boards. The cladding boards re hung horizontally or vertically, they are at least 15 thick, but the thicker ones (20 mm) will give better dimensional stability. Vertical cladding has fewer water traps and open joints. Horizontal boards should be profiled since overlapping of straight boards is not tight. Horizontal boards are usually tongue and grooved, and tongues are allways positioned on top (grooves on the upper edge of the boards would collect water). End grain joints of horizontally adjoined boards may collect water more than vertical boards, and overlapping is never tight and the water may penetrate at joints. In both vertical and horizontal fixing the end surfaces of the boards must be sealed or finished with opaque, high build coatings. Boards are fixed to their substrate battens which sould be 24 to 38 mm thick and impregnated. The space between the wall and the cladding (between the breather paper on the exterior wall surface and the back surface of the cladding) must be free for air circulation from below upwards. Wood cladding is vulnerable to wetting from outside (due to precipitation) but also from inside, when the wapour difussion through the walls may accumulate condensed water in the exterior wall layer. The circulation of air in the gap brings its relative humidity to equilibrium with exterior air, thus preventing the decay in the closed space. Vertical cladding is fixed to horizontal battens, which restrict ventilation unless the air gaps are left beteen them. Alternative arrangements include diagonal battens or battens across counter battens. Wood-based boar cladding is a relatively new tendency in architecture which follows the trend of building so called box-shaped houses. Boards geometry determines a clear, simple architectural form, still preserving an unique impression of a wood material, especially if the board surface reveals wood texture. It goes without saying that the boards must be finished since they are more sensitive to moisture and UV light than solid wood, especially at the edges. Wood based boards must have their edge surfaces finished with the same system as their exposed faces, and sometimes back faces should be coated too. This can prevent distortion and warping caused by differences in hygroscopic swelling and shrinking of outer and inner layers. Designing of wood-based boards does not allow sharp edges (e.g. corner jointing at 45 ) because of mechanical weakness of such exposed edges. Boards are usually fixed by nailing, in such a way that the nail penetrates only one board, not restricting the movement of the underlapping board. One of the most important design features is a possibility that the joints accommodate the movement which at 100 mm wide boards may be around 2 3 mm. Boards fixed without allowing for movement would split. Overlapping joints are a solution where a board has only one edge fixed, allowing for movement in the joint. Overlapping areas must be finished before assembly, so that shinking does not expose unfinished tongues or edge surfaces. Warping of boards causes the deformations where annual rings tend to straighten, therefore the fixings should be designed so that joints tighten rather than loosen under these circumstances. The favourable solution is laying the boards with their right faces (pith sides) are exposed, and bark sides turned inside. Tongue and grooved boards may be secret nailed so that only the tongue of the lower board is being fixed. Galvanised (rust-proof) nails should be used to prevent rust staininig under the transparent finishes. Better alternatives include aluminium nails (care must be taken with copper-chrome-arsenic preservatives, which can cause their corrosion) or stainless steel or bronze nails. They are fixed at 60 cm centre distances, and nails should be punched a few milimeters below the surface to prevent the heads standing proud after moisture movement. Indentations should be sealed. Thicker hardwood cladding and wood-based boards should be screwed rather than nailed, especially if the wood species has small dimensional and form stability (such as ash, elm, robinia or even oak). Screw heads should be either driven deeper than the surface and sealed, or fixed resting on wide, tight washers. Adequate and durable finishing Modern range of materials and methods for finishing built-in wood is various, ample and comes in a broad range of quality. It is almost impossible to comprise all the information about the products, their properties and application possibilities. The planners and users can not easily determine which finishing material can (or should) be used in particular building situation and in forseeable conditions in use. The choice is often dominated by aesthetic effect, although it can not be a sole criterion for specifying the surface finishing system for wood. Exterior wood finishing differs from the furniture finishing inasmuch as furnitures aesthetic characteristics predominantly determine the finishing system. Coating on exterior wood products has a notably important technical, protective function. The most important functions of the finishing system for wood in exterior conditions are as follows: Protection from light, expecially from the ultraviolet portion of the sunlight. Light destroys the surface layer of wood, leading to discoloration, checking, peeling of the transparent coatings and finally to wood erosion. Defense from moisture, i.e. reduction of the dimensional movement of wood (swelling and shrinking) as the consequences of the hygroscopic moistening and drying of wood depending on the conditions in use. Controlling the internal stresses caused by action of light and fluctuations of equilibrium moisture content of wood, which cause checking, cracks, deformations, weakening of the adhesive bonds and joints of wood with other materials In some cases a preservation against biological attact may be required, particularly against the fungal decay and change in colour of wood and coating caused by mould and fungi. Only the opaque and thick coating may meet these requirements and remain unaltered during long years of wood protecttion. Things are complicated by our contradictory expectations regarding the transparency for light and water vapour permeability of the coating. The right choice of the finishing system depends therefore on argumented analysis of the requirements that will be imposed on it. The more opaque the coating, the better its protection of the deleterious effect of light on wood. The best solution is that the coating does not transmit any light at all. This type of coating, however, does not allow us to see the surface of wood, and its aesthetic values, so important for our fondness of wood and its ecological values, become irrelevant. We simply can not say whether the white paint covers wood, plastic or aluminium. On the other hand the hell stains and transparent coatings, which reveal the wood colour and texture, are not suitable for exterior use because of their insufficient durability. Semi-transparent natural coatings (high-build stains) are a good compromise between technical requirements and aesthetics, but their durability depends on the quality of raw materials, on the type of pigment and incorporation of UV-stabilizing aditives. Unfortunately, the appearance of such finishing materials does not reveal its durability. Fortunately, there is a number of semi-transparent stains and lacquers in the market which guarantee good protection for wood, and new testing procedures will enable better specification of the finishing system for particular application. The thicker the coating, the better its protection against undesirable effect of water. Greate number of coats yields better properties than thick spreading rate. Thicker coating reduces the moisture uptake from the moist air and by precipitation, and therefore annual and multiannual fluctuations of highest and lowest equilibrium moisture contents are smaller. Complete impermeability of the coating, on the other side, is not acceptable because some of the water acuumulated in wood must evaporate through the film. Besides moisture from outside, wood is subject also to the water from inside, i.e. through the walls of the rooms with higher relative air humitidy. This vapour moves during winter from the interior, where the vapour pressure is higher, to colder (outer) layers of teh elements, where it can condense and accumulate under the impermeable coating. The coating must therefore be water-repellent, but also vapour-difusive, but these two properties are difficult to combine. General positive attitude towards the low-build stains, which was twenty years ago based upon presumption that it is advantageous for wood to breathe, turned out wrong because the highest moisture concentrations under such coatings in a three years period often exceeded 20 % which is an upper limit for the start of decay. Modern practice prefers high-build semi-transparent systems (so called high-build stains), which offer good protection against light and water and are applied at a minimum film thickness of 60 (m. Water-based paints should be applied in a dry thickness of 120 (m. These values of film thickness ensure better moisture excluding effectiveness, smaller fluctuations of moisture content, and consequently smaller occurance of cracks than is the case with thinner films. Thick coats also erode slower, which means that their thickness is less reduced during exposure. Just one additional coat on double-coated wood prolongs the period of maintenance for 30 %. Film thickness is particularly important on mechanically stressed arease, and the points of highest risk are the exposed edges. Surface tension of the coating may cause withdrawal of the material at sharp edges and thinner deposit of paint on these vulnerable places. Rounding the edges of exposed parts of wood elements helps to solve this problem. Architects often argue that the pigmented finishes for wood cover and hide the natural colour and texture of wood. Since the natural colour of unprotected wood changes very rapidly, it is almost unavoidable to add some pigment to weatherbording finishes if the desired colour and hue is to be kept stable for years. The surface texture of wood can though remain visible under semi-transparent and even under opaque coatings, especially if the wood is left rough sawn, sand blasted or brushed with steel wire. Rough texture does not impair the durability of the coating, as would be expected. On contrary, It seems that the service life of the coating can be enhanced on very rough surfaces. Chemical protection - only where neccesarry Former code of practice for wood technologists demanded that all the wood members which are built-in without access, and without air circulation, should be chemically impregnated. It implies that these members can not be physically reached, controlled, repaired or exchanged. In modern era the application of chemical preservatives against fungal and insect attacs is being avoided wherever possible, or reduced to absolute minimum. The reasons for this are increased public concern for toxicity of such chemicals in our close proximity and ecological awareness of modern users. The environmental protection prefers wood as a natural material which can be used with minimal (if any) dejmand for chemical and technical interventions. This attitude is hardly compatible with the permanent modification of wood by chemicals which are harmful for humans and animals. On the other hand, wood is progressively promoted as a recycling material that can be thoroughly utilised in various building applications, and chemical preservation practically eliminates the transformation of wood into other recycling products. Hence the application of biocides (that is, the compounds which are toxic for wood-destroying fungi and isects) is restricted to the following cases: Chemical preservation should be applied in minimum levels and only for those cases where it is unconditionally demanded. Preservatives could be introduced only to those parts of the buildings where the risk of permanent wetting and decay is great or conceivable (e.g. in ground contact, in water and for elements of large cross sections which are permanently exposed to exterior climatic conditions). Chemical protection may be applied only in those cases when other protective measures, like phisical protection and designing, could not be satisfactorily performed or are insufficient for ensuring the durable service life of wooden products Only those structural elements are impregnated which can not be accessed i.e. thowe which renovation is too expensive It is possible (though not always justified) to introduce chemical preservation in situations where wood would not be competitive building material due to its limited durability. Wood facades are the places where chemical preservation is rarely (if ever) required, and could be justified only for structural closed elements (such as substrate battens) on multi-story objects. Wherever applied, chemical means should be implemented professionally and thoroughly, so that its efficiency is assured. It is therefore allways recommended that the preservation is being performed by authorised and licenced companies like industrial plants for pressure treatments. This should ensure that the process is efficient and thoroughly supervised, and that leaching of the preservative in use is prevented or restricted. All the treated members which are cut to measure on the building must have their end grain re-treated, othervise the treatment is inefficient. Essential maintenance Maintenance, regular controlls and renovations of built-in wood products are unavoidable if a long lasting, durable favourable appearance and technical functionality are to be preserved. The frequency of actions, the measures and methods of renovation and maintenance depend on the expected results and predicted service life. There are no general rules and recommendations for this, because the frequency of controlls and the choice of renovation methods depend on too big number of factors, many of which are variable or unpredictable (exposure to elements, choice of wood species, wood materials and their properties, technolgy and design of elements, aesthetic requirements etc.). However, it is possible to determine on a general level that experience can help in rough estimation of the maintenance intervals. South or west facades in continental countries, finished with semi-transparent stains, should be renovated every two to four years. Unfinished wood is practically not maintained. Industrial wood products, such as windows, that are finished with opaque high-build paint, should be re-painted every five to eight years. However, some superb products have been monitored for twenty years their original properties remained virutallly unaltered. Wood constructions must be regularly checked. Members that are made of naturally non-durable species and are left unfinished, must be examined every two years and repaired if required. Structures of naturally durable species, if additionally chemically treated, can be examined every ten years. Although the functional durability of the wood on buildings is much debated, and frequent controlls and maintenance are recommended as precocious measures, examples of traditional use of wood in building should be remembered. It clearly shows that an adequate and appropriate installation of wood in building ensures its great durability and provides us with optimism regarding its future for use for building purposes. 8. Literature Graystone, J. (1985): The care and protection of wood (90 pp.): Slough (England): ICI Paints Division. Landolf, A.; Eggenberger, N. (2001): Dreischichtplatten als Fasadenverkleidnungen. Kompetenz Zentrum Holz 9 (2) 4 12. Ljuljka, B., Turkulin, H. (1986): Tradicionalna primjena hrastovine. (Traditional application of oak wood -in Croatian, Eng. Captions and Summary). Annales pro exp. For. Special edition 3, 415 - 437. Miller, E. R.; Turkulin, H. (2001): Standards for classification and testing of exterior wood finishes EN 927. I: 927 1 and DD ENV 927 2. Drvna ind. 52 (3): 117 123. Risi, V. (2001): Dreischichtige Massivholzplatten: Spannungen bei Klimawechsel. Kompetenz Zentrum Holz 9 (2) 1 4. Sell, J. (2000): Bedeutung des konstruktiven Wetterschutzes bei Holzfassaden. In: Die Gebudehlle (J. Blaich, ed.): 3 10. Dbendorf, Switzerland: EMPA Akademie Sell, J.; Fischer, J.; Wigger, U. (2001): Oberflchenschutz von Holzfassaden. Lignatec 13/2001, 27 pp. Zrich: Lignum. Turkulin, H.; Jirou-Rajkovi, V.; Grbac, I. (1997): Povrinska postojanost drvnih graevnih konstrukcija. (Surface durability of wood building constructions in Croatian, Eng. capt. and Summary).. umarski list 121 (11/12):617 - 630. Turkulin, H. ; Jirou Rajkovi, V.; Bogner, D. (1999): Structural effects of weathering on unprotected and painted wood. Proceedings: Surface properties and durability of exterior wood building components (H.Turkulin, Editor); 30 April 1999, Zagreb, Croatia. Paper 3: 1 20. Zagreb: Faculty of Forestry. Slika 1. Razlike u postojanosti meu vrstama drva nisu estetski izraene: nakon godine dana prirodnog izlaganja veina vrsta drva poprimi slinu sivu boju. Figure 1. Differences in durability between various wood species are not reflected in their appearance: after a year of unprotected, natural exposure most of the timbers get simmilar, gray colour. Slika 2. Na masivnim ploama mogu nastati razliite tete u ovisnosti o zatiti i stupnju izloenosti. 1 pukotine sljubnice, nezatieno drvo; 2 radijalne pukotine i 3- pukotine na granici goda kod lazurom obraenih ploa. Bonice na licima trpe znatno vea naprezanja od utezanja nego blistae. Doljnja ploa, zatiena neprozirnim pokrivnim premazom, neoteena je nakon godinu dana otrih vanjskih uvjeta. Figure 2. Solid wood boards may exhibit various damages depending on the type of protection and level of exposure. 1 delamination checks on unprotected board; 2 radial checks and cracks on the ring border on the stained surface. Flat-sawn outer lamellae suffer much greater shrinking stress than square-sawn boards. Lower board, protected with opaque high-build coating, remains unaffected after a year of severe exposure. Slika 3. Iskustveno razvijena pravila dobre fizike zatite drva na proelju vidljiva su na hrvatskoj korablji: drvo je odignuto od tla, vodoravni konstruktivni elementi zatieni su strehama i nadstrehama, obloni elementi (na zabatima i stubitu) poloeni su uspravno. Figure 3. Experience developed good code of practice for physical protection of traditional Croatian oak cottage: wood is elevated from the ground, horizontal (structural) elements are well protected by eaves and overhangs, non-structural cladding is hung vertically (gable, railing). Slika 4. Kua od arievine iz sedamnaestoga stoljea, nedavno obnovljena za stanovanje. Figure 4. Larch cabin from seventeenth century, recently renovated for living. I Slika 5. Posljedice izloenosti nezatienog drva nisu iste na svim stranama svijeta. Na zapadnoj strani zbog sunca i kie drvo najjae posivi, na junoj je manje vlaeno pa sporije sivi, a na sjeverim i istonim proeljima esto uope ne posivi. Figure 5. Weathering effects on unprotected wood differ regarding the geographic orientation. In western expositions the actions of rain and sunshine cause rapid development of gray wood surface; wood facing south remains dryer and therefore turns gray to a lesser extent. North and east expositions often exhibit no gray tint at all. Slika 6. Lo primjer primjene drva za proelja: drvo u dodiru s tlom, bez streha i bez odgovarajue povrinske obrade brzo propada i neugledno izgleda. Figure 6. Bad example of application of wood on faade: wood layed on the ground, witout overhangs and finishing decays pretty fast and gives poor appearance. Figure 7. Dobar primjer moderne arhitekture drvom: dobra fizika, konstrukcijska i povrinska zatita osiguravaju postojanost i lijep izgled zgrade. Figure 7. Good example of modern wood architecture. Appropriate physical protection, design detailing and finishing ensure durability and aesthetic appeal of the building. Slika 8. Finska kua pokazuje niz detalja dobre konstrukcijske zatite. Drvo je odignuto od tla, strehe tite proelje od oborina. Glavna daana obloga proelja uspravno je postavljena, ventilirane pozadine i pokrivenih gornjih krajeva. Spojevi uspravnih dasaka pokriveni su letvicama, vodoravna oplata je spojena utorom i perom. Sve je neprozirno lieno. Prozori su uvueni i ugraeni s drvenim pristupkom tako da je manja povrina izloena nevremenu. elni presjeci elemenata opava pokriveni su i premazani. Najiziloenije daske na elima krovnih ploha lahko su dostupne i izmjenjive. Figure 8. A house in Finland exhibits details of good structural protection. Wood is raised from the ground, eaves protect the cladding from rainfall. The main cladding is vertically hung, ventilated from behind, top end-grain is covered. Vertical joints are covered with laths, horizontal cladding is tongue and grooved; all is opaque finished. Windows are fitted with a wooden rebate, withdrawn from the faade, with reduced surface exposed to elements. Trimming exposed ends are protected and sealed. Intensely weathered boards on roof edges are easily accessed and replaced. Slika 9. Arieva indra odignuta je od betonske osnovice koja je s gornje strane skoena da odbija i odvodi kiu od drva. Figure 9. Larch shindles elevated from the concrete skirt; its upper surface is sloped to direct recoiled dropplets off the wood. Slika 10. Pravilan spoj drva i betonske podloge: drvo je odignuto postavljanjem na metalnu plou (spajalo ili sidrite) odignutu od betona. Cjelokupni prostor ispod drvene konstrukcije se prozrauje. Figure 10. Correct joint of wood and concrete slab: wood is elevated by mounting on the metal plate (fastening or anchor) distanced from the concrete surface. The area below the wooden structure is ventilated. Slika 11. Zazori izmeu vodoravnih elemenata su zatieni skoenom pokrovnom letvom, razmak izmeu letve i dasaka i skoenje doljnjih ploha spreava kapilarno uvlaenje vode. elni presjeci bi trebali biti premazani i/ili pokriveni. Figure 11. Gaps on horizontal weatherboarding are protected by a profield bar, the spacing between the elements and sloping of horizontal surfaces (upper and lower edges) prevent cappilary uptake of liquid into the joint. End grain surfaces should be sealed and/or covered. Slika 12. Ventilirana obloga od cementnih drvnih ploa iverica. Uspravne reke su pokrivene drvenim letvicama, a vodoravne su nainjene irokima 1 cm. Plohe reki su skoene prema van/dolje i povrinski zatiene kao i vanjska lica ploa. Figure 12. Ventilated cement-bonded particleboard cladding. Vertical gaps are covered by wooden laths; horizontal ones are left open, ca 1 cm wide. Their rims, sloping outwards, are finished in the same way as the exterior surfaces. Slika 13. Detalj kutnog spoja prozora i proelja: voda koja se slijeva nigdje se kapilarno ne zadrava, uspravna opavna daska je odmaknuta od proelja i klupice. Figure 13. Detail of the corner joint of the window and the faade: water running down the window can not penetrate the joint. Vertical trim is distanced from the rebate and the sill, thus cappilary uptake is prevented. Slika 14. Daana oplata, vodoravno postavlajena. Obloga je osjetljiva na djelovanje kie izvana i vlage iznutra, pa se prostor izmeu ljepenke na zidu i obloge mora prozraivati. Figure 14. Weatherboarding, horizontally hung. The cladding is vulnerable to moisture from outside and inside, so the space behind the cladding and a breather paper on the wall must be ventilated. Slika 15. Daana oplata, uspravno postavljena, ili obloga od ploa se mora postaviti na vodoravne letvice koje imaju elne razmake da se osigura uspravno strujanje zraka. Figure 15. Weatherboarding, vertically hung, or a wood board cladding, should be fixed to horizontal battens with air gaps left between them to ensure vertical circulation of air. Slika 16. Daske se trebaju uvrstiti s mogunou bubrenja i utezanja, a avli jedne daske ne smiju privrivati i susjednu dasku. Utori moraju biti okrenuti na dolje da ne skupljaju vodu. Bonice trebaju biti tako okrenute da njihovim deformacijama doe do stezanja spojeva, a ne do otvaranja nakon utezanja. Figure 16. Boards must be fixed with allowance for movement, and the nails should not restrict the adjacent board. Grooves must be positioned downward so that the water will not collect. Flat-sawn boards should be positioned so that the joints tighten at distortion, rather than loosen arter shrinking. Slika 17. Proelje s oblogom od drvnih ploa izvodi se privijanjem ploa na uspravne letve, obloene gumom. Vijci su uputeni i kitani ili privijeni preko iroke podlone ploice. Figure 17. Facade with a board cladding. The boards should be fixed on vertical battnes over a rubber strip. Screws should be punched below the surface and sealed, or fixed wtih a washer. Slika 18. Uspravni spojevi ploa: zatvoreni spojevi (utorom i perom, ili s aluminijskim perom) ili otvoreni spojevi (uvijanjem preko gumene trake). U svim sluajevima rubne povrine ploa moraju biti kitane ili povrinski obraene. Figure 18. Vertical joints of boards: closed joints ( tongue and grooved, with aluminum tongue), or open joints, fixed over a rubber strip. In all cases edge surfaces must be sealed or finished. Slika 19. Vodoravni spojevi ploa. Otvoreni spojevi trebaju imati skoene, povrinski obraene rubne plohe na meusobnom razmaku od 1 cm. Figure 19. Horizontal joints of boards. Open joints should have sloping edges which are finished as the board faces and distanced about 1 cm. Slika 20. Kutni spojevi ploa. Spojeve treba ostaviti otvorenima, ili ih zatititi drvenim ili metalnim profilima (lijevi stupac). Spojevi na desnoj strani su nepravilni: treba izbjegavati tupe sljubove i otre bridove drvnih ploa. Figure 20. Corner joints of boards. Joints should be left open, or protected with a wooden or a metal profile (left column). Joints on the right are incorrect: closed joints and sharp edges of the boards should be avoided. Slika 21. Povrina jelovine prije i nakon osam mjeseci prirodnog izlaganja u kontinentalnim uvjetima (Zagreb, Hrvatska, 45 prema jugu, od sijenja do kolovoza 1997). Vidljiva je promjena boje, velik broj povrinskih pukotina te erozija koja je intenzivnija na zonama ranog drva. Figure 21. The surface of fir-wood before and after eight months of natural exposure in continental conditions (Zagreb, Croatia, 45 facing south, from January to August 1997). Photograph reveals a colour change, large number of superficial cracks and erosion which is more intensive on the earlywood portions. Slika 22. Povrina jelovog uzorka koji je zatien lazurom te prozirnim lakom, a izloen osam mjeseci pri istim uvjetima kao i uzorak na sl. 2. Prolaz svjetla kroz lak dovodi do razgradnje povrinskog sloja drva, to ima za posljedicu ljutenje premaza. Figure 22. Surface of the fir sample finished with wood stain and transparent varnish. The sample was exposed for eight months under same conditions as the specimen on fig. 2. The penetration of the light through the varnish causes the degradation of the wood surface layer, which eventually leads to the film peeling. Slika 23. Smrekove daice s piljenom, blanjanom i bruenom povrinom (s lijeva na desno) te obraene pigmentiranim premazom ili neobraene. Karakter drvne strukture ostaje estetski dojmljiv i ispod neprozirnog premaza ako je povrinska tekstura grublja (lijevo). Figure 23. Fir plates having a rough-sawn, planed and sanded surface (from the left) being opaque painted white or left unfinished. The character of wood structure remains aesthetically impressive even under the covering coating if the surface texture is rough (left portions).  Hrvoje Turkulin je docent na umarskom fakultetu Sveuilita u Zagrebu, a Jrgen Sell je profesor na ETH sveuilitu u Zrichu i voditelj Drvnog odjela vicarskog Saveznog instituta za istraivanje materijala i ispitivanja u Dbendorfu. Hrvoje Turkulin is an assistant professor at the Faculty of Forestry of the Zagreb University; Jrgen Sell is a professor at the Eidgenssische Technicshe Hochschule (Zrich) and head of the Wood Department of the Swiss Federal Institute for Material Testing and Research (EMPA) in Dbendorf. .A  =>} 234|((`CCzV{VVVb\\3hIhppp?ABJK|} fg&$ h 4hhh! 4h$ h 4h.h !345["\"&&z({(|((()) 4$ h 4h.hh! 4hh)*],-//81^2_25588U<V<??_C`CCCFF GGHITJKKPPU$ h 4h.h$ h 4hh UUYY^\_\`\a\b\\\``}a~aacdwd*e+e/h0h1h2h3hIhJhjj5m6m]n$ h 4hh$ h 4h.h ]n^npppp|ppqlrrsstvvvv~wwwyzz{||R}}}}~%&g$ h 4h.h&ghvÈjkefƑǑ-.LMؗghQ01L,-Ȟɞҟء, K@Normala "@" Heading 1ac"@" Heading 2Uc@ Heading 3V"@" Heading 4Ua$@$ Heading 5Uac$@$ Heading 6Vac"A@"Default Paragraph Font@ Footnote Text &@ Footnote Referenceh"B@" Body TextVac$O"$ Body Text 2Uac"O2" Body Text 3VaRS)U]ngTUVWXYZ@JTimes New Roman Symbol "ArialTimes New Roman CE"h.LdF,MdFCd&5B3Hrvoje Turkulin, Jrgen Sell **  !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\aiRoot Entry F1`WordDocument  )CompObj/=!jSummaryInformation(  FMicrosoft Word Document MSWordDocWord.Document.69q Oh+'0   H T ` lxHrvoje Turkulin, Jrgen Sell A*obNormal*1AMicrosoft Word for Windows 95 @DocumentSummaryInformation8   FMicrosoft Word Document MSWordDocWord.Document.89qumarski FakultetB Hrvoje Turkulin, Jrgen Sell @@DC@X?5 ՜.+,0@Hdl t| umarski FakultetB Hrvoje Turkulin, Jrgen Sell