Many architects turn to our technical department for advice on how to carry out architectural projects that incorporate wood in their facades. Fear of the material’s behavior and false myths lead many to renounce the use of this material. If it is known, it can give great satisfaction. However, for the design of wood facades, we can find several points of interest that are crucial in the effectiveness of the creative and productive process of these elements.
“We may use wood with intelligence, only if we understand wood” Frank Lloyd Wright
In this article we want to collect some of the main guidelines that we have to take into account to make your facade a success in the future.
1- Recognize the type of use of the wood facades in question.
The use classes to which wood elements are subjected are included in the durability section of the CTE (DB-SE-M 3.2.1.2.) and in the UNE EN 335: 2013 standard. According to this, a facade would be classified in Use Class 3, exterior wood without contact with the ground.
Within this class of use there are two subclasses, 3.1 which refers to timbers in which the construction design facilitates the evacuation of water and thus the drying of the wood, so that the risk of moisture retention is limited. The 3.2 includes wood elements that can remain damp for a longer period of time, but never permanently.
2- Choose a wood species with adequate durability for the facades.
The attack of the different xylophagous agents will depend on the humidity that can be retained in the wood. Highly durable wood species with values between 1 and 2 should be used; the service life of the wood element will depend on this.
The UNE-EN 350-2:1995 standard, _which will soon be replaced by the PNE-pr EN350_ standard, establishes a system of categories to define the natural durability of each wood species against different agents: xylophagous fungi, coleoptera, termites and marine xylophagous_. This always refers to the heartwood, since the sapwood of any species is considered non-durable.
Hardwoods generally have a very high natural durability, ipé, teak, iroko, cumaru, chestnut…. However, the commitment to the use of softwoods has developed internal modification treatments, physical such as heat treatment or chemical such as acetylation. This results in softwoods that are completely suitable for this type of use. In addition, heat-treated bamboo is another excellent option.
These latest generation timbers are very stable, with very small contraction and expansion movements, so that thin, slender sections can be achieved. In the case of hardwoods, their higher density makes them more nervous woods, woods with more movements. In this case, a good selection of the wood is essential: sections from radial cuts, with straight directrix, belonging to the heartwood…
NOTE: The FD P20 651 standard, referring to the Durability of elements and timber works, includes in a very complete way the above mentioned questions, and can be very useful for the designer.
Any board composed of different layers is commonly known as phenolic plywood or marine plywood. Not all of them are suitable for exterior use, and obviously not all of them are marine. In this case it is very important to choose the glue that joins the different veneers, and it must be a Class 3 gluing, according to UNE 314.
3- Thinking about the construction detail in the design of facades
One of the main enemies of wood outdoors is water and avoiding it involves the design of a correct construction detail that has to:
– Facilitate the evacuation of water to prevent it from being retained at any point. The edges of any element will be rounded _which will also facilitate the penetration of the final finish_. Flat or concave surfaces will be replaced by other inclined or convex ones and meetings between pieces that prevent the water from flowing easily will be avoided.
– Ventilate wooden elements. The air space behind a facade cladding must be properly ventilated. So the key to this is the arrangement of battens and the meeting of the elements with the upper and lower parts of the area to be covered. The correct ventilation of the cavity will prevent the movement of the wood and the appearance of stains.
If in point 2 we talked about the durability of the wood cladding, we cannot forget the durability of the wood used as a batten. There is no point in placing something durable on the outside if what is not visible, which is the fixing of everything, can rot. There are those who, out of ignorance and to alleviate this, choose to use metallic battens, but we must not forget the differences in contraction and expansion of materials as different as metal and wood.
Our recommendation is to use autoclave-treated woods for risk class IV or even tropical woods _especially iroko because of its better workability_ if high density tropical woods are used in the final cladding.
– Allow the wood to move. The intrinsic hygroscopicity of wood causes it to contract or expand with changes in relative humidity, a very frequent phenomenon outdoors. Our constructive detail should not restrict the movement of the wood elements, in order to avoid the appearance of fissures or cracks that will contribute to the retention of water in them.
– Facilitate the replacement of any part. It is common that a small percentage of exterior wood elements can suffer damage and that is why it is very important to think about their possible replacement. There are multiple fasteners on the market that even being hidden allow you to remove or put any piece and thus facilitate that your facade is always like the first day.
– Take care of the fastening elements. There are woods with a high presence of tannins, chemically modified woods… that suffer a reaction when in contact with metallic elements that are not stainless steel, which can cause the staining of a facade and with it the aesthetic deterioration of the same.
4- Choosing a suitable finish for the wood facades
The other great enemy of outdoor wood is the sun. The action of the ultraviolet rays produces in the wood the destruction of the lignin, taking then this typical grayish aspect that nowadays is so fashionable. A wood that has not been treated outdoors will tend to this gray color and at the same time will facilitate the appearance of molds on the surface.
There are a multitude of exterior wood finishes on the market, but not all of them are as beneficial for future maintenance. The micronage of a protective element and dark tones will make your facade look better for a long time, but how do you proceed to replace the finish?
Our recommendation is not to use elements that create a film on the wood and thus avoid the tedious sanding that in certain design of wood facades can become very complicated. There are different wood saturators whose maintenance requires cleaning and a simple application.
It is equally important to seal the heads of exposed wood or the edges of marine plywood boards that may be part of a facade. Epoxy or acrylic elastic resins are ideal for this purpose.
5- A correct installation
A good construction solution is always based on a correct installation. For this final process, it is considered from the beginning of an efficient design of wood facades.
In order to prevent the possible irregularities that usually exist in the base support (leveling and plumbing), it is important to study the detail of the wood elements’ attachment to the wall and to use systems that facilitate this adjustment.
There are mechanical fastenings, bolted fastenings and even wood-wood connections such as the one used by GUBIA in the entrance atrium of the Chamartín Tower. A timber tie studied with high shear resistance values.
All this entails a costly study of evolving solutions, but our work philosophy must always be to do things well, as Mies said “God is in the details” and wood is not a problem if you know how to work with the material.
6- Perform an adequate maintenance of the facade
It is key to make the end customer aware that in the design of wooden facades, maintenance must be taken into account. Who does not moisturize after going to the beach? Depending on the orientation of the facade and the finish micronage applied to it, maintenance can be performed every two or even every five years. In that sense, there are firms that can offer such guarantees.
We should not trust the correct functioning of a facade only to the guarantees of a finish, since this only depends on the aesthetic result. As we exposed previously there are other five keys that are the ones that will assure that the wood does not rot.
We hope we have helped you with these recommendations. Contact our technical department for any questions you may have. We have been studying façade solutions for more than a decade, developing systems, carrying out various types of tests. All of that guarantees the design of wood facades as a complete success.
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