There are many myths and doubts about some uses of wood, mainly in the construction world. In this article we will try to clear up some doubts.
Is it true that wood is more resistant to fire than steel?
Yes. The fire resistance of wood, compared to steel or concrete, is another important parameter in the design of a building. The fact that wood is a combustible material need not be misleading. In case of fire, the wood evolves producing a slow combustion: the penetration of the carbonized zone in a section of wood is less than 1 millimeter per minute. And it decreases as the fire progresses because the carbonized layer itself insulates the interior from the heat From the play.
However, structural elements, such as steel, which are not combustible, enter the deformation zone plastic when the fire area reaches a certain temperature and causes collapse. Therefore, metallic structures must be protected with special paints against fire.
How long does a wooden roof last?
Always, if water ingress is avoided.
There are examples of wooden buildings that last for centuries: the Katsura Palace in Kyoto has maintained its original wooden structure since 1650. The “Stavkierke” – Norwegian churches – were built over a thousand years ago. Other examples of wooden buildings that are hundreds of years old are the vast majority of the domes and roofs of Christian cathedrals and churches, or the granaries, in wood, with specimens of more than five hundred years old in the central-eastern part of Asturias.
In addition, the evolution of fungicide and insecticide treatments means that wood faces an unstoppable evolution as a building material.
Does wood rot near the sea?
No. For wood, unlike steel, unaffected by saltpeter of marine environments, so its use in buildings, bridges or furniture in coastal areas is imposed.
Only if it is in contact with sea water, for example, in pontoons or docks, in certain areas can it be attacked by the larva of a mollusk, the “teredo marine”, also known as “joke”. The wood that will be in this class of use, number 5 of the CTE, must be treated in depth in an autoclave with copper salts.
Another important aspect is the corrosion resistance: the wood does not react or is attacked by chemical agents such as chlorine. This is one of the reasons why, currently, its use is practically mandatory in covering swimming pools.
What treatment should be applied to a laminated wooden roof structure?
This is a very frequent question, since in many occasions in the execution of projects, in-depth treatments are prescribed in an autoclave, when these types of treatments are indicated for exposure to the weather and in the open.
A ventilated wooden roof structure (laminated or sawn) with waterproofing blade will be of Class 2 Use, according to the Technical Construction Code – Structural Safety – Wood. Under these conditions, the level of protection required must be superficial, with an open pore product (lasur) with insecticidal and fungicidal properties (NP1).
I want to make an pergola in my garden. I heard that you shouldn’t use fir wood. Is right?
Yes. Uncovered pergola is an element that will be outdoors (Class of use 3.2, according to CTE) and will require deep treatment in an autoclave with copper salts.
Highly impregnable wood must be used for the treatment to be effective against mold and rotten insects, and the spruce is not. The most recommended wood is pine. Although the mechanical properties of these two species are very similar, they have very different behaviors in terms of impregnation.
In addition, for protection against ultraviolet radiation, we recommend that you apply an open pore paint or stain of an intermediate color. Otherwise, it will turn gray over the years.
Why does wood rot? Can this be prevented?
Wood rots because it is attacked by decaying fungi (they eat wood). There are several types: brown, white rot … It can be easily avoided by keeping the moisture content of the wood not exceeding 18 – 20% moisture: just do not get wet for a long time. The only way for fungi to progress on wood is above these moisture levels.
Another way to prevent rotting, when predicted humidity levels are going to be high, is by application of fungicidal protectors, by brushing or in an autoclave, depending on the exposure.
Or also using highly rot-resistant woods, like the tropical ones. Unfortunately, this type of wood may come from illegal logging in developing countries. In these cases it is not an ecological alternative, so we advise you to check its origin or look for alternatives.
… The project includes laminated wood treated in an autoclave. Can this wood be treated like this?
Yes, but treating only sheet by sheet separately, drying and laminating with treated and dried wood sheets. Note that in an autoclave the protectors are applied by pressure and vacuum cycles, which can cause delamination of the parts.
In any case, if the use is for a ventilated roof (Class of Use 2) and not for the open air (Class 3 or higher), this level of chemical protection is not necessary. The Technical Construction Code prescribes as recommended the superficial application of a stain with fungicidal and insecticidal properties.
What is C18 or C24 wood? And the GL24h?
The first two refer to lumber of conifers (C) with resistance to flexion fm, k of 180 and 240 Kg / m2 respectively.
The third for glued laminated wood (GL means “Glue”) with fm, k of 240 Kg / m2 and homogeneous composition (H). In the Technical Construction Code, the strength classes of each species of wood for structural uses are defined.