Wood is a natural and living material that changes under the influence of biological agents, particularly fungi and insects. These risks depend on wood's exposure to humidity.
The durability of wood used in deck construction can be determined by identifying the external stresses and humidity that will affect the structure. Wood is divided into four main grades. The important thing is to select (treated or untreated) wood that is sufficiently durable to withstand the different types of stress. Most modern wood decks designs call for Grade 4 woods, which means they are permanently exposed to humidity with moisture content over 20%. For more information, see reference books on the topic.
Wood develops a number of distinct characteristics as it grows and ages (knots, cracks, etc.). These distinct characteristics on a wood board do not necessarily prevent it from being used as planned, because the impact on its technical characteristics is often non-existent.
Maximum width/thickness ratio of board section:
To avoid excessive board deformation, there is an upper limit to the width/thickness ratio. The limit depends on the natural inherent stability of the wood species. Thus, the more stable the wood during expansion and contraction, the higher the authorized upper limit can be.
Authorized values are listed in the table below.
For safety and stability reasons, boards should not be thinner than the minimum thickness values listed in the table below.
Top edges of boards should be cut square. For top edges that are rounded, the radius of the curve should be greater than or equal to 2 mm.
Wood deck boards undergo expansion and contraction cycles during their lifespan according to climate fluctuations. During these cycles, wood is subject to warping, splitting, cracking, etc. Whatever the species selected, the designer needs to know the characteristics of each species to produce a high-quality structure. The following scale was used in the table below.
Low stability (LS) / Medium stability (MS) / Stable (S)
Depending on the client's preferences, the comparative concepts of hardness among species should be taken into account. The following scale was used in the table below:
Low hardness H1 -> Very high hardness H5
The values in the table below are for information purposes only and may under no circumstances be used as a reference in contracts.
Species |
Hardness |
Stability |
Max. width/thickness ratio* |
Minimum thickness |
Douglas (Pseudotsuga menziesii)traité ou non traité |
D2 |
MS |
6 |
20 |
Mélèze
(Larix decidua) |
D3 |
MS |
6 |
20 |
Sapin épicéa traité classe 3a |
D2 |
S |
6 |
20 |
Pin Maritime
(Pinus pinaster) |
D3 |
MS |
6 |
20 |
Pin Sylvestre
(Pinus sylvestris) |
D2 |
MS |
6 |
20 |
Western red cedar (Thuja plicata) |
D1 |
S |
7 |
27 |
Western Hemlock
(Tsuga heterophylla) |
D2 |
MS |
6 |
20 |
Châtaignier (Castanea sativa) |
D3 |
MS |
5 |
22 |
Pin jaune (Pinus strobus L.) |
D2 |
MS |
6 |
20 |
Chêne Rouvre ou pédonculé |
D3 |
MS |
5 |
22 |
Robinier (Robinia pseudoacacia) |
D4 |
PS |
4 |
22 |
Species |
Hardness |
Stability |
Max. width/thickness ratio* |
Minimum thickness |
Azobe (Lophira alata) |
D5 |
PS |
4 |
50 |
Angelim vermelho (dinizia excelsa) |
D5 |
PS |
4 |
50 |
Basralocus (Dicorynia guinensis) |
D4 |
MS |
6 |
20 |
Bilinga (Nauclea diderrichii) |
D4 |
MS |
5 |
27 |
Bangkirai (Shorea spp. Section Shorea) > 850kg/m3 |
D4 |
MS |
6 |
19 |
Cumaru (Dypterix spp.) |
D5 |
MS |
6 |
19 |
Doussié (Afzelia spp.) |
D4 |
S |
6 |
19 |
Garapa (Apuleia leiocarpa) |
D4 |
MS |
5 |
20 |
Gonçalo Alves (Astronium spp.) |
D4 |
MS |
5 |
19 |
Greenheart (Chlorocardium rodiaei) |
D5 |
MS |
4 |
27 |
Ipe (Tabebuia spp.) |
D5 |
S |
7 |
19 |
Iroko (Milicia spp.) |
D4 |
MS |
5 |
21 |
Itauba (Mezilaurus itauba) |
D3 |
MS |
5 |
19 |
Jatoba (Hymenaea spp.) |
D4 |
MS |
5 |
19 |
Kapur (Dryobalanops spp.) |
D3 |
MS |
6 |
19 |
Keruing(Dipterocarpus spp) |
D3 |
PS |
4 |
27 |
Maçaranduba (Manilkara spp.) |
D5 |
PS |
5 |
21 |
Makoré(Tieghemella spp.) |
D3 |
MS |
6 |
27 |
Merbau (Intsia bijuga & palembanica) |
D5 |
S |
7 |
19 |
Moabi (Baillonella toxisperma) |
D4 |
MS |
6 |
19 |
Mukulungu (Autranella congolensis) |
D4 |
PS |
4 |
50 |
Padouk (Pterocarpus soyauxii) |
D4 |
S |
7 |
19 |
Piquia (Caryocar sp.p) |
D3 |
MS |
4 |
27 |
Pynkado (Xylia dolabriformis) |
D5 |
MS |
4 |
27 |
Tali (Erythrophleum spp.) |
D5 |
MS |
4 |
27 |
Tatajuba (Bagassa guianensis) |
D4 |
PS |
4 |
21 |
Teck (Tectona grandis) |
D3 |
S |
7 |
19 |
Under the sun's ultraviolet rays, the natural colour of wood progressively changes to grey. The change in colour is only at the surface level and can take more or less time depending on exposure. The colour of wood boards that are not UV protected begins to change within six months of installation.
This natural phenomenon has no impact on wood strength or durability.
The same applies to treated wood. However, the wood changes more slowly.
