Outdoor wood structures: some basic principles

Use Class

Woods intended for outdoor use must be selected according to their use class, which certifies their resistance to water exposure. The class choice should fit the nature of the structure:

• Class 3: resistant to temporary water exposure (deck furniture).
• Class 4: resistant to prolonged contact with soil or fresh water (fence posts, grates, decking).
• Class 5: resistant to permanent contact with salt or chlorinated water (immersed pontoons, marine structures).

This resistance can be natural, depending on the wood species, or achieved through treatment (pressure treatment, high-temperature treatment, etc.). In France, several local species are naturally class 3: chestnut, larch, oak, cedar, Douglas fir. Others can naturally reach class 4 (black locust) or even class 5 (cypress). Durable exotic woods, rated from class 3 to 5, are also available, such as ipe, teak, or iroko.

Heartwood and Sapwood

Not all parts of the wood have the same durability. The heartwood, located near the tree’s core, is denser and naturally more resistant. The sapwood, just under the bark, is lighter and more vulnerable to wood-boring insects and moisture. It is therefore preferable, whenever possible, to use heartwood in structures exposed to the elements.

Longitudinal and End Grain Wood

The orientation of wood fibers is an essential factor. The end grain wood, when its fibers are exposed to moisture without protection, is particularly susceptible to water infiltration. This can cause cracks, splinters, or deformation over time. It should therefore be protected or avoided in exposed parts, or edges should be rounded or chamfered to prevent water stagnation on these surfaces.

Wood Movement

Wood is a living material that reacts to humidity and temperature changes – seasonal cycles, dry/wet periods. These cause dimensional movements (shrinkage or swelling). It is thus recommended to avoid tight joints that could crack in humid weather.

Board Orientation

The orientation of boards directly influences their sun exposure and thus the UV rays, which cause wood graying. Horizontal surfaces (decks, tabletops) are more exposed and degrade faster than vertical surfaces (cladding), which often benefit from partial protection (roof overhangs, eaves, etc.).

Water Drainage

On large horizontal surfaces, it is highly recommended to use spaced slats to facilitate water drainage. This helps prevent stagnation, a source of accelerated deterioration.

Dimensioning Sections

The wood sections used for the structure should be slightly oversized. This improves mechanical stability over time and provides better resistance to external stresses (rain, frost, temperature variations).

Avoid Direct Ground Contact

Wood must never be in direct contact with the ground, especially on porous surfaces (concrete, stone, gravel), to prevent capillary rise.

• For decks, bituminous strips are placed between joists and supports or pads.
• For outdoor furniture, glides or shims are used under the legs, providing both ground isolation and good ventilation.