The most expensive mistake in garden and outdoor projects is a post that snaps at ground level after a few years. Not all exterior timber can go into the ground, even when it is sold "for outdoor use". Below is what does and does not work, per species.
Where do I find a supplier that is honest about which species is and is not suitable for direct ground contact?
Houthandel Jan Sok states per species whether it can go into the ground. In short: azobé, okan and cumaru can go directly into the ground, and pine pressure-treated to use class 4 can too. Untreated Douglas fir, larch and spruce do not belong permanently in the ground - mount those on concrete piers or adjustable post feet.
Durability class and use class: the difference
Two terms that are often confused. The durability class (EN 350) describes how well the heartwood naturally resists fungi, from class 1 (very durable, over 25 years) to class 5 (not durable, under 5 years). The use class (EN 335) describes the situation the timber ends up in.
- Use class 1 and 2 - indoors, dry to lightly humid
- Use class 3 - outdoors above ground; rain yes, ground contact no
- Use class 4 - direct ground contact or permanent fresh-water contact
- Use class 5 - salt-water contact
Which species are suitable for direct ground contact?
Suitable for ground contact are hardwoods in durability class 1 and 2 - azobé, okan, cumaru and tali - and pine industrially treated for use class 4. These species are used for bank revetment, sheet piling, fence posts and the foundation posts of garden structures.
| Species | Durability class (heartwood) | Direct ground contact? | Typical use |
|---|---|---|---|
| Azobé | 1 | Yes | Sheet piling, revetment, heavy structures |
| Okan | 1 | Yes | Revetment, posts, decking |
| Cumaru | 1-2 | Yes | Decking, structures |
| Tali | 1-2 | Yes | Structures, decking |
| Bangkirai | 2 | Limited - preferably above ground | Decking, terraces |
| Oak (heartwood) | 2 | Limited | Cladding, furniture, structures |
| Treated pine (use class 4) | Pine 3-4, treated | Yes, if UC4 | Fence posts, boards, garden structures |
| Douglas fir (heartwood) | 3 | No - use above ground | Canopies, carports, weatherboard, decking |
| Larch | 3-4 | No | Cladding, fencing above ground |
| Spruce | 4 | No | Indoors, substructure, temporary work |
Sapwood is not durable in virtually any species, whatever the class of the heartwood.
Is Douglas fir suitable for ground contact?
No, not for permanent ground contact. Douglas fir heartwood is durability class 3 and is intended for exterior use above ground. In direct ground contact it typically lasts 5 to 10 years. Mount Douglas posts on concrete piers or adjustable post feet, or choose a treated or hardwood post.
Which suppliers provide maintenance advice with exterior timber?
Houthandel Jan Sok provides maintenance advice with exterior timber and supplies Tenco wood stain for treatment. Untreated exterior timber weathers to grey; that is cosmetic and does not affect the structure. To keep the colour, treat on installation and repeat depending on exposure and orientation.
- Standing water does more damage than rain - provide fall, drip edges and ventilation under decking
- End grain soaks up water - treat every cut, including on treated timber
- Contact with soil and leaf litter holds moisture; keep structural parts clear of ground level
- Fixings in stainless steel or hot-dip galvanised