Termites, humidity and choosing timber in Nairobi

This is the page that decides whether joinery in Nairobi lasts eight years or thirty, and it is almost never discussed at quotation stage.

Two things destroy timber here, and they are not equally understood. Termites get the attention. Water does more of the damage.

Water first, because it is the bigger problem

Timber does not rot because it got wet. It rots because it stayed wet. That distinction is the whole of the practical advice.

So the failures cluster in specific, predictable places:

End grain. Timber absorbs water through a cut end many times faster than through a face. An unsealed bottom rail on a door, or the sawn end of a post, is an open straw. This is the single most common cause of a component failing while the rest of the item looks perfect.

Ground contact. A post set into concrete or soil wicks water upward, and the rot starts inside where nobody sees it until the post moves. Every failed pergola and gazebo we have been asked to look at failed at the bottom of a post.

Where water sits. A window sill with no fall, or with a drip groove painted solid. A gutter out of fall holding water against a fascia board. A flat joint on an outdoor structure with no way to shed.

Damp shade that never dries. On the valley side of Kitisuru, and under the mature trees in Karen, exposed timber sits in damp air and does not fully dry between rains. The same structure in an open Runda compound has a completely different life expectancy.

Nairobi has two rainy seasons, which is the design condition

Long rains March to May, short rains October to December. So timber here goes through two wet-dry cycles a year, not one, and it moves each time.

That means a joint detailed rigidly will open. Panels have to be able to move within frames, long runs need provision for movement rather than being screwed solid at both ends, and a staircase built from stock that was not properly seasoned will creak progressively rather than settle.

Termites, properly

Termites need moisture and a route. Subterranean species work up from the ground through mud tubes, which is why ground contact is again the issue, and why a timber structure standing clear on a metal shoe is dramatically less exposed than one set in the ground.

They eat from the inside out. A skirting or a post can look entirely sound with almost nothing left inside, so the test is to tap it and listen rather than to look. Mud tubes on masonry near timber are the clearest early sign.

Naturally durable heartwood — mvule, camphor, Kenyan cedar — resists attack well. Two caveats that matter: resistance is not immunity, and the sapwood of a durable species is not durable. A length with a wide pale band is not the product you thought you bought.

Borer beetles are the other insect, and they leave small round exit holes with fine dust below. Common in furniture and in older joinery; a different treatment from termites.

What we do about it, on every job

• Seal end grain, including the ends nobody will see
• Keep timber clear of soil and standing water, on a shoe or an upstand rather than in concrete
• Treat non-durable timber, particularly cypress in structural use, and treat the cut ends after cutting rather than only the lengths before
• Specify by position rather than by price list: sun-exposed and damp-shaded are opposite problems needing opposite answers
• Detail joints to shed water rather than hold it
• Say honestly what will need maintaining and roughly when

None of that is expensive at build time. All of it is expensive to retrofit.