Sub Floor Ventilation And Timber Rot

A suspended timber floor sits over a void, and that void has to stay dry.

When it does not, the timber in contact with the moist air goes first: bearers, joists, the ends where they sit on the brickwork. By the time the floor feels soft upstairs, the damage below has usually been running for years.

What you notice first

A musty smell in a room, strongest near the floor and worst after rain.

Springy or bouncy floorboards, particularly in one area rather than throughout.

Cupping or lifting boards, where the underside is taking up moisture the top side is not.

Skirtings separating from the wall at floor level, which is often read as a settlement crack and is sometimes a floor dropping. The distinction matters, and it is the same reading problem as [ceiling and wall cracks, what they mean](/blog/ceiling-and-wall-cracks-what-they-mean).

Paint failing on skirtings and door frames near the floor only.

Why the void goes damp

Blocked or painted over vents. The sub floor was built with vents in the brickwork, and over decades they get rendered over, painted shut, blocked by garden beds, or covered by a new path. Each one removed is airflow removed.

Ground levels raised outside. A new path, a deck, a garden bed or added topsoil lifts the external level above the vents or above the damp proof course, and the brickwork starts drawing moisture up. One of the most common causes and the least obvious.

Water entering the void. A leaking pipe, a downpipe discharging at the wall, or stormwater with nowhere to go. Which is why [stormwater pits and grates on a property](/blog/stormwater-pits-and-grates-on-a-property) and sub floor dampness are frequently the same job.

No cross flow. Vents on one wall only means air comes in and sits. The void needs openings on opposite sides so air actually moves through.

An internal obstruction, where a later addition, a pier wall or a slab poured inside the void has split it into pockets with no path between them.

The order the work goes in

Air first, then timber. Replacing rotted joists in a void that is still wet produces new timber with the same future.

Clear and restore the vents. Cut back render, remove paint build up, clear vegetation, and add vents where the cross flow is missing. The simplest step and usually the largest single improvement.

Fix the ground levels. Lower the external level below the vents and the damp proof course, and get the paving and garden beds away from the wall.

Fix the water sources. Downpipes discharged properly away from the building, surface water directed to a pit, and any plumbing leak repaired. A dripping outdoor tap at the wall is a sub floor problem, which is part of why [outdoor taps, irrigation and garden plumbing](/blog/outdoor-taps-irrigation-and-garden-plumbing) is worth looking at on an older property.

Add mechanical ventilation where passive airflow cannot be achieved, on a tight site where the clearances are too small or the void is landlocked.

Then repair the timber, once the void is drying. Bearer and joist ends, the pier bearing points, and the boards above.

What gets missed

The pier and bearer bearing points, because they are the hardest places to see and the wettest. A joist that looks sound in the middle of its span can be gone where it sits on the brickwork.

Which is why the inspection is a crawl rather than a look through a vent, and why the scope is written after that crawl. It is the same reason the carpentry findings in [the carpentry faults that hide inside other jobs](/blog/the-carpentry-faults-that-hide-inside-other-jobs) show up late when nobody went under the floor.

The other thing that gets missed is that the repair crosses trades. Clearing vents and adjusting levels is bricklaying and site work, the drainage is plumbing, the structural timber is carpentry, and the floor finish and skirtings are another trade again. One team scoping all of it at once is the difference between a repair and four visits that each uncover the next.

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