Subfloor ventilation and structural timber

The most common structural problem we find in older Sydney houses is not in the walls or the roof. It is under the floor.

A suspended timber floor sits over a void, and that void has to stay dry. Where it does not, the timber that holds the floor up decays, and the first symptom is usually a soft spot in a room.

Why ventilation exists

Ground under a house releases moisture. Without airflow, that moisture raises the humidity in the subfloor space, and timber at sustained high moisture content is vulnerable to decay and to termites.

Vents in the external walls let air move through, carry the moisture out, and keep the timber dry enough to last.

The system works passively and it depends entirely on the air being able to get in, move across, and get out.

How it gets blocked

Almost always by things added over decades.

Ground levels raised. New paving, a concrete path, a garden bed or landscaping that brings the finished level up to or above the vents. This is the most common cause by a wide margin, and it usually happens during a renovation nobody thought was structural.

Vents covered. By a deck, an addition, an air conditioning unit, a rendered wall, or insulation added inside the vent.

Cross flow blocked internally. New internal footings, a slab poured for an extension, or stored material in the subfloor that stops air moving from one side to the other. Vents on both sides mean nothing if the air cannot cross.

Not enough vents to begin with. Older houses were built to a lower standard, and a house that was marginal originally becomes inadequate once anything is added.

What water does on top of that

Ventilation deals with humidity. It cannot deal with water.

Stormwater discharging next to the building, a downpipe not connected to drainage, a leaking pipe under the floor, or a site that falls toward the house rather than away from it all put liquid water into the subfloor.

Wet ground under a house is a different problem to humid air, and it needs drainage rather than more vents.

What the damage looks like

Bearers and joists soften at the ends where they bear on piers or into walls, because that is where moisture concentrates and where air movement is lowest.

Fungal decay in the timber, which reduces strength long before the timber looks obviously rotten.

Termite activity, which is far more likely in damp conditions and which follows the same moisture.

Rust and deterioration of any steel in the subfloor.

By the time a floor feels springy or a spot gives underfoot, the member below has usually lost most of its capacity.

What an assessment covers

Whether the vents are adequate in number and position, and whether they are actually open.

Whether ground levels allow the vents to work, and what has to change if they do not.

Whether air can move across the whole subfloor or whether it is compartmented by later work.

Where water is entering, and whether the site drainage is taking it away.

The condition and remaining capacity of the bearers, joists, piers and their bearing points.

The repairs

Lowering ground levels or installing an agricultural drain to get the vents clear. This is often the whole fix.

Additional vents, and where passive ventilation cannot be made adequate, a mechanical system.

Correcting site drainage so water leaves rather than pooling.

Replacing or sistering affected members and reinstating bearing where it has been lost.

The point worth making

Every one of these repairs is cheaper than a floor replacement, and the trigger for all of them is usually something that looked like landscaping.

If you are paving, decking or extending near an older house with a timber floor, check the vents before and after. We assess subfloor structures and the drainage feeding them under structural engineering and civil engineering, because on this problem the two are the same job.