PERTHThe little grille in the brickwork is doing a job
Subfloor ventilation in Perth's timber-floored and suspended-slab homes: why the void has to breathe, the 150mm rule, how vents get blocked
Most of Perth is built on a concrete slab poured on the ground, which has nothing underneath it to ventilate. But two kinds of house do, and on those the subfloor is frequently the most informative half hour of an inspection: the older timber-floored stock in the inner suburbs, and any house built on a suspended slab over a void.
Two floors, one problem
A suspended timber floor sits on stumps or piers above the ground, with a ventilated void beneath. A suspended concrete slab does the same thing in concrete. Either way there is a space under the floor, and either way that space has one job it must not fail at: staying dry.
Building and Energy has published specifically on the concrete version, because it is the one people assume is immune. Its bulletin reminds industry that ground moisture must be considered in the design and construction of a suspended concrete slab, so that the slab does not become a path for damp to reach the building. Concrete is not waterproof, and a slab over damp ground with nowhere for the moisture to go will carry it upward.
Why the void has to breathe
Ground gives off moisture constantly. In an unventilated void that moisture has nowhere to escape, so it condenses on the underside of the floor and on the timber in it. Sustained dampness is what rot needs, and it is what draws termites to establish without ever touching the soil outside.
The fix is airflow. Vents in opposing external walls let air cross the void and carry the moisture out. The standard clearances are specific — a void of at least 150 mm below suspended floor members, vents placed so air has a clear cross-path to every part of the void, and no dead corners where damp can pool. Where a supporting wall crosses the void it has to be built open, honeycomb fashion, so air is not trapped on one side of it.
What goes wrong, and it is almost always the same thing
The vents get blocked. That is the finding, over and over.
A garden bed built up against the wall over the airbricks. A new path or paved alfresco poured across them. Render bridging them. A renovation that added a room and sealed one side of the void without ducting air through to it. Every one of these is an ordinary improvement made by someone who did not know the little terracotta grille in the brickwork was doing a job.
Once the air stops moving, the void goes damp, and the damage begins out of sight. By the time it shows as a springy floorboard or a musty smell in a room, the timber below has usually been working on the problem for years.
Perth's own version
Two things make this sharper here than the textbook suggests. Across the Bassendean sand belt through the middle suburbs, parts of the system sit over the Gnangara Mound with the water table close to the surface, so a void that would stay dry elsewhere does not. And in the older river-terrace suburbs on the Guildford Formation, clay holds moisture near the surface long after rain. Both put more water under the floor than the vents were sized for.
Every suburb page on this site opens with which ground the houses there sit on, and for a subfloor that is not background detail — it is the whole risk.
What we do down there
Where there is safe and reasonable access we go into the subfloor and look: the condition of stumps, bearers and joists, moisture readings in the timber, whether the vents are clear and cross-flowing, and any sign of termite workings or fungal decay. Access is the limit. Under AS 4349.1 a subfloor with a crawl space below the reasonable minimum, or blocked entirely, is recorded as not inspected, with the reason stated.
An inspector who reports a clean subfloor they could not physically enter has told you nothing, and on a timber-floored Perth house the subfloor is exactly where the expensive surprises live. It is one of the reasons a pre-purchase inspection on older stock is worth more than the same inspection on a modern slab, and why the timber pest scope matters most on precisely these houses.
Why a subfloor has to breathe
In a house with a suspended timber floor, there is a void between the ground and the floor above, and that void has to be ventilated for a simple reason: the ground beneath it is a source of moisture, and if that moisture is allowed to build up in a still, unventilated space, it keeps the timber of the floor structure damp, and damp timber decays. The subfloor vents, the openings around the base of the house, exist to let air move through the void, carrying moisture out and keeping the space dry enough that the bearers and joists stay sound. When the ventilation works, the void breathes, the timber stays dry, and the floor structure lasts. When it fails, the void becomes a damp, still space, and the conditions for timber decay and for attracting termites are exactly what develops.
This is why subfloor ventilation, which sounds like a minor technical detail, is actually one of the things that most determines whether the floor structure of an older Perth home stays sound over the decades. The floor structure is out of sight, in a space few people ever enter, and its condition depends heavily on whether the void it sits in is dry, which depends on whether the ventilation is doing its job. A well-ventilated subfloor is a quiet success nobody notices; a poorly ventilated one is a slow problem developing where nobody is looking.
What goes wrong, and it is almost always the same thing
The striking thing about subfloor ventilation problems is how consistent their cause is: it is almost always blocked or inadequate vents, and the blocking is almost always something added around the house after it was built. A garden bed banked up against the wall, covering the vents. A new path or paving laid across them. A concrete or brick addition that sealed the airflow. Landscaping that raised the ground level over the vents. Debris and vegetation accumulated against them. In each case the ventilation the house was built with has been compromised not by a fault in the original design but by something done later that blocked the air, and the result is the same: the void stops breathing and starts holding moisture.
This is worth knowing because it means the problem is usually both preventable and fixable. Keeping the subfloor vents clear, not banking garden beds or paving over them, not letting the ground level rise across them, not letting debris accumulate against them, is ordinary maintenance that keeps the void breathing. And where the vents have been blocked, clearing them, or where necessary improving the ventilation, restores the airflow the timber depends on. The trap is that the blocking happens gradually and invisibly, a garden bed built up over years, a path laid one summer, and by the time the consequences show in the floor structure, the connection to the blocked vents is not obvious to an owner who never goes under the house.
What an inspection does down there
The subfloor is one of the places where the difference between a thorough inspection and a cursory one is starkest, because a proper inspection gets into the subfloor void where it is accessible, and a poor one glances from an access hatch or skips it entirely. Getting into the void, where access allows, is where an inspector can actually see the condition of the floor structure, the bearers and joists, look for the signs of timber decay and termite activity that concentrate in damp subfloors, and assess whether the ventilation is adequate or has been compromised. So much of what matters about an older home's floor structure is only visible from underneath that a subfloor inspection, where the void is accessible, is one of the highest-value parts of the whole exercise.
There are limits, and an honest report states them: a subfloor too low to enter safely, or access that is genuinely blocked, means the space cannot be fully inspected, and the report should say so rather than imply it was assessed. But where the void is accessible, a careful inspector reads a great deal: the state of the timber, the moisture conditions, whether the vents are clear, whether there are signs of decay or termites, whether the ground and drainage under the house are keeping it damp. In Perth, where the older suburbs hold the suspended-floor stock and where blocked vents are the near-universal cause of subfloor problems, that look underneath is often where the real condition of an older home is decided, and it is worth making sure any inspection actually includes it rather than skipping the space where the important things hide.
Common questions
Why does a subfloor need ventilation?
Ground gives off moisture. In an unventilated void it condenses on the floor timbers, and sustained dampness causes rot and draws termites. Cross-ventilation through opposing wall vents carries the moisture out.
What blocks subfloor vents?
Usually ordinary improvements: garden beds, paths or paving built up over the airbricks, render bridging them, or a renovation that sealed one side of the void without ducting air through.
Do concrete-floored houses need subfloor ventilation?
A suspended concrete slab over a void does. Building and Energy's Industry Bulletin 120 reminds industry that ground moisture must be managed in suspended concrete slab construction.
This comes up on every pre-purchase building inspection and building & timber pest inspection we carry out.
Read next: the 25mm gap that decides whether your termite barrier works and wet areas are the most commonly failed part of a new build.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.