Subfloor & Roof Void Mould Treatment, Wollongong & the Illawarra
The house smells musty — worse after rain, worse when it’s been shut up — but you can’t find mould anywhere you look. That’s the classic signature of growth in the spaces you don’t live in: under the floorboards or up in the roof cavity. In the Illawarra’s older housing stock it’s one of the most under-diagnosed problems we see — it won’t announce itself until floorboards cup or the smell becomes impossible to ignore.
We inspect these hidden spaces properly, treat the growth, and fix the airflow and moisture conditions that caused it, with all work fulfilled by qualified, licensed local remediation professionals in our partner network.
Why Illawarra Subfloors and Roof Voids Grow Mould
Much of the housing from Corrimal through Woonona and Bulli to the coastal villages was built as raised-floor weatherboard or double-brick on blocks that slope off the escarpment. Those subfloors were designed to breathe through perimeter vents — vents that decades of garden beds, paving, rendering and extensions have often buried. Add escarpment stormwater running under the house after heavy rain and you get damp soil, still air and cold timber: exactly what mould wants.
Roof voids fail differently: small leaks around flashings that never quite reach the ceiling, exhaust fans ducted into the cavity instead of outside, and condensation on the underside of sarking or metal roofing on cold clear nights — a problem we also see in newer, tightly sealed builds around Flinders and Shell Cove.
What the Service Involves
- Physical entry and inspection. A remediation professional gets into the subfloor or roof void — not a torch shone through a vent — and maps the extent of growth on bearers, joists, flooring undersides, roof timbers and sarking.
- Moisture profiling. Timber moisture readings, soil dampness and drainage patterns under the house, cavity humidity, and any active leak paths.
- Treatment of affected timbers. Growth on structural timbers is treated in place — typically HEPA vacuuming and appropriate agents — because you clean structural timber, you don’t rip it out for surface mould. Timber showing decay or structural concern is flagged for a licensed builder.
- Debris and material removal. Mould-affected insulation, stored junk, builder’s off-cuts and fallen leaf litter are bagged and removed, since they hold damp and feed regrowth.
- Ventilation and moisture correction. Clearing or reinstating blocked subfloor vents, mechanical subfloor fans where passive airflow can’t do the job, correcting exhaust ducting so it terminates outside, and referrals to licensed plumbers or drainage contractors where stormwater is getting under the house.
- Verification. Follow-up readings so you know the cavity is trending dry, with a written summary of what was found, done and recommended.
This inspect–treat–ventilate sequence follows the same source-first logic as our whole-home remediation, applied to the parts of the house nobody sees.
When You Need This Service
- A persistent musty or earthy smell with no visible mould in the living spaces
- Cupping, lifting or springy floorboards, or damp-feeling carpet on a timber floor
- Mould that keeps appearing at skirting-board level or on the lowest sections of walls
- Ceiling mould that returns after treatment, suggesting the cavity above is feeding it — often found via our bathroom and ceiling service
- A building or pre-purchase inspection that flagged subfloor damp or poor ventilation
- Older raised-floor homes that have never had the subfloor looked at, particularly after the run of wet east-coast-low years
If you want confirmation before committing to treatment, a standalone mould inspection and moisture investigation covers these cavities and gives you a written report first.
Our Process, Step by Step
- Describe the problem. Call (02) 0000 0000 or use the quote form. For hidden-cavity work, photos help less — tell us the smell pattern, the age and construction of the house, and your suburb, and we’ll know a lot already.
- Site inspection. Cavity work is never priced sight-unseen. The inspection establishes access, extent and cause, and produces an itemised scope and formal quote.
- Access check. Subfloor clearance varies wildly under older Illawarra homes — crawl spaces can shrink to a couple of hundred millimetres at the low side of a sloping block. Where there’s no workable entry, a licensed builder can cut an access hatch, priced before anything proceeds.
- Older-home safety screen. Pre-1990 homes can have asbestos-containing fragments in subfloors and eaves linings. Suspect material stops the job at that point — assessment and removal is exclusively for licensed asbestos professionals.
- Treatment day(s). Timbers treated, affected insulation and debris bagged out, vents cleared. Waste leaves through licensed disposal channels, with any regulated waste handled by appropriately licensed transporters.
- Ventilation works. Passive vent reinstatement usually sits within the same scope; mechanical subfloor or roof ventilation needs a licensed electrician for wiring and is quoted as its own line item.
- Drainage referrals. Where escarpment runoff or a broken stormwater line is the real driver, we say so plainly and refer you to a licensed plumber or drainage contractor; treating timber while water still flows under the house is wasted money.
- Re-check and handover. Follow-up moisture readings and a written record — useful for your own files and for pre-sale documentation.
What Affects the Cost
Access is the big one: a subfloor you can crawl through comfortably costs far less to treat than one that has to be worked in sections at minimal clearance. After that: the area affected, how much insulation or debris must come out, whether mechanical ventilation is needed, and what the moisture source turns out to be.
| Job type | Indicative range* |
|---|---|
| Subfloor or roof void inspection with moisture report | $300 – $800 |
| Roof void treatment (timbers, sarking, ducting correction) | $1,000 – $2,500 |
| Subfloor treatment (joists, bearers, debris and insulation removal) | $1,500 – $4,500 |
| Treatment plus mechanical ventilation installation | $2,500 – $6,000+ |
*Indicative guide only — confirmed after inspection with a formal itemised quote. The mould removal cost guide explains the drivers in more detail.
What’s Included vs What May Cost Extra
Included in a standard cavity treatment: entry and full-extent inspection, moisture readings, treatment of affected timbers, removal and licensed disposal of mould-affected insulation and debris, clearing of accessible existing vents, and a written completion summary with recommendations.
Quoted separately when required: cutting new access hatches (licensed builder), mechanical subfloor or roof ventilation systems and their electrical work (licensed electrician), new insulation, drainage and stormwater repairs (licensed plumber/drainage contractor), structural timber repairs (licensed builder), asbestos assessment or removal (licensed asbestos professionals), and treatment of living areas if growth has spread beyond the cavity.
Related Services and Where We Work
Cavity mould often travels: if it has reached bedroom walls and ceilings, whole-home remediation may be the honest scope, and if the trigger was a specific leak or storm event, start with our water damage response. The heaviest demand for this service comes from the older housing stock through Corrimal and the northern suburbs and Thirroul and the coastal villages, though raised-floor homes around Kiama, Jamberoo and Gerringong keep us busy too.
Subfloor & Roof Void Mould FAQs
How do I know it’s subfloor mould if I can’t see anything?
You mostly smell it: a musty, earthy odour strongest after rain, near floor level, or when the house has been shut up. Cupping boards and skirting-level mould are the visual tells. The only way to confirm is physical inspection with moisture readings — which is why our quotes start with one.
There’s white fluffy growth on the timbers under my house. Is that mould?
Possibly mould, possibly a timber-decay fungus — different problems with different fixes, and decay fungi can matter structurally. The inspection distinguishes the two, and anything structural is referred to a licensed builder.
Will treating the roof void fix the mould on my ceilings?
If the cavity is the source — a leak, condensation on the sarking, a fan dumping steam up there — then yes, treating the void and correcting the cause is what makes ceiling treatment finally hold. The two are often quoted together for exactly that reason.
Can you get into a really tight subfloor?
Usually. Our partner professionals work low-clearance subfloors regularly, and the inspection confirms what’s workable. Where a section genuinely can’t be reached, we say so and price an access hatch rather than pretending it was treated.
Will the musty smell go away?
Once the growth is treated, damp materials are removed and the cavity is drying, the odour fades — typically over days to a few weeks as airflow does its work. If it persists, something is still wet — the follow-up readings exist to catch exactly that.
Get the Hidden Problem Looked At Properly
If your house smells damp and you’re out of places to look, call (02) 0000 0000 or use the Get a fast quote form — tell us the suburb, the age of the house and where the smell is worst, and we’ll organise an inspection that gives you answers in writing.
quote form