Problem
Garage floor moisture
If moisture is coming up through the slab, every coating you put on top will fail. The fix starts with a test, not a tin.
Diagnostic indicators
How to recognise this on your floor.
None of these are diagnostic on their own. Two or three together usually means we are looking at the same thing.
Damp or darker patches
Damp or darker patches that come and go with the weather
White powdery deposits (efflorescence)
White powdery deposits (efflorescence) on the slab surface
A musty smell in
A musty smell in the closed garage
Condensation on cold mornings
Condensation on cold mornings that takes hours to clear
A previous coating that
A previous coating that lifted in the same spot more than once
Mould or staining at
Mould or staining at the wall-floor junction
Plain English
What is going on.
Concrete is permeable. Even a sound slab can move several grams of water vapour per square metre per day, and a slab without a working damp-proof membrane can move many times that. Every coating system has a moisture vapour transmission rate it can tolerate. If the slab pushes more water than the coating can breathe, the coating lifts.
Moisture is the single most-missed cause of coating failure in Melbourne. It is also the most testable. A relative humidity probe and a calcium chloride test are 30-minute jobs that tell us exactly what the slab is doing on the day we read it.
The path forward depends on the number. Some slabs need an epoxy moisture-mitigation primer before any decorative system. Others need passive ventilation work. A small minority need an injected damp-proof course. We will say which one applies before we recommend a finish.
Engineering cause
Why slabs sweat
Three structural causes, often layered together. The damp-proof membrane below the slab was punctured during the pour, or never installed, or has degraded. Surface drainage outside the garage directs water toward the slab edge. And the garage envelope is sealed tightly enough that what does come up cannot evaporate, so it pools at the surface.
None of those are visible from above. That is why the diagnostic tests matter. A coating crew that recommends a finish before testing is guessing.
The path
What we do about it
Recommended path: Tier 3 Expert assessment
Bill runs a Tier 3 Expert assessment on site. Calcium chloride test, relative humidity probe, visual inspection of the slab perimeter and any existing coatings. You get a one-page report with the test numbers, a photographed failure map, and the engineered specification for a system that will tolerate what your slab is doing.
If the slab is dry enough to coat, we say so and the path opens to a normal garage finish. If it needs a moisture-mitigation primer first, we specify which one and price both layers. If the slab is too wet to coat at all, we will tell you that too, and point you at the structural fix that has to come first.
Brief Bill
Send the brief, we will read the slab.
Tier 3 Expert engagement. Fee is $900 and credits against the eventual remedial contract. One business-day reply from Bill.
Common questions
Before you ask, this is what most people want to know.
You do not, with certainty, until we test. The signs above are strong indicators, but the only way to be sure is a calcium chloride test or a relative humidity probe. Both are part of the Tier 3 assessment.
$900 fully credited against any remedial work you commission with us. The fee covers Bill's site visit, the moisture tests, the photographed report, and the engineered specification.
Sometimes, with a moisture-mitigation primer engineered for the test reading on the day. Sometimes not. The number on the test sheet decides; the marketing on the can does not.
If the cause is a structural drainage issue we have specified a fix for, then no. If the cause is a buried damp-proof membrane that has failed and you have chosen to live with it under a moisture-mitigation primer, the primer carries the load and the topcoat performs normally.
On site, about 90 minutes. The calcium chloride test takes 60 to 72 hours to settle, so the written report follows by email within 5 business days of the visit.
New slabs are wet by definition. Concrete typically needs 28 days per 25 mm of thickness before it is dry enough to coat. We can advise without a formal Tier 3 if the pour date is recent and known.
Ready when you are
Most rescues end on a properly engineered garage finish.
Three finish tiers, prices on the page, fixed before we start.