How do I test my garage floor for moisture before applying a coating in NB?

Test your garage floor for moisture before coating by taping down plastic sheeting and waiting 24-48 hours — if condensation forms under the plastic, you have a moisture problem that needs to be addressed before any coating will adhere properly.

This simple test is called the plastic sheet test (sometimes called the polyethylene sheet test), and it's the most practical method for NB homeowners to use before applying epoxy or polyaspartic floor coatings. Cut a 45x45 cm piece of 6-mil poly, tape all four edges to the concrete with duct tape, and leave it for 24-48 hours. Check both the underside of the plastic and the concrete surface beneath it. Moisture on the underside of the plastic means water vapour is migrating up through the slab from the ground below. Moisture on top of the plastic (between the plastic and the tape) suggests the problem is condensation from humid air rather than ground moisture — a different issue with a different fix.

Why this matters so much in New Brunswick comes down to two realities: the Maritime climate brings persistent humidity, and many older NB garages were poured without a vapour barrier beneath the slab. Without that 6-mil poly under the concrete, ground moisture wicks upward continuously through the porous slab. In NB's shoulder seasons — particularly April through June when the ground is thawing and saturated — this moisture drive can be significant. Epoxy and polyaspartic coatings bond to concrete through a chemical and mechanical adhesion process that moisture completely disrupts. Even a coating that looks fine at first will bubble, peel, and delaminate within one or two freeze-thaw cycles if applied over a damp slab.

For a more precise reading, you can use a calcium chloride test kit (available at flooring supply stores for $20-$40 per test). You place a small dish of calcium chloride under a sealed dome on the concrete surface for 60-72 hours, then weigh the dish to measure how much moisture it absorbed. Results are expressed in pounds of moisture emission per 1,000 square feet per 24 hours. Most coating manufacturers specify a maximum of 3 lbs for epoxy and up to 5 lbs for polyaspartic — which is one reason polyaspartic coatings have become the preferred choice for NB garages. They tolerate more moisture vapour transmission than traditional epoxy, and they cure in cold temperatures, making them a better fit for NB's climate and older slab conditions.

Practical steps if your test shows moisture:

If the moisture is coming from above (condensation from humid air), the fix is timing — coat the floor during a dry stretch in late summer or early fall when humidity is lower and the concrete has had the summer to dry out. Running a dehumidifier in the garage for several days before coating also helps.

If the moisture is coming from below (no vapour barrier under the slab), you have a few options. A penetrating concrete sealer applied first can reduce — but not eliminate — moisture vapour transmission. Moisture-tolerant primers designed for wet slabs are another option, and your coating contractor should be specifying one if your slab shows any vapour drive. For severe cases, the honest answer is that a surface coating may not be the right solution — interlocking garage floor tiles or a floating floor system that allows moisture to pass beneath it will outlast any bonded coating on a slab with no vapour barrier.

Timing your coating project in NB matters as much as the moisture test itself. The ideal window is late May through September, when concrete temperatures are above 10°C and humidity is manageable. Avoid coating in early spring when the ground is saturated, or in fall when overnight temperatures drop and slow curing. The concrete surface temperature must be at least 3°C above the dew point at application time — your coating contractor should be checking this before they start.

If you're planning a DIY coating project, the moisture test costs almost nothing and takes two days. Skipping it and having to strip and recoat a failed floor costs $1,500-$3,000 in materials and labour — a painful lesson that NB's climate makes more common than in drier parts of the country. For professional coating installation, find local contractors through the New Brunswick Construction Network at newbrunswickconstructionnetwork.com.