Under-Slab Vapour Barriers: Why 6-mil Poly Isn't Enough (GTA Guide)

Water moves. A slab on grade sits directly on the ground, and the ground is almost never dry. Moisture in the soil and granular fill below the slab evaporates upward, travels through the concrete, and comes out the top — where it wrecks flooring adhesive, cups hardwood, bubbles vinyl and epoxy, and feeds mould under anything that traps it. The concrete looks fine. The floor on top of it fails a year later.

The fix is a vapour barrier under the slab. It’s cheap insurance on a problem that’s expensive and disruptive to chase after the floor is down. The catch is that the material most crews reach for — a roll of 6-mil poly from the general shelf — is the wrong material for the job.

Why 6-mil construction poly isn’t the answer

Standard 6-mil polyethylene is a fine temporary cover, a curing sheet, or a dust barrier. It is not an under-slab vapour retarder, for two practical reasons:

• It punctures. Granular fill is angular. Between the crew walking on it, the rebar and chairs being set, and the concrete being placed and screeded, thin poly gets pinholes and tears you’ll never see. Every hole is a path for moisture.
• It isn’t rated or tested for the job. The standard that matters here is ASTM E1745 — the specification for water-vapour retarders used in contact with soil or granular fill under concrete slabs. General-purpose 6-mil poly isn’t manufactured or tested to it, so an engineer or spec writer who calls for a Class A, B, or C barrier isn’t allowed to accept it.

That’s why the right product is a purpose-made under-slab barrier, usually 10-mil or 15-mil, that carries an ASTM E1745 class rating.

What ASTM E1745 actually tells you

ASTM E1745 rates under-slab barriers on three things: permeance (how little water vapour gets through), tensile strength, and puncture resistance. All three classes have to hold a low permeance; where they differ is toughness:

• Class A — the highest tensile strength and puncture resistance. The default for anything demanding, or where the barrier will take abuse before the pour.
• Class B — mid-range.
• Class C — the minimum, for lighter-duty conditions.

For most GTA slab-on-grade work, contractors run a Class A 15-mil sheet because the extra puncture resistance is what actually survives the site. Permeance matters, but on a real job the sheet fails at the holes long before it fails at the material — so puncture resistance is the number that protects you.

10-mil vs 15-mil: pick for the site, not the spec sheet

Both thicknesses can meet the standard. The practical difference is durability:

• 15-mil is the workhorse for slabs on angular granular fill, jobs with a lot of foot traffic before the pour, and anywhere the barrier sits exposed for a few days. It shrugs off the abuse.
• 10-mil is lighter and can be right for smaller or protected pours, but it’s more prone to pinholes when the crew is busy on top of it.

When in doubt, go heavier. The cost difference between thicknesses is small next to the cost of pulling up a failed floor.

Where it goes, and how to seal it

A vapour barrier only works if it’s continuous — one unbroken skin between the ground and the slab. Follow your drawings, but the standard practice (ASTM E1643 for installation) looks like this:

• Directly under the slab. The barrier goes on top of the granular base. If you’re also running under-slab rigid insulation, the common approach is to place the barrier directly under the concrete, on top of the foam — check your energy-code detail, since some assemblies call for it below the insulation. (See our guide to rigid foam insulation for concrete slabs for how the two layers stack.)
• Lap the seams at least 6 inches and seal them with the manufacturer’s tape. An untaped lap leaks.
• Seal every penetration. Pipes, drains, conduit, and columns each need a boot or a taped, sealed collar. The perimeter gets sealed to the footing or wall per your detail.
• Patch every tear before the pour. If a sheet gets holed after it’s down, tape a patch over it — don’t pour over an open puncture.

Get this layer right before the rebar goes in, because once the steel and forms are set, you can’t fix what’s underneath.

Where it fits in the pour

The vapour barrier is one of the last things to go down before you place steel and pour. It belongs in the same planning conversation as your foundation prep — base prepared and compacted, insulation set if the drawings call for it, barrier down and sealed, then reinforcement. Sequence it so nobody’s walking across a freshly laid sheet with a wheelbarrow.

Get it from one yard

2AZ Group stocks 15-mil under-slab vapour barrier and the matching seam tape at our Mississauga and Brampton yards, alongside the rigid insulation, rebar, form materials, and everything else that goes down before the pour — so you load one truck instead of chasing four suppliers.

• Mississauga: 3330 Ridgeway Dr, Unit 7
• Brampton: 2084 Steeles Ave E, Unit 1

Request a quote or call 647-926-2597 for current sizes and pricing. See our full Supplies range for everything else on the pre-pour list.