How much clearance should be left between drywall and a concrete basement floor in a Toronto home?

You should leave a 1/2-inch to 3/4-inch gap between the bottom edge of your drywall and the concrete basement floor. This clearance is one of the most important details in basement finishing, and getting it wrong is one of the leading causes of mould and water damage in GTA basements — which is saying something, given how common basement moisture issues are in Toronto's older housing stock.

The reason for this gap is straightforward: concrete basement floors wick moisture upward through capillary action, and any drywall in direct contact with the concrete will absorb that moisture like a sponge. Even in basements that appear dry, the concrete slab is constantly drawing ground moisture upward. In Toronto's spring thaw and during heavy summer rainstorms, this moisture transfer increases significantly. Drywall paper facing is an ideal food source for mould, and a damp drywall edge sitting on concrete creates perfect conditions for mould growth inside the wall cavity — often hidden behind the baseboard where you will not see it until the damage is extensive.

The standard practice among experienced GTA basement drywall crews is to use temporary spacers — scraps of 1/2-inch or 3/4-inch plywood placed on the floor — to hold the drywall sheet up while it is being fastened to the studs. Once the sheet is secured with screws, the spacers are removed, leaving a consistent gap along the bottom. This gap will be completely hidden by the baseboard trim, so it has zero impact on the finished appearance of the room.

In Toronto homes built from the 1950s through the 1980s — the bungalows and split-levels across Scarborough, North York, Etobicoke, and the inner suburbs — basement moisture is particularly common because many of these homes lack modern waterproofing membranes on the exterior foundation walls. Even with interior insulation and a 6-mil polyethylene vapour barrier (which is required by the Ontario Building Code for basement wall assemblies), moisture can still migrate along the slab. The 1/2-inch to 3/4-inch gap provides a critical buffer.

Some contractors in the GTA go a step further and install a pressure-treated bottom plate for the framing rather than standard spruce, specifically because the bottom plate sits directly on the concrete and is the most vulnerable to moisture. The Ontario Building Code requires that any wood framing in contact with concrete must be pressure-treated or separated by a moisture barrier (sill gasket or poly). If your framing contractor used untreated lumber for the bottom plate, this is a problem that should be corrected before drywall goes up — not after.

If your basement has a history of water intrusion — even minor seepage during heavy rains — consider increasing the gap to a full inch and installing moisture-resistant drywall (green board at $20–$28 per sheet) or mould-resistant drywall (purple board at $24–$32 per sheet) for the bottom course of drywall. This does not make the drywall waterproof, but it provides significantly more resistance to occasional moisture exposure than standard paper-faced drywall.

For the bottom plate area, some GTA contractors also apply a bead of acoustical sealant along the inside of the bottom plate before installing the drywall. This creates an airtight seal that reduces air leakage from the wall cavity to the room, which improves both energy efficiency and vapour management — an important consideration in Ontario's Climate Zone 6 where the temperature difference between the heated basement and the cold exterior can drive significant moisture movement through wall assemblies.

A full basement finish in the GTA (800–1,200 square feet) typically costs $5,000–$12,000 for the drywall scope alone — framing, insulation, vapour barrier, hanging, taping, and finishing. Getting the floor clearance detail right from the start is far cheaper than tearing out mould-damaged drywall two years later, which can easily cost $3,000–$8,000 for remediation and replacement. If you are planning a basement finish, get matched with a drywall contractor through Toronto Drywall Installers for a free estimate — an experienced basement finisher will get these details right.