How do I protect a new addition foundation from frost heaving in Ottawa?

Frost heave is one of the forces an addition foundation has to be designed against in Ottawa, where winters regularly reach well below -20°C. It happens when water in the soil freezes and the ground lifts. Clay and silt soils are frost-susceptible, so they heave, while sands and gravels generally don't. Since much of the Ottawa area has clay soil, frost protection deserves real attention.

The first line of defence is footing depth. The Building Code rule for clay or soils that are not clearly defined is that footings must be at least 1.2 m deep, and not less than the depth of frost penetration. There's no official Ottawa frost-depth number, because frost depth is set by local experience, so your designer or engineer confirms the depth for your lot and the City checks it at the footing inspection before concrete goes in. On sensitive clay, including Leda clay, footing sizes must also be engineered.

Free-draining backfill helps. Replacing frost-susceptible clay next to the wall with clean stone or gravel gives water a path down to the footing drain instead of sitting against the wall where it can freeze. Your designer will specify the backfill.

Exterior foundation insulation can keep the soil next to the wall warmer and reduce frost reaching the footing. Some designs also add horizontal insulation extending outward from the footing. How much insulation and where it goes is a design decision for your site.

Drainage removes the water frost needs. A perimeter footing drain in clear stone and filter fabric, discharging to an approved outlet, keeps water from collecting at the base of the foundation. Ask your contractor whether they test the drain before backfilling.

Small attached elements like stoops, porches and entry landings are often the first things to heave because they're built on lighter foundations. Make sure the drawings show how these are protected from frost, whether with frost-depth footings or an engineered shallow insulated design the City accepts.

Grading finishes the job. The ground should slope away from the foundation and downspouts should discharge well away from the wall, so water from fall rains isn't sitting against the foundation when it freezes. Your designer and the City can confirm any grading requirements.

Costs depend on soil, depth and the measures used, so ask for itemized quotes. You can browse addition contractors in the Ottawa Construction Network directory to compare them.

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Looking for experienced contractors? The Ottawa Construction Network connects homeowners with qualified professionals:

• Homeupgraders
• RenoMotion Inc.
• Nic’s D.U.C.T Works Inc
• Vanguard Environmental
• L.L. Renovation