What type of foundation waterproofing do I need for an addition in Ottawa?

Foundation moisture protection for an addition is not a place to cut corners. Ottawa's freeze-thaw seasons, spring snowmelt and the clay soils found across much of the area can all keep water sitting against a foundation wall, and a leak in a new addition is far more expensive to fix after it's finished than to prevent during construction.

Start with the difference between dampproofing and waterproofing. Dampproofing is typically a thin coating that resists moisture but won't hold back liquid water under pressure. Waterproofing is a continuous membrane system designed to stop water itself. Which one the Building Code requires depends on site conditions such as groundwater, and your designer will specify it on the drawings. Many owners choose to go beyond the minimum on a basement they plan to finish, so ask contractors to price both options.

A common waterproofing approach is a self-adhered rubberized asphalt membrane on the exterior of the foundation wall, often paired with a dimpled drainage board that protects the membrane from backfill and gives water a path down to the footing drain. Ask what membrane each contractor proposes and how they protect it during backfill.

At the footing, a perimeter drain (weeping tile) set in clear stone and wrapped in filter fabric carries water away, discharging to a sump or another approved outlet. On clay, which drains slowly, this matters. Your designer and the City will confirm where the drain can discharge.

Inside, a sealed slab with a moisture barrier underneath and, where there's a sump, a pump with a battery backup add protection against an outage during a storm.

Pay the most attention to the joint between the new foundation and the old one. It's the most common leak point in an addition. Look for membrane lapped onto the existing wall, a flexible sealant, and where the designer calls for it, a waterstop at the connection. Ask each contractor to describe how they detail that joint.

Costs depend on wall area, depth, soil, access and the system you choose, so ask for an itemized line for waterproofing, drainage board and footing drain rather than one lump sum. You can browse addition contractors in the Ottawa Construction Network directory and compare those itemized quotes.

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• Justyn Rook Contracting
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