What does it cost to waterproof a new addition foundation in Ottawa?

Waterproofing the foundation of a new addition is far cheaper to do properly during construction than to fix later. As broad perspective only, comprehensive waterproofing and drainage for a typical addition foundation can range from roughly $8,000 to $20,000, depending on the size of the foundation and the system chosen. Treat that as perspective, not a quote.

Why it matters: spring runoff, freeze-thaw cycles and clay soils that drain slowly all keep water around foundations. Clay and silt soils are also frost-susceptible, meaning they can heave when they freeze, which puts stress on walls and joints.

The parts of a good system:

• An exterior membrane applied to the foundation walls before backfilling, such as a spray-applied or sheet rubberized membrane. Ask each contractor which type they use and how it is detailed at corners, penetrations and the footing.
• A drainage layer such as a dimpled membrane or drainage board over the waterproofing, to relieve water pressure and direct water down to the drain.
• Foundation drainage at the footing, typically perforated pipe in clear stone, leading to a sump or approved outlet. Ask how the new drain ties into the existing house's system.
• Free-draining backfill. Backfilling with granular material rather than the excavated clay costs more but reduces water pressure against the wall. Ask whether it is included.
• A sump pit and pump, where groundwater or drainage outlets call for one. Ask about a backup if the basement will be finished.

The most vulnerable spot is the tie-in where the new foundation meets the existing one. Water readily finds that joint, so ask exactly how it will be sealed, for example with waterstops, extra membrane overlap or other detailing. A poorly sealed tie-in can let water into your existing basement in the first spring.

Interior coatings can add a layer of protection but shouldn't be the only waterproofing on a new foundation, where the exterior is accessible during construction.

Your designer specifies the dampproofing or waterproofing and drainage on the permit drawings, and the City's inspector will review the foundation work as part of the permit process. On clay or poorly defined soils, footings must be at least 1.2 m deep and not less than the depth of frost penetration, and the waterproofing should protect the wall down to the footing.

When comparing quotes, ask for waterproofing, drainage board, foundation drain, backfill and sump as separate line items. You can browse foundation and addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare.

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