How do I handle drainage around a new addition on Ottawa's clay soil?

Clay soils, which are common around Ottawa, drain slowly. Water tends to sit on the surface or against a foundation instead of soaking away, and clay and silt soils are frost-susceptible, which means they can heave when they freeze. When you build an addition you disturb the drainage patterns your property has settled into, and the loosened backfill around the new foundation can collect water and channel it toward the walls. Good drainage design deals with all of this from the start.

The first piece is a perimeter drainage system at the base of the footings: a perforated drain pipe surrounded by clean stone and wrapped in filter fabric so fine clay doesn't clog it. Your designer or contractor will specify the pipe and stone, and you can ask why they chose the materials they did. The drain needs somewhere to go, typically a sump pit with a pump or another approved outlet; ask what's allowed on your property.

Surface grading is just as important and often overlooked. The finished ground should slope away from the foundation so rainwater and snowmelt run away from the house. The Building Code sets requirements for this, and your designer will show final grades on the drawings. On clay, where the ground can still be frozen and saturated in spring, a swale or shallow channel to intercept and redirect surface water can help. Ask how your contractor plans to handle grading at the junction between the addition and the existing house, and include topsoil and seeding in the quote.

On a lot with known drainage problems, consider exterior waterproofing on the foundation walls rather than basic damp-proofing alone. A waterproofing membrane with a drainage board or dimpled drainage mat gives water a path down to the perimeter drain instead of pressing on the wall. Ask contractors to price both options so you can compare.

Don't forget downspouts. The addition's downspouts should discharge well away from the foundation onto a splash pad or into an extension that carries water away from both your house and your neighbour's.

Foundations on sensitive clay need extra care. Under the Ontario Building Code, on filled ground, peat or sensitive clay, including Leda clay, footing sizes must be engineered. Ask whether a soil investigation makes sense for your lot.

Ask each contractor to itemize drainage, waterproofing and grading so you can compare like with like. If you have permit questions, Building Code Services can help (buildingpermits@ottawa.ca). You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare quotes.

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