What snow load rating does my addition roof need in Ottawa?

The roof on your addition has to carry Ottawa's snow, and the exact design load isn't something to guess. Snow loads for a specific location come from the Ontario Building Code's climate data, and your designer or structural engineer applies them to your roof's shape, slope and exposure. What you can do as a homeowner is understand the factors that drive the design so you can ask good questions.

The first factor is the basic snow load for the location, which reflects how much snow can accumulate on the ground. The roof design load is then adjusted for things like roof slope, exposure to wind and the possibility of rain falling on an existing snowpack. Flat and low-slope roofs, which are common on kitchen bump-outs and rear additions, generally don't shed snow the way steeper roofs do, so they tend to need more robust framing.

The factor many people overlook is drifting where the addition meets the house. When wind blows snow off a higher roof onto a lower addition roof, it can pile up far deeper in that area than elsewhere. Snow can also slide off a steep upper roof onto the lower one. The Building Code requires these drift and sliding loads to be accounted for, which often means heavier framing near the junction. Ask your designer to show you where the drift zone is on the drawings.

Your framing options depend on the span and the loads. Engineered trusses, site-framed rafters and engineered joists or beams are all common, and the right choice depends on the roof shape, ceiling design and budget. Ask for the framing as a separate line item so you can compare approaches between quotes.

Snow load design also connects to ice damming. Good insulation and air sealing, plus proper ventilation, reduce the melt-freeze cycle that builds ice and concentrated weight at the eaves. For an addition, the verified Zone 1 value for a ceiling with an attic is R-60.

When you hire, ask who is doing the structural design, whether the roof framing will be engineered or follow prescriptive tables, and how drift at the junction has been handled. Your building permit drawings will need to show the roof structure, and Building Code Services (buildingpermits@ottawa.ca) can answer questions about what they need to see. You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare quotes.

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

• Justyn Rook Contracting
• RenoMotion Inc.
• JMY Renovations
• Steven Labelle - Your Complete Home Renovator
• Titley Construction