What size beam do I need to span an opening between my house and new addition in Ottawa?

The right beam size for the opening between your house and an addition comes from a structural calculation, not a rule of thumb. Your designer or a structural engineer will size it, and the beam, posts and connections will be shown on the drawings submitted with your permit application. The City of Ottawa lists structural alterations among projects that need a building permit. Knowing what drives beam size helps you anticipate costs and have better conversations with your engineer and contractor.

Four things decide the beam. The first is the span, meaning how wide the opening is. The second is the tributary area, meaning how much floor and roof the beam holds up. The third is the loads: the weight of the structure itself, plus live loads from people and furniture and the snow load on the roof. Ottawa's snow load is a major factor in any beam that supports roof framing. The fourth is the material: solid wood, engineered lumber or steel.

Span has an outsized effect. As an opening gets wider, the bending force on the beam grows much faster than the width does, so going from a modest opening to a wide one can mean a much deeper or stronger beam. For shorter openings carrying only roof loads, built-up engineered wood beams are common and can often be lifted into place by a framing crew without equipment. For wider openings, engineered wood can get very deep and heavy, and engineers often switch to steel, which is stronger for its size but may need a small crane or boom truck to place.

If the beam supports a second storey as well as the roof, the loads increase and the beam, posts and footings all get larger. That is why an engineer needs to know exactly what sits above the opening.

The beam is only part of the system. It sits on posts or columns that carry concentrated loads down to the foundation, and the footings under those posts often need to be enlarged or newly poured. Ask your engineer whether new pad footings are needed and include them in your budget.

Connections matter too. Steel and wood move differently with temperature and moisture, and the engineer's details should let the structure move without cracking finishes.

Since the beam, posts, footings and installation can add up, ask for itemized quotes that break each out. Contractors in the Ottawa Construction Network directory can price the work once your engineer has specified the beam, so you can compare quotes on the same design.

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

• Justyn Rook Contracting
• RenoMotion Inc.
• Capital City Drywall
• Dump n Dash Hauling
• Geerts Roofing Inc