Can metal roofing go over old shingles in Calgary?

Installing metal roofing over existing asphalt shingles is technically possible, but most Calgary roofers will advise against it, and the cost savings rarely hold up once you understand what gets hidden underneath.

The Case Against an Overlay

The main argument for going over old shingles is skipping tear-off labour, which runs roughly $1 to $4 per square foot. On a typical Calgary house that might save $1,500 to $4,000 upfront. The problem is that metal roofing installed over shingles sits on whatever is underneath, and you lose the chance to inspect the decking for rot, soft spots or hail-damaged sheathing before it is sealed away for the next 40 or 50 years. Calgary's freeze-thaw cycle, roughly 128 cycles a year on average, is hard on decking, and a hidden soft spot will eventually telegraph through a metal panel or cause a fastener to pull.

There is also a moisture question. Metal does not breathe the way shingles do, and trapping an old shingle layer underneath can hold moisture against the deck. Roofers install a vented or drainage-mat underlayment between the old surface and the new metal panels specifically to address this, which adds cost and partially offsets the tear-off savings.

On the permit question: the City of Calgary does not require a permit for like-for-like re-roofing with no structural change, under Alberta's Permit Regulation. The provincial exemption covers re-roofing with a similar type of material. Whether a switch from asphalt shingles to metal falls within that exemption is not something to assume either way. Ask the City of Calgary directly before signing anything, and make sure the contract states clearly who is responsible for pulling any permit that turns out to be needed.

The National Building Code (2023 Alberta Edition) requires eave protection extending at least 900 mm up the slope and to a line at least 300 mm inside the inner face of the exterior wall. On an overlay, the roofer needs to confirm that requirement is met at the eave, which can be tricky when the existing shingles are already in place.

If the house was built before 1990, the old shingles may contain asbestos. Alberta OHS requires that removal be done by trained, competent workers with waste taken to an approved landfill. An overlay avoids disturbing those materials, which is one legitimate reason some owners consider it, but it is worth getting a professional opinion on the condition of the existing roof first.

Metal roofing installed properly in Calgary runs roughly $11 to $20 per square foot, putting a typical house between $20,000 and $50,000. Stone-coated steel sits toward the lower end; standing seam toward the upper. Class 4 impact-resistant metal panels are worth asking about given Calgary's position in what the City calls "hail alley," and the Insurance Bureau of Canada notes that a majority of insurers offer discounts of up to 50% for hail-resistant materials. Ask your insurer what discount applies before you choose the product, because that conversation may change the math on the upgrade.

Before calling a roofer, check whether your insurer has a position on metal-over-shingle installations, since some policies or claim settlements specify a full tear-off. That call takes five minutes and could save a dispute later. Then contact the City of Calgary about the permit question, and after that, ask two or three roofers for written quotes that spell out whether tear-off is included, what the decking allowance covers, and exactly which metal system they are pricing.