You can lay new asphalt shingles over one existing layer, and metal over one layer with an underlayment, where the municipality and the product manufacturer allow it. Most Lower Mainland roofers still recommend a tear-off, and on this coast they're usually right: the savings are about $1 to $3 per sq ft, and what a layover hides (wet sheathing, failed flashing, no eave membrane) is what causes the next leak. Here's when a layover is defensible, when it isn't, and what metal over shingles involves.
What a layover saves
Tear-off and disposal of a single shingle layer costs about $1 to $3 per sq ft in the Lower Mainland, so on a 2,000 sq ft roof a layover saves roughly $2,000 to $6,000, or 10 to 25% of an $11,000 to $20,000 shingle job. It also saves a day and a dumpster. That's the whole upside.
What a layover costs you
- No look at the deck. Soft plywood, delaminated edges and rot at the eaves stay hidden. Rotten sheathing under new shingles is a roof that fails from below.
- No new eave membrane or underlayment. Ice and water shield goes on the deck, not on old shingles. BC's roofing association considers eave membrane required under its guarantee program, and most local roofers treat it as standard.
- Flashing stays or gets botched. Step flashing at walls and chimneys is woven into the shingle courses; with a second layer it's either reused or improvised on top.
- Telegraphing. The new shingles follow the bumps, curls and ridges of the old ones, and look it.
- Heat and weight. Two layers hold more heat (hard on shingles) and add about 2 to 3 lb per sq ft the structure wasn't necessarily designed for.
- Warranty. Some shingle manufacturers reduce or exclude warranty coverage on layovers; the roofer's workmanship warranty often does too.
- Double tear-off next time. Removing two layers costs more ($0.50 to $1.50 per sq ft extra) and the second layer usually fails sooner, so the saving is partly borrowed.
When a layover is defensible
A single flat-lying layer of shingles with no curling, on a dry deck confirmed from the attic, on a simple roof with few penetrations, in a municipality that allows it, on a house you'll sell or demolish within a decade. That's a narrow set, and roofers who do good work will usually still decline. If the roof has moss, two layers already, curled shingles, soft spots, or has leaked, it's a tear-off.
Never roof over cedar: shakes are too uneven and sit on skip sheathing. See replacing a cedar shake roof.
Metal over existing shingles
This is the more common question, because metal panels and metal shingles are light and rigid and several manufacturers approve installation over one layer of flat asphalt shingles. The Building Code regulates the new roofing, not what's under it; whether a layover is allowed is a question for the municipality and the panel manufacturer's installation manual. Industry guidance adds conditions:
- One layer only (some jurisdictions accept two), lying flat, with the roof structure sound: no sagging deck or buckled trusses.
- Existing leaks fixed first, including the flashing.
- A synthetic underlayment over the shingles, because shingle granules abrade and can corrode the back of the metal and the old shingles trap moisture against it.
- On many systems, wood strapping (furring) screwed through the shingles to the rafters, giving a flat, ventilated base and something to fasten to. Strapped installs are common on exposed-fastener panels and some metal shingle systems; standing seam usually wants a solid, flat deck.
- New flashing at every wall, chimney and penetration, cut into the shingle layer.
Done that way, metal over shingles is a legitimate system and some BC metal shingle manufacturers advertise it. The trade-offs are the same as any layover: nobody has looked at the plywood, the eave membrane isn't on the deck, and a future tear-off is two systems deep. Many Lower Mainland metal roofers quote both ways and recommend the tear-off on a house they want to warranty for 40 years. The metal cost guide covers metal pricing with and without tear-off.
Flat roofs: recover versus replace
On torch-on and single-ply roofs, a "recover" (a new membrane over the old one) is standard practice when the old membrane is dry and well adhered, and a tear-off is required when insulation below is wet. A roofer will cut a test core to find out. Recovering a wet flat roof seals the water in. See the membrane guide.
What to ask if a roofer proposes a layover
- Have you been in the attic, and what did the deck look like?
- How will you handle the step flashing at walls and chimneys?
- Where does the eave membrane go?
- Does the shingle or panel manufacturer's warranty cover installation over existing roofing? Show me.
- What's your workmanship warranty on a layover versus a tear-off?
- Does our municipality have a position on second layers?
If the answers are vague, take the tear-off quote. Compare quotes on the same basis (both tear-off or both layover) using the quote comparison guide, and remember the cost guide ranges assume a tear-off.
Common questions
How much does a layover save?
The tear-off and disposal, about $1 to $3 per sq ft, or $2,000 to $6,000 on a 2,000 sq ft roof. Against that: you can't see or fix the deck, flashing is reused, and the new roof's life and warranty are shorter.
Can metal go over shingles?
Industry guidance allows it over one flat-lying layer with an underlayment, subject to the municipality and the panel manufacturer. Many Lower Mainland roofers still recommend a tear-off so they can check the deck.