Yes, in most cases you can install steel roofing directly over an existing layer of asphalt shingles, and thousands of Ontario homes and outbuildings have been re-roofed exactly that way. The question worth asking isn’t whether it’s allowed. It’s whether the deck underneath is sound enough to justify covering it up for the next forty years. That’s a judgment call, and it’s the one we make standing on the roof, not over the phone.
If you’re comparing a recover against a full tear-off, it helps to understand what a steel roof install actually involves on a residential or farm building, and if the structure in question is an older barn with more going on than just a tired roof, what’s involved in bringing an aging barn roof back into service rather than patching over a problem.
Does a metal roof over shingles need a building permit?
Usually not, if you’re only swapping the roofing material and touching nothing structural. Most municipalities in our service area treat a like-for-like re-roof as maintenance. The moment the job involves replacing sheathing over a wide area, altering rafters or trusses, changing the roof line, or adding a dormer, you’re into permit territory, and so is any building where the roof assembly is part of a change of use.
The catch is that “usually” varies by building department. City of Kawartha Lakes covers an enormous rural area out of its Lindsay office, and farm buildings there get looked at differently than a subdivision bungalow: an agricultural structure over a certain size, or one being converted from storage to livestock or workshop use, can trigger a review that a simple house re-roof never would. Five minutes on the phone with your local building division before the material is ordered is worth more than any advice you’ll read online, ours included.
How many layers of roofing are allowed?
Two is the practical ceiling. One original layer plus one recover is standard; a second recover over two existing layers is something most building departments and most steel manufacturers won’t accept. If someone already shingled over shingles on your house, common on properties that changed hands in the eighties and nineties, you’re tearing off before the metal goes on, no matter how good the surface looks from the driveway.
Weight is rarely the reason. Steel roofing panels are considerably lighter than the asphalt they replace, so a recover generally reduces the dead load on the framing rather than adding to it. The concern with stacked layers is fastening and flatness: screws need to reach solid wood with predictable penetration, and every additional layer of soft, compressible shingle between the panel and the deck makes that less predictable.
Do the old shingles cause problems under a steel roof?
Two of them, and both are avoidable. The first is telegraphing. Asphalt shingles create a ridged, granular surface, and a metal panel screwed flat to that surface picks up every high spot. On a low-slope run in raking afternoon light you’ll see it as waviness, oil-canning, that no amount of tightening will pull out afterward.
The second is abrasion and heat. Panels sitting directly on granules can wear their backside coating over years of thermal movement, and a shingle layer sandwiched tight against steel traps heat that shortens the life of both. Neither problem shows up in year one, which is why cut-rate recovers look fine at handover and fail at year twelve.
Do you need strapping between the shingles and the metal?
This is the part homeowners get talked out of, and it’s the part that matters most. Running horizontal strapping, furring, purlins, whatever your installer calls it, over the old shingles gives you a flat, consistent plane to fasten to, a defined air gap for ventilation, and a drainage path for any condensation that forms on the underside of the panel. It also gets the steel off the granules entirely.
That air gap is not optional in a Canadian climate. Steel is an excellent conductor: warm, moist air escaping from the attic hits a cold panel and condenses on the back of it. With a vented cavity, that moisture dries. Sealed tight against shingles, it sits there. Many manufacturer warranties are written around an approved underlayment and a ventilated assembly, and installing without them is a quiet way to void the paperwork you paid for.
Which brings up the third piece: attic ventilation. If the soffits are stuffed with insulation and the ridge vent is undersized, a new roof of any material is going to underperform. We check that before we quote.
Is a recover a bad idea on an old roof deck?
Sometimes, and it’s the honest reason we talk people out of recovers more often than we talk them into one. Going over the shingles means nobody ever sees the sheathing. Any soft plywood, any rot around an old vent flashing, any spot where a chimney has been leaking slowly for a decade, it all stays buried under the new roof, and it keeps doing what it’s been doing.
Tear off instead of recovering when you see any of these:
- Visible sag, springiness underfoot, or daylight through the deck from inside the attic
- Existing water staining on rafters or the underside of the sheathing
- More than one existing layer of roofing
- Curled, cupped, or badly deteriorated shingles that won’t lie down
- Ice-dam damage at the eaves, which usually means the problem is insulation and ventilation, not the shingles
Ice damming deserves a note of its own around here. Inland properties north of the lake, through the Kawarthas and up around Lindsay, hold snow on the roof longer than the shoreline stretch near Port Hope does, and the ground snow loads the Ontario Building Code assigns are set municipality by municipality for exactly that reason. Ask your installer what figure applies to your address before anyone decides the existing framing is fine. A metal roof sheds snow faster than asphalt, which is genuinely useful, but it also means that snow has to land somewhere: doorways, walkways, and anything parked below want snow guards or a rethink of where the load comes off.
Will going over shingles shorten the life of the metal roof?
Done properly, strapped, vented, screwed into solid material, no. A recover of that quality performs like a tear-off installation and lasts as long as the panel and fastener system are rated for. Done as a shortcut, with panels laid straight onto granules and no air gap, you can lose a meaningful share of that service life to condensation and coating wear, and you’ll never know until the underside starts telling you.
The cost difference between the two approaches is smaller than most people expect, because the tear-off, disposal, and deck repair are what a recover actually saves, not the strapping. Skipping the strapping to save a few dollars is the wrong economy on a roof meant to outlast the mortgage.
If you’re weighing this on a house, shop, garage, or barn anywhere within about ninety minutes of our shop in Campbellcroft, we’ll get up there, look at the deck, check the ventilation, and tell you plainly whether your roof is a recover or a tear-off. Both answers come up regularly. The wrong one is expensive.