Metal roof over shingles
Can you? Usually. Should you? Read this first.
Usually possible. Frequently not the right call.
It’s the most common question we get about reroofing, and the honest answer is that it depends on things you can’t see from the ground.
Layer limits vary by municipality. Confirm yours before planning around it.
Going over the existing shingles saves real money. It also means covering up the one thing worth looking at. Both of those are true at the same time, and which one wins depends on your roof.
The short answer
Usually possible. Frequently not the right call.
It can be done, plenty of roofs have it, and there are situations where it’s clearly the sensible option. But it’s a judgement made after looking at the roof, not a default, and anyone who tells you yes over the phone hasn’t earned the answer.
What going over saves you.
The savings are real and worth naming:
- No tear-off labour. Stripping a roof is a day or more of crew time.
- No disposal. Shingle waste is heavy and dump fees are charged by weight.
- Faster. The building is never open to weather.
- Less mess. No debris across the yard, no nails in the driveway for the next two years.
On a straightforward roof that’s a meaningful reduction in the job. It isn’t nothing.
What the old roof hides.
- The inspection. A reroof is the only opportunity in twenty or thirty years to see the deck. Soft sheathing, previous leaks, inadequate fastening, framing that’s moved — all of it is visible for one afternoon and invisible again forever after. Going over shingles means choosing not to look. If there’s a problem under there, it doesn’t stop. It continues under a roof that will now outlast the deck holding it up.
- Flatness. Metal telegraphs. Shingles have a texture, lapped courses and often a wave or two of their own, and every bit of that reads through a metal panel far more visibly than it would through more shingles. Oil canning gets worse, not better, over an uneven substrate.
- Ventilation. Adding a layer doesn’t fix an under-vented assembly, and metal changes the thermal behaviour of the roof. If ventilation was marginal before, it’s worth resolving now rather than sealing it in.
- Fastening. Your fasteners now pass through shingles before reaching the deck, which affects holding and means the layer thickness has to be accounted for rather than ignored.
What actually decides it
- How many layers are already up there. Most jurisdictions cap the total. Two existing layers usually means tear-off regardless of preference. Confirm with your municipality — the limit and how strictly it’s applied both vary around the Island.
- The condition of what’s underneath. Curling, cupping, granule loss and any soft spot underfoot are arguments for stripping. A single layer of flat, sound, relatively young shingle is the case where going over is defensible.
- Whether the deck is suspect. Any leak history, any staining visible from the attic, any sag — strip it.
- The substrate plan. Going over shingles well normally means furring strips or a purlin system to create a flat plane and an air gap, rather than laying panel straight onto the old surface. That adds material and labour, and it eats into the saving you were chasing.
- Your insurer and your warranty. Worth a phone call before you commit. Some product warranties have conditions about the substrate, and some insurers take an interest in layer counts.
How to do it properly.
If you’re going to do it, do it properly:
- One existing layer only, sound and flat
- Deck confirmed good from below, or from a partial lift
- Furring or purlins to create a plane and a vent gap
- High-temperature underlayment over the furring
- Ventilation resolved as part of the job
- Fasteners sized for the added thickness
- Layer count confirmed with your municipality first
That’s a real assembly with a real service life. Panel laid straight onto old shingles because it was quicker is a different thing, and it’s the version that gives the practice its reputation.
What we’d tell you about your roof
We look at it. If it’s a single sound layer on a good deck with a simple geometry, we’ll say so and price both options.
If there’s leak history, more than one layer, ventilation problems or anything that makes us think the deck needs eyes on it, we’ll tell you to strip it — and we’d rather lose the job than bury a problem under a roof that will outlast it.
Going over shingles is a saving today and a bet on what you didn’t look at. Sometimes that bet is fine. You should know you’re making it.
Price your roof in about two minutes
Find your house on the satellite map, outline the roof and see an estimated range straight away. It is an estimate rather than a quote, and you see it before we ask for any details.
Frequently asked
Going over shingles.
Usually possible, frequently not the right call. It depends on how many layers are already there, the condition of the deck underneath, ventilation, and whether the surface can be made flat. It’s a judgement after looking, not a default.
Most jurisdictions cap the total number of roofing layers. Confirm with your municipality before planning around it — the limit varies and so does enforcement.
Usually. Furring or purlins create a flat plane and an air gap, which is what stops the old surface telegraphing through and gives the assembly somewhere to breathe. It costs material and labour and reduces the saving.
Yes — you save the tear-off labour and the disposal fees. Furring and the extra fastener length claw some of that back.
You’re covering the deck without inspecting it. If there’s rot, previous leak damage or inadequate fastening, it stays there under a roof that isn’t coming off again soon.
It can. Metal telegraphs whatever it sits on, and shingle courses are not a flat surface. Furring reduces it considerably. Laying panel straight onto old shingles is the version that looks wavy.
Let us look at it
We’ll price both options.
Then tell you which one we’d do.
Strip it or go over it — the roof decides, not the quote.