An overlay is cheaper on the day. That is a real advantage and it is worth saying plainly before the argument against it.
What you are buying is the removal of two costs: the tear-off labour and the disposal. What you are giving up is the only opportunity anybody gets to look at the deck.
Whether that trade is sensible depends almost entirely on which end of this service area you live in.
What the manufacturer actually permits
This is the part that decides it, and it is not a matter of opinion.
The Ontario Building Code requires roofing materials to be installed in conformance with the manufacturer’s written instructions. That single requirement pulls the shingle maker’s application sheet into the code, and the application sheet is where the overlay conditions live.
IKO, whose shingles are made in this province, puts it directly: where the roof has one layer of shingles lying flat and the deck is in good condition, the existing shingles typically do not have to be removed — and you should check with local officials that building codes are being followed.
Read the three conditions separately
One layer. A roof already carrying two is a tear-off, and no preparation changes that.
Lying flat. Curled, cupped or buckled shingles are not a surface you may cover. They telegraph through.
Deck in good condition. Which is the awkward one, because nobody can confirm it without pulling the old roof off. You are being asked to certify the thing the overlay prevents you from seeing.
How many layers of shingles can a roof have?
Confirm that with your municipal building department before you plan anything, in the town you actually live in. Permit and code questions are administered locally across the twelve municipalities here, and what triggers a roof permit varies with them.
The preparation is not optional
An overlay done properly is a real job rather than a shortcut, and the preparation is where the corners get cut.
| Step | Why |
|---|---|
| Split and re-nail curled or buckled shingles | Anything proud telegraphs through the new surface |
| Replace missing shingles | Voids show as depressions within a season |
| Remove loose or protruding nails | A raised nail head becomes a leak |
| Sweep the surface clean | Grit under a new shingle prevents it lying flat |
| Check the ventilation while you are up there | Re-roofing is the cheapest moment to correct it |
That last one is worth more than it looks. A re-roof is the one time the soffit intake, the ridge vent and the path between them can be sorted for very little extra, and in the snowbelt half of this corridor it is what decides whether the ice dam cycle continues.
What an overlay actually costs you
Three costs, none of which appear on the quote.
Heat. A second layer insulates the new shingles from the deck, so they run hotter through summer. Heat ages an asphalt mat, which is why an overlay generally has a shorter life than the same shingle laid on a clean deck. That shortens the interval before you are reading how long a roof lasts again.
Weight. A second layer of asphalt is dead load on the same framing, permanently. In the snowbelt end of this service area that load is added to the heaviest snow the corridor gets — Stratford at roughly 218 cm a year, Listowel at about 192 cm, the country inland of Goderich more again. Down on the Erie shore at Chatham, around 79 cm, the same weight argument is far weaker.
The unseen deck. Soft sheathing, previous water damage, a nailing surface that will not hold — all invisible, all still there, all now under two layers.
The next tear-off costs more
Worth pricing into the decision rather than discovering later. When the overlay reaches the end of its life, two layers come off instead of one. More labour, more disposal, and no way to defer it a second time.
Where an overlay is genuinely defensible
It is not never. The honest case looks like this:
- One existing layer, flat, with no lifting or curling
- A deck with no history of leaks, sag or previous repair
- The Erie shore end of the corridor, where snow load is the lowest in the province
- A simple roof plane with few valleys, dormers or chimneys
- A genuine budget constraint that would otherwise mean deferring the roof another two winters
A leaking roof deferred is worse than an overlay. That is a real trade and we will say so.
Where it is not
Anywhere in the snowbelt with two layers already on, or with any question about the deck.
On a complicated roof. Every valley, dormer and chimney is flashing work, and flashing detail over an existing layer is compromised before it starts.
On a roof with an active leak whose source nobody has traced. Covering a roof leak you know how to spot is not repairing one.
What we will tell you not to spend money on
An overlay to avoid finding out about the deck. If that is the actual reason, the deck is the problem and it does not improve under cover.
A tear-off on a sound single-layer roof in Leamington because a national article said always tear off. Your snow load is the lowest in the province and the arithmetic is different here.
And do not accept either recommendation without deck photographs or, on an overlay, a written roof inspection statement of what was done and how. The cost comparison only means something once you know which roof you actually have.