A choice made at the quote
Among the steps of a roof replacement, this choice comes very early. The price depends on it, so does the number of days, and above all what the roofer will see while working.
An overlay means giving up seeing the wood
Under an overlay, the deck stays hidden. A deck that is rotten in one spot stays rotten under the new roof, without anyone having noticed.
The overlay has other drawbacks, to weigh with the roofer.
- The roof structure carries one more layer, along with the winter snow.
- Bumps in the old surface show through the new one. Curled shingles underneath distort the ones on top.
- Heat trapped between two layers can wear the roofing faster.
- Flashing is harder to redo properly over a double thickness.
- At the next replacement, there will be two layers to remove, with more time and more cost.
What the overlay has going for it
The job is shorter and the dumpster fills up less. The old roof stays in place, and the house is never uncovered.
What the roofer can do once the wood is exposed
- Replace the boards or panels affected by rot.
- Straighten a surface that is no longer even.
- Install a new waterproofing membrane where the roof is most exposed.
- Redo all the flashing, instead of making do with the old pieces.
The new roofing then rests on a straight, solid base. This is the condition for it to have every chance of lasting as long as the manufacturer states.
Signs that work against an overlay
In the following cases, an overlay should be ruled out.
- Water has already come in, or a ceiling shows a stain.
- The roofer felt the deck give way underfoot, or saw it ripple.
- The roof already has more than one layer of roofing.
- The old roofing curls, cracks or comes loose.
- Moisture or mould is visible in the attic.
If a quote still proposes an overlay, ask the roofer why they rule out a tear-off. Also ask them what base the new product's manufacturer accepts, and how many layers it allows.
On a flat roof, insulation first
On a flat roof, the risk changes in nature. An old membrane that let water through may have soaked the insulation below it. A new membrane laid on top would trap that moisture.
We can check the insulation before deciding, with test cuts for example. Have us specify the method, and what the quote plans if a wet area is found.
Frequently asked questions
Who can tell how many layers are on my roof?
The roofer can count the layers. On a sloped roof, they lift the edge of the roofing, on the gutter side or at the rake. On a flat roof, they make a test cut.
Does the manufacturer's warranty hold on an overlay?
The warranty does not always hold under the same conditions. Some manufacturers impose specific ones when their product is installed over old roofing. Read the warranty text with the roofer before giving your approval.
Only one roofer out of three suggests an overlay. What should I make of it?
An overlay takes less time and less labour, and the price is often much lower. Ask the other two about their choice to tear off. What they answer will tell you what they noticed on your roof.