Two ways to redo a roof
A tear-off means removing all the old covering down to the decking. An overlay means installing the new covering directly over the old one.
The decision is made early among the steps of a roof replacement, at the quote stage. This choice changes the price, the length of the job and what we will be able to check.
What a tear-off allows
With the wood exposed, we can:
- replace rotten boards or panels
- correct an uneven surface
- install a new waterproofing membrane at the sensitive spots
- redo all the flashing instead of working around the old pieces
The new covering then rests on a flat, solid surface. It has every chance of reaching its stated lifespan.
What an overlay saves
An overlay is faster and produces less debris. The old roof is never removed, and the house stays covered at all times.
The risks of an overlay
The first risk is the unknown. Rotten decking stays rotten under the new roof, and no one has seen it.
Other drawbacks follow:
- extra weight on the structure, added to the snow load
- defects in the old surface that show through, such as curled shingles that warp the new ones
- heat trapped between the layers, which can age the covering faster
- flashing that is harder to redo properly
- a longer and more costly tear-off next time, with two layers to remove
The warranty also deserves a check. Some manufacturers set conditions when their product is installed over an old covering. Ask us to show you what the warranty says in that case.
When an overlay is a bad idea
Rule out an overlay in the following situations:
- the roof has already leaked, or the ceiling has stains
- the decking feels soft underfoot during our visit, or it ripples
- the roof already has more than one layer
- the old covering is curled, cracked or lifting
- the attic shows moisture or mould
Ask us what the manufacturer of the new covering accepts as a base, and how many layers it allows.
The case of a flat roof
On a flat roof, the question is different. Water that got through an old membrane can soak the insulation underneath. Laying a new membrane over wet insulation traps that moisture.
Ask us how we will check the condition of the insulation, for example with test cuts, and what we plan if part of it is wet.
Frequently asked questions
How do I know how many layers my roof already has?
We count the layers by lifting the edge of the covering, near the gutter or the rake. On a flat roof, the layers are counted with a test cut.
Does an overlay change the warranty?
Yes, an overlay can change the warranty. The conditions depend on the manufacturer and the product. Have the warranty text shown to you before you accept this option.
Why does one quote offer an overlay and not the others?
An overlay is faster and takes less work. Its price is often much lower. Ask the other roofers why they planned a tear-off. Their answer will tell you what they saw on your roof.