When it works
- The old roof is dry (checked, not guessed)
- Loose gravel is removed and the surface is cleaned
- Blisters and soft areas are cut out and repaired
- Building code allows another layer
When it doesn't
- Wet felts across large areas
- Rotted or soft decking
- Already multiple layers
Foam over wet felt traps water. It can blister the new roof and keep rotting the deck you can't see.
The upside
Done right, foam over a dry built-up roof avoids tear-off cost, adds insulation and lets the crew build slope toward the canales. See tar and gravel replacement.
Common questions
How is moisture checked?
By probing, core samples and moisture meters in suspect areas.
Does all the gravel have to come off?
Loose gravel does. Embedded gravel is handled during surface prep.