An overlay depends on the condition, type and number of existing roof layers. GAF discusses situations where tear-off is needed and why uneven or unsuitable surfaces cannot simply be hidden.
GAF technical bulletin: Roofing over existing shingles.
What this means for your roof
An overlay depends on the condition, type and number of existing roof layers. GAF discusses situations where tear-off is needed and why uneven or unsuitable surfaces cannot simply be hidden.
A useful question for your contractor
Ask what can and cannot be inspected if the old roofing stays in place.
Ask for the answer to be tied to the actual roof condition and the exact product being installed. Keep the agreed repair or installation detail with your estimate and project photos.
Bulletin edition: 2021 edition. The PDF pages contain different revision dates within 2021; no exact original release day is asserted.
Read GAF: Roofing over existing shingles
This homeowner summary explains the cited bulletin. The complete manufacturer document and current product instructions govern the installation details. A requirement for one manufacturer or product should not be transferred to another without checking.