NRCA's Technical Services Section frequently receives inquiries regarding reroofing situations and whether existing self-adhering underlayments originally applied directly over roof decks need to be removed before installation of new steep-slope roof coverings. If you are involved with reroofing steep-slope roof systems, you should be aware of some of the considerations applicable to these situations.
What is recommended
It generally is understood when tearing off an existing roof covering and replacing it with a new roof system that the tear-off is intended to include removal of all layers of existing roofing materials down to the roof deck surface. However, once a self-adhering underlayment has been in place for some time, it usually becomes firmly bonded to wood substrates and cannot be readily removed. Removing firmly adhered self-adhering underlayment often results in delaminating or otherwise damaging wood substrates.
Addressing this situation, The NRCA Roofing Manual: Steep-slope Roof Systems—2009 states: "During reroofing, self-adhering polymer-modified bitumen sheet underlayment may not be able to be removed completely from the existing roof deck without damaging the roof deck."