Can a roofer lay new peel-and-stick over an old bitumen roof? What the instructions say

GAF's current instructions for the two-ply Liberty system name the substrates it goes on: untreated APA-rated exterior-grade plywood or OSB, or tongue-and-groove wood plank, all clean, smooth and dry. Step 1 on an existing building is to remove the existing roof or roofs, then check the deck and replace damaged sections. Old bitumen is not on the list. The same document limits the system to slopes from 1/4:12 to 3:12, says it may suit roofs of 2,500 square feet or less, and gives 45°F to 95°F as the ideal installation range.

Section R908 of the International Residential Code treats a new covering over an old one as an exception. According to a Seamless Home explainer on R908, the usual route is written approval from the new product's maker in its installation instructions. A recover is banned outright when the existing roof is water-soaked or too deteriorated to serve as a base, or already has two or more layers. When the instructions call for a tear-off, the manufacturer route is closed.

The warranty follows the instructions too. GAF's limited warranty on Liberty materials, in effect for installs after April 1, 2026, excludes leaks caused by any condition that does not strictly follow GAF's published instructions, by poor workmanship or by ponding water. It pays only for the GAF materials and excludes interior damage. The longest term, 20 years, requires the two-ply system on a primed approved substrate with GAF's primer and flashing cement.

Walls need their own check. Where the roof meets a wall, GAF calls for a cant strip, base sheet run at least 8 inches up the wall, cap sheet nailed every 6 inches and metal counterflashing at the top. If the old roofing runs up behind the siding, ask the roofer how that detail gets built and get the answer in writing, along with which Liberty system and warranty term the quote covers. GAF Technical Services takes calls at 877-GAF-ROOF.