How to Re-Roof a Flat Roof With GAF

GAF is North America's largest manufacturer of both commercial and residential roofing.
It was founded in 1886 and today offers a full line of roofing products for all types of roofs, including several options for flat or low-slope roofs. Flat roofs traditionally were "built up," composed of alternating layers of roofing paper and hot asphalt, topped with gravel to weight down the material. Today's flat or low-slope options tend toward membranes that can be laid down or rolled out and are installed without hot tar. GAF offers some self-adhering materials for the do-it-yourselfer.

Step 1

Remove the old roofing material with a roof scraper or flat shovel. Strip the roof down to its decking, usually plywood or oriented strand board. Clean the decking of any adhesive or other material and replace any rotted or damaged material. Drive in any protruding nails with a hammer.

Step 2

Install a self-adhering base layer from GAF recommended for the type of finish material chosen on plywood decking, or on GAF polyiso insulation, which also is self-adhering. GAF roofing materials may be single-ply or multiple-ply style.

Step 3

Work from the low point of the roof. Lay out base sheets on the roof with at least 1 inch extending down over a fascia or facing board. Align the sheets according to directions with the material and let them sit for at least 30 minutes. Remove the release liner from the adhesive side, half of a strip at a time, and put sheets down on the deck. Overlap rows according to lines on the material. Seal the base by rolling with a lawn or linoleum roller.

Step 4

Install metal edge flashing on all roof edges. Fasten edging with roofing nails driven at 3-inch intervals with a hammer or using recommended roofing cement. Let any cement dry thoroughly. Prime the metal face on the top of the roof.

Step 5

Align cap sheets. Lay the first sheet 1/4-inch from the bottom of the lowest point of the roof with its selvage edge on the high side of the roof. Let it sit for 30 minutes. Remove the release strips over the adhering side, working from top to bottom and removing one-half at a time. Let it stick to the roof.

Step 6

Overlap succeeding layers by aligning the selvage edge. Overlap ends where two sheets join by at least 6 inches and in a bed of GAF Matrix roofing cement. Remove the release strips, starting at the selvage edge, then in half of each section, working from top to bottom.

Step 7

Secure the sheets with a weighted roller to firmly press the adhesive to the roof. Make sure all edges and overlaps are sealed with the appropriate roofing cement. Be especially careful to seal any overlaps with granular material on the underneath side.

Things You Will Need

  • Roof scraper or flat shovel
  • Polyiso insulation (optional)
  • Base layer, type varies
  • Metal edge flashing
  • Shingle nails
  • Hammer
  • Roofing cement
  • Roller


  • Check GAF's Internet sites for detailed instructions on specific materials. GAF offers several styles of self-adhesive roofing, and installation techniques can vary.

About the Author

Bob Haring has been a news writer and editor for more than 50 years, mostly with the Associated Press and then as executive editor of the Tulsa, Okla. "World." Since retiring he has written freelance stories and a weekly computer security column. Haring holds a Bachelor of Journalism from the University of Missouri.