How to Build & Seal a Flat Roof

Written by bob haring
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Flat roofs are popular in some Southwestern styles of architecture and some modern designs. Flat roofs also are frequently found in connection with more traditional roofs, like gables or hip roofs. They are common on carports, patios and decks. The problem with flat roofs is that water does not drain from them the way it does from pitched roofs. Standing or pooling water over time can damage a roof surface and cause leaks. Modern materials and techniques make it possible to build flat roofs that are durable and leakproof.

Skill level:

Things you need

  • 2-by-12-inch rafter boards
  • Oriented strand board (OSB) or plywood
  • Roofing nails
  • Hammer
  • Utility knife or shears
  • Foam underlayment/ISO board
  • Rubber roof membrane
  • Rubber membrane cement
  • Paint-type roller with extension handle
  • Uncured rubber
  • Rubber roofing cement

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  1. 1

    Frame the flat roof with 2-by-12-inch rafters nailed 16 inches apart rather than the 24 inches standard in pitched roofs. Build overhangs by placing double rafters at the wall and building lookouts, either by making a boxlike structure with short rafters running opposite the main rafters or by installing a diagonal rafter from the double rafter to the roof end and bracing it with short rafters on both sides. The box method is easier for installation of drainage downspouts.

  2. 2

    Sheath the framed roof with oriented strand board (OSB) or plywood nailed to the rafters. Some builders use a foam underlayment or small wedges nailed to rafters to give the roof a slight pitch so water will drain better. Leave a one-eighth inch gap at joints of the sheathing to allow for expansion and contraction. Lay down a substrate called ISO board, a special half-inch thick foam with a fibreglass backing. Stagger the joints and fit the pieces tightly together at seams. Cut the ISO board to fit with a utility knife.

  3. 3

    Cover the roof with EPDM (ethylene propylene diene monomer) rubber mats, after clearing the roof of any dust or debris. Cut pieces to fit with shears or a utility knife. Leave an extra nine inches all around. Fold a sheet in half and spread glue with a paint-style roller over the sheathing and the folded-back piece of rubber. Then carefully unfold the glued rubber back in place, working from the fold forward. The glue will bond as soon as it touches, so work slowly and carefully and smooth the rubber as you go. Don't lift the rubber or it will tear.

  4. 4

    Work over the roof in stages until the entire area is covered with EPDM. Cover any seams with a foot-wide strip of uncured rubber---not EPDM---fastened with a rubber roofing cement. Finish edges and corners by laying the extra nine inches over the roof edge and gluing it down. Install a galvanised steel drip edge flashing called gravel stop on all edges.

Tips and warnings

  • Add downspouts if desired at corners. Cut through the EPDM and sheathing for the spout, and then seal it by gluing EPDM around the opening and sealing with a roofing caulk.
  • Flat roofs also can be made with alternating layers of tar paper, roof cement and gravel. That was the standard for many years, but new rubber materials are more waterproof, longer-lasting and easier to install.
  • If you have never used EPDM, experiment with a small section off the roof first, until you master the technique.

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