How to Apply Rubber on a Flat Roof


Flat roofs cause problems for traditional roofing applications such as asphalt shingles because they allow water to sit and pool instead of being pulled down the roof by gravity. Hot tar and gravel are sometimes used to seal flat roof surfaces, but these materials are messy and can be dangerous to roofers if not used properly. To reduce costs and avoid the drawbacks of tar roofing, many roofing companies choose to apply rubber roofing on flat roofs. When properly applied, a rubber roof provides a high level of leak protection at a reduced cost.

Things You'll Need

  • Push broom or other flat broom
  • Roofing rubber rolls
  • Roofing rubber adhesive
  • Roofing tape
  • Screw gun and roofing screws
  • Flashing cement and applicator
  • Flashing rubber strips
  • Roofing roller
  • Terminating bars
  • Waterproofing caulk

Clean the flat roof surface by brushing it with the push broom or other flat broom. This removes any gravel, loose nails or other debris that could otherwise damage the roofing rubber as you apply it.

Unroll the rubber sheeting in the approximate area where it will be installed, letting it hang over edges or up the sides of air conditioning units or other roof installations by several inches. Allow the rubber to sit unrolled for at least 30 to 60 minutes to relax and expand.

Fold back a portion of one sheet of rubber, applying adhesive to the roof underneath it. Allow the adhesive to set if instructed by the adhesive's packaging, then stretch the rubber over the section of adhesive and lay it down. Pull, brush and make other necessary adjustments to the rubber to remove wrinkles before the adhesive fully sets. Repeat this process with the other side of the rubber sheet, resulting in the entire sheet being glued down.

Continue gluing the rubber sheets, allowing sheets to overlap by 3 to 6 inches. Place a layer of roofing tape between the sheets where they overlap, then use roofing screws to secure the edges of the sheets along this seam. Place a screw every 12 to 18 inches.

Fold the rubber back whenever you encounter a pipe, air conditioning unit or other object that needs to be cut around. Use chalk to mark the edge of the object on the bottom of the sheet of rubber, then measure it and make a shape with similar dimensions on the rubber. Cut through the rubber diagonally and place it over the object, making adjustments to the cut as necessary so the rubber sheet lies flat on the roof with a portion pressing up against the object on all sides. Trim the rubber so it goes up the side of the object just a few inches.

Apply flashing cement adhesive along all seams, including seams created by cuts in the rubber around objects. Remove the plastic backing from flashing rubber strips, placing the strips bottom-up beside the previously cemented seams. Apply additional flashing cement to the strips, allowing it to set according to package directions.

Flip over the flashing strips, placing them smoothly over the previously cemented seams and rolling them with a hand-held roofing roller. As the two layers of flashing adhesive combine and dry, a waterproof seal will cover all of the seams.

Place terminating bars along the edges of the roof and screw them into place. Caulk the terminating bars to prevent leaks caused by the screws.

Tips & Warnings

  • Use an industrial forklift or crane to lift rubber, flashing cement and other roofing materials onto the roof.

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