How to Put Flashing on Flat Roofs

Written by harvey birdman
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How to Put Flashing on Flat Roofs
Flashing lays under the tiles on your roof; in the case of a flat roof it would lay under the asphalt layer. (roof image by Eldin Muratovic from

Flashing refers to the system of metal sheets that roofers put on the seems of a roof to prevent leaks. On a normal slanted roof, the flashing is installed where the roof angles meet to provide an extra layer of protection for the weaker joints. On a flat roof flashing is used where joints in the roof panels meet. The only difference is that you need to be extra careful about caulking the sheet metal. Flat roofs are not actually flat; they slope towards a central drain point and thus require flashing as well.

Skill level:

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Things you need

  • Sheet metal
  • Sheet cutters
  • Roofing cement
  • Weatherproof caulk
  • Caulk gun
  • Safety goggles
  • Work gloves
  • Measuring tape
  • Water
  • Trowel
  • Metal saw
  • Mallet
  • Brick

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  1. 1

    Put on your safety goggles and work gloves whenever working with sheet metal. The edges of sheet metal are incredibly sharp and can cut your hands easily. Measure the seams on the flat roof that you want to cover with flashing. Mark on your sheet metal with a grease pen what panels you need to cut out to cover these seams. You need to have at least 4 inches to either side of the seam line covered with flashing to completely seal it, so account for this when you mark up the sheet metal.

  2. 2

    Mount the sheet metal on a pair of sawhorses. Cut the sheet metal on the grease metal lines with a metal saw or with sheet cutters if it is a smaller installation. It is OK to let the flashing panels just fall; they will be hammered into place, which will correct any distortions. Open the roofing cement bucket and pour in water until it reaches the line marked on the inside; if it goes over the line, then the cement will be very runny and hard to work with. Stir the cement with a trowel until it is a muddy consistency.

  3. 3

    Lay the flashing panel over the seam and hammer it with a mallet to make it conform to the shape of the roof. No flat roof is perfectly flat, especially older buildings that have settled unevenly. By making the panel conform to the roof you are helping water run off it easy and keeping debris from getting caught on it. Once the panel is shaped, take it off and trowel cement on the underside--just a light covering will do. Press the flashing panel firmly down on the seem and place a brick on it to keep the pressure up; multiple bricks may be needed for larger panels.

  4. 4

    Load a tube of weather-resistant caulk into your caulking gun once all the panels are set in place. Put down a steady line of caulk on all the edges of the flashing, even where the panels overlap each other. If any spot is left uncaulked, then water can seep under and leak through the seam. When all the sides are caulked you will be done flashing and can lay another layer of roofing material over it in two hours once the cement has solidified.

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