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How do I Repair my Asphalt Flat Roof?

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Built up roof repair

Repair made to built-up roof system with fabric and cement.

Photo © Jurin Roofing Services www.jurinroofing.com

Many homes and commercial buildings have flat or low-sloped roof assemblies. Many of those flat roof assemblies have an asphalt based roof system installed on them. These built-up roof systems can be either smooth surfaced or have a gravel surface on them to help protect them from weathering. As these roof systems age, they begin to break down with cracks and splits appearing in the perimeter, the penetration flashings and the general field of the roof. In order to maintain these roof systems and maximize the life of the roof while protecting the interior of the home or building from water damage, it is necessary to repair these roofs as leaks as they occur.

General repairs to an asphalt built-up roof system can be accomplished quickly and effectively in order to minimize the water entry into the home or building. By using the following list of materials and hand tools, you can install a repair to correct the problem associated with the roof leak in your home or building.

Confirming Your Roof Type

An important step in completing an effective repair to your low-sloped built-up roof system is to confirm that your roof system is in fact an asphalt based roof system. The following method of repair is only applicable to roof systems that are asphalt based. If your roof assembly is a single-ply membrane such as EPDM or TPO, the application of asphalt repair material to the surface of the membrane can damage the membrane requiring a much larger patch or complete replacement.

Tools and Materials Required

  1. Tools
  • Trowel
  • Flat Pry Bar
  • Caulk Gun (if using asphalt repair material in a caulk tube)
  • Utility Knife with Straight Blade
  • Dust Brush
  • Scissors
  • Materials
    • Asphalt Primer
    • Roof Cement
    • Reinforcing Fabric – 4” or 6”
  • Skills
    • No fear of heights
    • Ability to safely utilize hand tools
    • Ability to set up ladder safely
    • Ability to walk on roof surfaces

    Step One -- Prepare Repair Area

    The first step in repairing your built-up asphalt roof system after you have located the source of the roof leak is to clean the surface of the roof to allow you to apply the patch directly to the surface of the membrane. This is a critical step and will ensure that your patch adheres properly to the surface of the roof and not to the dirt or stone that is on or imbedded in the surface of the roof.

    This is a more difficult step if the surface of the roof is covered with a gravel surfacing. If the surface of the roof is covered with gravel, the gravel must be removed prior to applying the patch to the roof surface. The gravel surfacing allows water to flow through and between the gravel. If the patch is applied to the gravel without removing the gravel water may be allowed to flow beneath the patch allowing the leak to continue.

    In order to do this, utilize the flat pry bar. Utilize the flat portion of the pry bar upside down. Scrape or “spud” the gravel from the surface of the roof by scraping the gravel from the surface of the roof. The pry bar should be used only in one direction and not drug over the surface in both directions. The gravel should be removed a minimum of 4 to 6 inches in all directions from what will be the leading edge of the patch to allow for proper adhesion of the patch.

    After the gravel is removed use the dust brush to sweep the surface of the membrane free of all loose dust and dirt. Asphalt repair materials will not stick to the roof surface through dust and dirt. The patch will actually stick to the dust and dirt and will not seal the hole. Water will penetrate under the patch allowing the leak to continue.

    Step Two -- Primer Repair Area

    Once the surface of the area where the patch is to be applied is free of all dust, debris and moisture apply the liquid asphalt primer to the roof surface. The liquid asphalt primer may be either brush or roller applied. There are products available in spray can form which work equally as well. Allow the primer to flash off so that there is no excess primer on the surface of the roof. Primer that is too wet to the touch will inhibit the adhesion of the patch.

    The application of the asphalt primer is important because older built-up roof systems may be brittle or lack the oils present in a newer roof system. By applying a primer to the surface of the roof, the patch can then bind itself to the roof surface increasing the long-term performance of the roof patch.

    Step Three -- Apply Fabric Patch

    After the primer has dried, measure your asphalt impregnated cotton fabric so that it extends a minimum of three inches past the edge of the area to be patched. It is recommended that the patch be installed in a two-layer application. The first layer of the patch should be a 4” wide fabric. The subsequent layer of the patch should incorporate a layer of 6” wide fabric. Measure and cut the fabric with either a straight blade utility knife or a pair of scissors.

    Step Four -- Cement in Patch

    The final step is to cement the fabric patch. Utilize the trowel to apply a bed of asphalt flashing cement to the surface of the roof. The flashing cement should be applied at a uniform rate over the surface of the area to be patched exceeding the entire area where the cotton fabric will be applied. After the flashing cement has been applied, apply the 4” cotton fabric into the cement. Utilize the trowel to imbed the cotton fabric into the roof cement.

    After the first layer of 4” cotton fabric has been applied, apply an additional layer asphalt flashing cement over the patch extending the mastic beyond the portion of the patch already applied. After the second layer of asphalt flashing cement is applied imbed a single layer of 6” cotton fabric over the patch. Ensure that the patch extends beyond the first layer of 4” cotton fabric in all directions. Utilize the trowel to fully imbed the additional layer of cotton fabric into the flashing cement.

    After both layers of fabric have been installed, apply a finish coat of asphalt mastic to the roof patch. If the roof patch is being applied to a gravel surfaced roof system, it is recommended to reset the gravel over the area that has been repaired to conceal the repair and maintain the uniformity of appearance of the overall roof.

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