4 Steps for Repairing a Built-Up Asphalt Flat Roof

Worker spreading roofing cement over flat roof

Many residential and commercial roofs have flat systems composed of asphalt-based built-up roofing materials. Discover the tools, skill sets, and set of instructions to properly repair an asphalt-based built-up roofing system.

Many residential and commercial roofs have flat systems composed of asphalt-based built-up roofing materials. Built-up roofs can be smooth-surfaced or have gravel on top to protect from weathering. The older built-up roofs get, the more likely they are to start cracking around the perimeter, particularly around flashings and the outer field of the roof.

Thankfully, built-up roofs can be maintained to maximize their lifespan as long as leaks are repaired as they occur.

With the right materials, tools, and knowledge, asphalt built-up roof repairs can be done quickly and with lasting results. Before you plan on making any kind of repair to your low-sloped built-up roof, make sure your roof is actually an asphalt-based system.

If you follow through on the following type of repair on a single-ply membrane, like EPDM or TPO, the asphalt repair material will damage the membrane. Worst-case scenario you’ll have to spring for a full replacement.

Skills and tools you need for the repair

It’s critical that you feel confident stepping out onto your roof. A fear of heights or a lack of trust in your abilities could lead to a much larger repair problem, or worse, personal injury or death. So before you buy any repair materials, we recommend being confident in the following areas:

  • Comfortable with heights

  • Ability to walk up and down ladders

  • Safely use hand tools

  • Ability to walk on the roof’s surface comfortably

If any of these steps make you uncomfortable, hire a professional roofing contractor. If not, you’ll need the following tools:

  • Trowel

  • Caulk gun

  • Flat pry bar

  • Utility knife

  • Asphalt primer

  • Roof cement

  • Scissors

  • Reinforcing fabric

FInd a qualified built-up roofing repair contractor

1. Dutifully prepare the repair area.

The first step in repairing a built-up asphalt roof (and the majority of other roofs) is to thoroughly clean the damaged surface. Cleaning the area around the patch will ensure it seals correctly to the membrane and not to loose gravel or dirt.

If your roof is covered with a gravel surface, this becomes slightly more difficult.

Built-up roofs are blasted with gravel to allow water to flow through and between the pieces, but the gravel needs to be removed before you can apply the patch. If the patch is applied to the gravel without clearing it away first, water will infiltrate below the patch and your roof will continue to leak.

a) Use the flat portion of your pry bar to scrape away the gravel from the leaking section. Draw the pry bar in one direction—not back and forth—and push the gravel at least 4 to 6 inches past the leaking section.

b) After you’ve pushed away the gravel, use the brush to sweep away any loose dust and debris. Asphalt-based repair materials won’t adhere properly to the membrane if a layer of dust and dirt is between them.


2. Prime the repair area.

apply asphalt primer with a roller

a) After you’ve cleared the leaking section of all dust, dirt, and moisture, apply the liquid asphalt primer with a brush or roller.

b) Wait a few minutes to allow the primer to flash off so there’s no excess on the roof.

A new asphalt primer will allow the patch to bind successfully to the roof’s surface, increasing the longevity of the patch. Older built-up roof systems typically lack the oils present to allow for a successful bind.

3. Carefully measure and cut the fabric patch.

Measure twice, cut once! 

a) While the primer is drying, measure your asphalt fabric to extend at least 3 inches past the edge of the leaking section.

b) Cut the fabric with scissors or a straight-blade utility knife.

Professional contractors recommend that two layers of patches be installed to ensure proper leak-protection. The first layer should be a 4-inch wide fabric, and the second layer a 6-inch wide fabric.


4. Cement the patch.

Apply a bed of asphalt flashing cement to the surface of the leaking section with a trowel. The cement needs to be applied at a uniform rate over the patch area, extending past the section where the cotton fabric will be applied.

Once the flashing cement has been applied, press the 4-inch cotton fabric into the cement, using the trowel to push the fabric into place.

a) Apply the second layer of asphalt flashing cement over the 4-inch patch, and well beyond it. Press the second layer of cotton fabric over the patch, checking that it extends past the 4-inch patch in every direction. Press firmly into place.

b) Once both layers of fabric are pressed into place, apply a finishing coat of asphalt mastic to the patch. If you moved gravel to start this process, reset the gravel over the patch area to conceal the repair and keep a uniformed appearance of the roof.

Hometown Roofing Contractors helps customers find qualified built-up roof repair contractors. Customers can research multiple contractors, request quotes, and see what other consumers have to say about their services.

Find a qualified built-up roof repair contractor