How to Hot Mop a Roof

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Hot-mopped asphalt is applied at approximately 400 degrees Fahrenheit.
Hot-mopped asphalt is applied at approximately 400 degrees Fahrenheit. (Image: Hemera Technologies/Photos.com/Getty Images)

Hot-mopped roofs are durable, low-cost options compared with others types of roofs. They are designed for roofs with shallow pitches, where the rise is no more than 4 inches in every 12 inches. A built-up hot-mop roof consists of layers of felt and asphalt over a bed of rosin paper. Flashing is used around roof features as required. Reflective gravel or a cap sheet limits sun damage and improves the roof’s appearance. Because of the fumes released by liquefying asphalt, hot mopping contributes to smog production. Hot mopping may not be allowed in every jurisdiction, or it may be allowed only at certain times of the year.

Things You'll Need

  • Rosin paper
  • Cant strips
  • Roofing nails and hammer (or nail gun)
  • Staple gun
  • Base sheet (fabric mat)
  • 3 to 4 layers of building felt
  • Asphalt primer
  • Plastic roof cement
  • Asphalt and kettle
  • Protective clothing
  • Bucket
  • Fabric roofing mop
  • Gravel or cap sheet (felt coated with aggregate)
  • Flashing (as needed)

Nail 45-degree cant strips at all inside corners, 12 inches on center, using a hammer and roofing nails. Includes all roof corners and roof features with interior corners (chimneys, curbs and parapets).

Use a staple gun to layer rosin paper lengthwise over the roof and cant strips. If short-handed, staple one end of the rosin paper, then stretch your paper to the other side of the roof and staple that end before adding more staples along the seam.

Layer and nail base sheets shingle style over the rosin paper, with a 2-inch overlap between courses.

Cut and install flashing as needed around roof fixtures and penetrations. Set flashing in plastic roofing cement to reduce leakage, and spray with asphalt primer.

Pour molten asphalt from the asphalt kettle into a bucket or pump for larger roofs. Transport the bucket or pump to the roof. When working with others, yell “hot” when moving asphalt around the roof, due to the potential for serious injury if splashed.

Use a fabric roofing mop to lay up the roof, from lower elevation to high. Apply asphalt to the base sheet, then building felt, then asphalt. Repeat for three layers (four including the base sheet). The felt should be staggered similarly to the base sheet application, shingle style, using two-thirds coverage.

Sprinkle light-colored gravel evenly over the final hot-mopped layer of felt to limit UV damage. You may use an aggregate-infused cap sheet instead of gravel to reduce overall weight.

Tips & Warnings

  • A three-layer hot-mop roof should last approximately 15 years.
  • Molten asphalt is extremely hazardous. Take it slow and use safety precautions when applying a hot-mop roof.

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