How do I install torch down roofing on a commercial building?

Written by mona culberson
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How do I install torch down roofing on a commercial building?
Torch down roofing should be applied by trained professionals. (hot tar, roofing image by Greg Pickens from

Torch-down roofing is the most common type of roofing used on large commercial roofs. Modified bitumen combined with asphalt and resin makes torch-down roofing highly resistant to damage. This type of roofing is extremely durable, low maintenance and has a life expectancy of 15 to 20 years. Although torch-down roofing is more expensive than traditional asphalt you can save a substantial amount of money on repairs in the long run.

Skill level:

Things you need

  • Safety glasses
  • Roofing gloves
  • Push broom
  • Hammer
  • Capped roof nails
  • Flashing material
  • Utility knife
  • Propane torch
  • Spark igniter
  • Spray primer
  • Roof insulation
  • Roofing base felt
  • Modified-bitumen roofing material
  • Tape measure

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  1. 1

    Sweep the roof of all dirt and debris.

  2. 2

    Lay flame resistant roofing felt down over the existing roof. Hammer capped roofing nails 15 cm (6 inches) apart in straight rows down the sides and centre of the sheets.

  3. 3

    Install flashing along the edge of the roof. Attach the flashing with a hammer and roofing nails every 20 cm (8 inches).

  4. 4

    Spray the inside flange of the edge flashing with the primer.

  5. 5

    Roll a sheet of modified-bitumen roofing material out on the surface of the roof. Leave a 5 cm (2 inch) lap over the side of the roof. Cut the end of the material with the utility knife. Leave a 5 cm (2 inch) overhang.

  6. 6

    Straighten the sheet down the flashing. Leave an overhang of 1.2 cm (1/2 inch) along the flashing. Ensure that the roofing material is flat. Roll the roofing material halfway back. Keep the material tight as you roll.

  7. 7

    Light the propane torch with the spark igniter. Adjust the flame of the torch until it turns blue.

  8. 8

    Melt the roofing material and the felt together along the flashing by running the flame of the torch down the intersection of the roofing roll with the sheathing. When heat bubbles begin to rise, roll the roofing material forward slowly along the width of the roll. Press down on the flashing with your foot as you heat the material to seal the edge.

  9. 9

    Roll the remaining sheets out to overlap the edges of the previously rolled sheet. Continue to torch and roll the sheets. Take special care to fully heat and bond the seams. Press down on the seam with your foot to tighten as you work.

  10. 10

    Cut strips of roofing material to fit the flashing edge. Lay the strips over the edges of the flashing. Leave an overhang of 1.2 m (1/2 inch). Torch the strips onto the flashing and the roofing. Press the strips down to seal.

  11. 11

    Measure around the vents and cut strips of roofing material to fit. Torch and seal the strips around vents.

Tips and warnings

  • Smooth the roofing material when rolling it out to prevent creases.
  • Wear safety equipment when applying torch down.
  • Ensure that the base felt is heat and flame resistant.
  • Use extreme caution when using a propane torch.

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