How to Avoid Condensation in Flat Roof Construction

Written by mona culberson
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How to Avoid Condensation in Flat Roof Construction
Adequate ventilation is the key to preventing condensation on a flat roof. (hot tar, roofing image by Greg Pickens from

Venting is the way to control or remove moisture in flat roof construction. Ventilation is key to preventing and controlling condensation. An air barrier system is a vital part of the interior construction of the building enclosure so that air pressure within the building can be controlled. Air barrier systems control concentrated condensation between the ceiling and the roof. Make allowances for the escape of moisture from all possible sources through adequate drainage and venting. Repair all leak areas and seal all penetrations to prevent condensation when constructing a flat roof.

Skill level:

Things you need

  • Closed cell foam insulation
  • Spray foam sealant
  • Roofing cement
  • Caulking gun
  • EDPM roof insulation
  • Roof breathers
  • Roof vents

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  1. 1

    Repair all leaks in the roof membrane with repair and sealing tape. Areas include vents, at low walls projecting from the edge of the roof and at any other flashing material. Caulk all seams with the caulking gun and roofing cement.

  2. 2

    Install a continuous air and vapour barrier above the ceiling. Closed cell foam insulation controls airflow through ceiling penetrations such as holes around a ceiling fans or light fixtures.

  3. 3

    Spray foam sealant around ducts, vents and roof drains to make these penetrations airtight.

  4. 4

    Install roofing insulation prior to laying the EDPM to create a trap for water and vapour.

  5. 5

    Build the roof at a slope to encourage draining. A level roof will result in water pooling.

  6. 6

    Install one way roof breathers and vents every 1000 square feet to prevent moisture from condensing. Breathers and vents will clear excess moisture and relieve vapour pressure continuously.

Tips and warnings

  • Adequate ventilation is key to preventing condensation.

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