Water ponding on sheet membrane roofing

Causes

Drainage is poor due to insufficient fall to outlets

The drainage outlet is blocked 

The outlet is too small for the volume of water on roof

The roof slope is not towards the drainage outlet

The substrate has slumped, resulting in localized ponding

Listed under: membranes, sheet membranes, Roof, cladding, moisture, roof cladding, sheet membrane roofing, leaks

Cause

Drainage is poor due to insufficient fall to outlets

Repair

  • monitor the water drainage as well as the membrane and joints affected by the ponding. If the water tends to sit for long periods and the membrane has deteriorated, consider having the roof rebuilt with a greater slope.
  • where the membrane has deteriorated to the point of replacement, rebuild the roof to incorporate more slope and fall towards the outlet.
  • each outlet of a low-slope roof should have an overflow pipe to prevent the possibility of water entry into the building in case the main outlet or the downpipe gets blocked.

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Cause

The drainage outlet is blocked 

Repair

  • clear the blockage
  • flat roofs that are fully enclosed by parapets should have at least two outlets and overflows.
  • each outlet of a low-slope roof should have an overflow pipe to prevent the possibility of water entry into the building in case the main outlet or the downpipe gets blocked.

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Cause

The outlet is too small for the volume of water on roof

Repair

  • increase the size of, or install an extra outlet
  • flat roofs that are fully enclosed by parapets should have at least two outlets and overflows.
  • each outlet of a low-slope roof should have an overflow pipe to prevent the possibility of water entry into the building in case the main outlet or the downpipe gets blocked.

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Cause

The roof slope is not towards the drainage outlet

Repair

  • install extra outlets so water falls to outlets
  • where the membrane has deteriorated to the point of replacement, rebuild the roof to incorporate more slope and fall towards the outlet.
  • flat roofs that are fully enclosed by parapets should have at least two outlets and overflows.
  • each outlet of a low-slope roof should have an overflow pipe to prevent the possibility of water entry into the building in case the main outlet or the downpipe gets blocked.

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Cause

The substrate has slumped, resulting in localized ponding

Repair

  • if the substrate has slumped, remove the membrane and the substrate, replace the substrate using H3-treated, 17.5 mm plywood and replace the roofing membrane.

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