Water Pooling On A Flat Roof

What's happening?

A flat roof is not dead level. It is laid to a slight fall so rain runs to an outlet or gutter. Designing Buildings gives a design fall of 1:40, which allows for site error and should leave at least 1:80 once built. Ponding is water that stays on the surface after the rain stops. It happens where the deck has sagged, where the fall was never enough, where the covering has bunched and made a dip, or where silt and leaves have blocked the outlet so water cannot leave. Standing water is not a leak by itself. It is a load on the deck and a constant test of every seam, and it grows algae and silt that block the outlet further.

What usually causes it?

• A fall that was too shallow when the roof was built, or never laid at all
• A deck that has sagged or deflected between joists, leaving a dip
• An outlet, gully or gutter blocked with silt, leaves or moss
• Insulation or covering that has compressed or bunched into a low spot
• An outlet fitted higher than the surrounding surface
• Debris and silt left by earlier ponding raising the next puddle

Is it dangerous?

Not immediately. The risk builds up: weight on a deck that is already sagging, a covering whose seams and edges are wet all the time, and a blocked outlet that can overflow into the building. If the ceiling below is bowing, stained or the deck feels soft, treat that as urgent and keep people off the roof.

Can I fix it myself?

From the ground or a window, note where the water sits and how long after rain it takes to clear. Take a photo each time. Check that the gutter or outlet is not visibly blocked. Do not walk on the roof to sweep a puddle: wet felt and skylights are a fall risk. Do not drill, patch or paint over the area.

When should you call a roofer?

Call a roofer if water is still standing two days after rain, if it is the same spot every time, if the surface has softened or dipped, or if there is a stain below. Ask them to check the fall and the outlets rather than only patching the surface.

What will the roofer actually do?

• Check the roof from a safe position, finding where water stands and how deep
• Clear the outlets, gullies and gutters and prove the water can run away
• Measure the falls and look for a sagged deck or a compressed insulation layer
• Build up the low spot with a tapered fill, or fit a new outlet where the position is wrong
• Repair any split or open seam that the standing water has exposed
• Advise on a re-cover with a proper fall where the whole roof is flat or failing

Who fixes this

• Flat roof repair
• Roof inspection

Questions people ask

Sources & References

1. Designing Buildings: flat roof
2. LABC Warranty: ponding on flat roofs
3. NFRC: choosing a roofing contractor