Ponding on a flat roof

A flat roof is never truly flat — it is laid to a shallow fall so water runs off. When water still sits there 48 hours after rain, either the fall has been lost through deck deflection or the outlet is not doing its job.

What it looks like from above

• Visible standing water or a distinct tide-mark ring on the membrane
• Silt and debris deposits marking the outline of a former pool
• Blistering, wrinkling or split seams inside the ponded area
• A blocked or undersized outlet with debris around the grating

What happens if it's left

• Standing water finds every weakness in a seam and freezes there in winter
• The extra weight deflects the deck further, deepening the pond
• UV and constant saturation shorten the life of felt and single-ply badly

How it's normally fixed

Clearing the outlet first, then addressing the fall — tapered insulation or a localised build-up — and repairing or overlaying the affected membrane. Aerial photographs after rain show the extent of the pond precisely.

Common questions