In short
Water that stays on a flat roof long after rain is called ponding. It usually means the roof does not fall enough towards its outlet or the deck has sagged between joists. It puts extra load and wear on the covering, so persistent ponding is worth fixing rather than living with.
This is a job for our Flat Roofs service.
What ponding is
A flat roof is never truly flat. It is built to a slight fall so that water runs to an outlet or gutter. When the fall is too shallow, has been lost as the structure moved, or is blocked at the outlet, water collects in pools and stays there. The National Federation of Roofing Contractors publishes a guidance note on ponding on flat roofs with exposed coverings, which sets out the problems it causes and how to avoid it.
Common causes
- A blocked outlet or gutter at the low edge.
- A deck that has sagged or gone soft between joists.
- A roof built with too little fall in the first place.
- Previous patches that have built up and dammed the water.
Why it matters
Standing water keeps the covering wet, adds weight, and finds any small defect. A split in a dry area of roof may never leak; the same split under a pond will.
What we do about it
We clear the outlet first, because that is sometimes all it is. If the problem is the deck or the falls, the fix is in the structure under the covering, and we will explain that before quoting a flat roof repair or replacement.