Normal puddle or ponding water
In the trade, the usual benchmark is 48 hours. After that much dry weather, it is called ponding water.
The causes
A lasting puddle is the symptom of flat roof drainage that no longer does its job. The causes fall into five families.
- A blocked drain. The puddle is centred on the drain.
- A drain set too high. After a re-cover or settling around it, the drain is no longer at the low point. Water has to rise to get out.
- Weak slopes. The roof was built almost dead flat, or a past replacement did not correct the slopes.
- Crushed or wet insulation. A board compressed by foot traffic or soaked with water creates a local dip.
- A sagging structure. Over the decades, wood joists take on a curve. The middle of a span becomes a low point that did not exist at first.
Effects on the membrane
A membrane is made to get wet, not to stay under water.
In winter, the puddle becomes a sheet of ice that expands and pulls on the seams. Each freeze-thaw cycle widens an existing weak spot.
Above all, a puddle changes how serious a defect is. A slightly open seam on an area that dries lets in a few drops. The same seam under a puddle feeds a leak for days.
Effects on the structure
Water is heavy, and a wide puddle adds a load the roof carries constantly.
The danger is a vicious circle. The load makes the joists sag, the dip deepens, the puddle grows and the load increases.
If you notice a visible dip in the roof, a sagging ceiling or new cracks on the top floor, call in a structural engineer. A roofer cannot rule on the strength of a structure.
The corrections
- Have the drain cleaned. We clear the strainer. If the puddle then disappears, the problem is solved.
- Lower or replace the drain. When the drain sits too high, we redo the connection at the right height.
- Add a drain or a scupper. This correction applies when a low point is far from the existing outlet.
- Correct a dip locally. We open the area, replace the damaged insulation, restore the slope and close it with new membrane.
- Redo the slopes during a roof replacement. Tapered insulation laid over the whole roof solves the problem at the source.
Frequently asked questions
Is a small puddle serious?
A shallow puddle, away from the seams, on a membrane in good condition can simply be watched. If the puddle spreads or reaches a seam or an upstand, ask us to come and look at it.
Can ponding water cause a leak even without a visible hole?
Yes, ponding water can cause a leak without a visible hole. Water that sits finds tiny defects that ordinary rain would not have time to get through.
Does my insurance cover damage caused by a puddle?
Coverage for damage caused by a puddle depends on your insurance policy. Only your insurer can tell you what is covered. Ask before a problem occurs.