Ponding Water on a Flat Roof: What It Means and What to Do

What counts as ponding

A little water on a flat roof right after a storm is normal. Water that is still sitting there 48 hours later is generally considered ponding. It usually collects in low spots, around clogged drains, or where the deck has sagged.

Why it is a problem

Standing water adds weight, speeds up UV and chemical breakdown of the membrane, and keeps pressure on seams. In freezing weather, it turns to ice that expands into any crack. Ponding areas are often where flat roof leaks start.

Common causes

Clogged or poorly placed drains and scuppers, a deck that has deflected over time, compressed or wet insulation, and roof equipment that blocks drainage paths are the usual suspects. Figuring out the cause comes first.

Fixing ponding

Clearing drains is the easy win. For low areas, tapered insulation or crickets can redirect water. Wet insulation should be cut out and replaced.

When it comes to coating, silicone systems are often chosen for roofs with ponding areas because they hold up to standing water better than many acrylics. The right product depends on the membrane and the building.

Get it inspected

If you see water standing on your roof days after rain, have it looked at before it becomes a leak inside the building.