Why is my flat roof ponding?

The three causes, in order of how easy they are

Blocked or undersized drainage. The cheap one, and worth ruling out first. Outlets, scuppers and internal drains fill with debris, and a roof that drained fine for years stops.

Insufficient fall. Flat roofs are not actually flat — they are built with a slight slope toward the outlets. If that was never adequate, the roof has ponded since day one and will keep doing so. The fix is tapered insulation or a re-pitch, not a new membrane.

Structural deflection. The roof deck has sagged, usually between supports or under the weight of equipment added later. That creates a low point nothing drains out of, and it is a structural question before it is a roofing one.

Why it matters that you know which

Because the most common expensive mistake on a flat roof is replacing the membrane while leaving the cause alone. The new roof ponds in exactly the same place, and in five years the owner concludes flat roofs are simply bad.

So before anyone quotes a membrane, the fall should be measured and the drainage checked. If a quote arrives without either having happened, it is a price for a product rather than a solution to your problem.

How much ponding is too much

Water that clears within a day or so after rain is generally tolerable. Water still sitting a couple of days later is the point at which it is doing real work on the seams and adding load you did not design for.

It is also worth knowing that ponding voids or limits some manufacturer warranties, which is a reason to deal with it rather than live with it. Check what yours says.