Ponding wateron a flat roof

The National Roofing Contractors Association (NRCA) defines ponding water as water that collects at low spots on a roof and is still there more than 48 hours after rain, under conditions that dry a roof.1

Short ponding is normal. NRCA and the membrane makers accept water that sits for a short time, and some residual water at drains and joints is expected.2 Water that stays past 48 hours can harm the roof assembly.2

Many roof makers set 48 hours as the most ponding they accept. In NRCA's 2012 poll of makers, those making built-up asphalt, polymer-modified bitumen, and EPDM roofs named 48 hours. TPO and PVC makers ranged from 48 hours to indefinite.2

Ponding time roof makers accepted, NRCA poll, 2012

Roof type	Ponding accepted
Built-up asphalt	48 hours
Polymer-modified bitumen	48 hours
EPDM	48 hours
TPO	48 hours to indefinite
PVC	48 hours to indefinite

NRCA, Professional Roofing, July 2012, from maker websites and phone interviews. The figures are old, and makers decide some cases one at a time, so read your own warranty.

A scupper is the opening through a parapet wall where roof water leaves. Many seams and attachments meet at a scupper, so NRCA says to keep the area free of standing water.4

The scupper can sit too high. Where stripping plies or gravel raise the roof surface at the opening, water can't drain out, so NRCA recommends a drainage sump at the scupper opening and soldered or welded scupper joints.4

The parapet is the short wall around the roof edge. NRCA names faulty flashing, where the membrane meets a parapet or another feature higher than the roof, as one of the major sources of leaks on flat and low-slope roofs.5 Its guideline puts base flashing at least 8 inches above the roof surface.6

A real fix changes where the water goes. NRCA lists sloping the deck or building slope with tapered insulation, which works on a reroof and where the deck can't supply slope.2 Crickets and saddles steer water around equipment and between drains.2

Drains and scuppers belong at the roof's low points, and they need regular inspection to stay clear.3 Sprayed foam can be built thicker to improve drainage patterns, as the Insurance Institute for Business & Home Safety (IBHS) notes.7 Our foam roofing page covers that option.

A coating doesn't change where water runs, so we answer the coating question in our elastomeric coating guide.

As a roofing company in El Paso that is family owned, Lifetime works with its own crew, not subcontractors. We quote a repair when a repair will do, and if a leak comes back on our work, we come back and fix it at no charge.