How to Fix Ponding Water on a Flat Roof

Flat roofs should never be completely flat — they require a minimum fall of 1:80 (about 1.25%) to drain rainwater off the surface. When water pools and sits for more than 48 hours after rainfall, this is known as ponding, and it's a problem.

Why Ponding Water is Dangerous

Standing water adds significant weight to the roof structure. Over time, it degrades many roofing membranes and encourages algae and moss growth. It also puts pressure on joints and flashings, increasing the risk of leaks.

Common Causes of Ponding

• Insufficient falls built in during original construction
• Roof deck has sagged or settled over time
• Blocked or inadequate drainage outlets
• Deflection in the structural decking under load

Solutions

The right solution depends on the cause. Blocked drains are the simplest fix — clear them and install regular maintenance. Insufficient falls can be corrected by adding tapered insulation during a re-roof, which creates drainage falls without structural work. Structural deflection may require a more significant intervention.

When You Re-Roof, Address the Drainage

If you're having a flat roof replaced, always discuss drainage with your contractor. Tapered insulation schemes are now commonly specified and offer an excellent long-term solution to ponding problems, while also improving the roof's thermal performance.