March 2026 · 5 min read

Flat Roof Drainage Problems: Causes, Signs, and Fixes for Houston Buildings

Houston gets an average of 50 inches of rain per year. Some years, we get that in three months. If your commercial flat roof can't move water off fast enough, every storm becomes a structural event.

The most common drainage failure we see is clogged internal drains. Leaves, roofing granules, HVAC debris, and general dirt accumulate around drain baskets over time. In a slow rain, the drain handles it. In a Houston downpour, the clog creates a dam. Water backs up, spreads across the membrane, and adds hundreds or thousands of pounds of load that the structure wasn't designed to hold.

Scupper blockage is the second most common issue. Scuppers are the openings in parapet walls that let water flow off the edge of the roof. They work great when they're clear. But debris piles up against the parapet wall during storms and blocks the opening. Regular clearing is the fix, and it takes about 10 minutes per scupper during a maintenance visit.

The trickier problem is inadequate slope. Building code requires a minimum 1/4 inch per foot slope toward drains on flat roofs. But over time, insulation compresses, decking settles, and the slope diminishes. Areas that used to drain start holding water. Re-sloping with tapered insulation during the next re-roof is the permanent fix. In the meantime, adding crickets (small sloped diversions) around problem areas can redirect water toward functioning drains.

Signs you have a drainage problem: visible ponding water 48+ hours after rain, algae or vegetation growing on the membrane surface, water stains on the underside of the deck inside the building, or sagging visible from the ground. Any of these warrant a professional inspection.

For Houston buildings specifically, we recommend drain maintenance every 6 months. Before and after storm season. It's the single cheapest maintenance task with the highest return on investment.