What Causes a Commercial Roof to Drain Slowly?
Last updated October 26, 2026
The short answer
Debris and Clogged Drains
Leaves, twigs, gravel, and windblown trash collect on commercial roofs and wash toward the drains. Over time, this debris forms a mat that blocks water from entering the drain opening. Even a small layer of silt can slow the flow enough to leave puddles that last for days.
Roof drains often have a dome or strainer that keeps large objects out, but fine particles still slip through and build up inside the pipe. If the pipe narrows or becomes partially blocked, water backs up onto the roof surface. You can often see the problem by looking for standing water near the drain after a storm.
- Check drains after heavy rain for visible blockages.
- Remove leaves, gravel, and trash from the drain opening.
- Look for silt or sediment buildup around the strainer.
- Inspect the drain pipe outlet at the ground level for clogs.
- Trim overhanging branches that drop leaves onto the roof.
Flat Roof Ponding and Slope Issues
Commercial roofs are often flat or have a very slight slope. If the roof does not have enough pitch toward the drains, water naturally pools in low spots. Ponding water can happen even when the drains are clear, especially after an intense downpour.
Over time, the roof structure can settle, creating new low areas that did not exist before. A sagging deck or compressed insulation can redirect water away from the intended drain path. This makes slow drainage worse and can lead to leaks if the standing water sits for long periods.
- Look for puddles that remain more than 48 hours after rain.
- Check if the roof surface appears uneven or has visible dips.
- Note where water collects and whether it moves toward the drain.
- Consider whether the roof was recently repaired or modified.
- Watch for cracks or blisters in the roofing material near ponding areas.
Blocked Scuppers and Gutters
Scuppers are openings in the parapet wall that let water flow off the roof. If they get covered by leaves or debris, water cannot escape and backs up onto the roof. Gutters that run along the edge can also clog at the downspout, causing overflow and slow drainage.
On commercial buildings, scuppers often have a metal box or screen to keep pests out. These screens can become clogged with dirt and pollen, reducing the flow. Regular cleaning of the scupper openings and gutters is a simple way to keep water moving off the roof.
Internal Roof Drain Pipe Blockages
Many commercial roofs have internal drains that carry water through pipes inside the building to the sewer or storm system. These pipes can become blocked with sediment, roots, or even construction debris. A blockage deep in the pipe can cause water to back up at the roof drain, making the roof appear to drain slowly.
If you have checked the drain opening and it is clear, the problem might be further down the line. You may notice gurgling sounds in the building or water stains on the ceiling near the drain pipe. A plumber can inspect the pipe with a camera to find the exact location of the clog.
Heavy Rain and Oversized Storms
During a heavy rainstorm, the volume of water can overwhelm the roof drainage system even if it is mostly clear. The drains can only handle so many gallons per minute, and intense rainfall can exceed that capacity. This is more likely in regions with hot, rainy summers, where downpours can be sudden and heavy.
If slow drainage only happens during severe storms, the roof may not have enough drain capacity for the local rainfall intensity. Building codes often set minimum drain sizes based on the expected rainfall, but older roofs may not meet current standards. A roofing professional can evaluate whether additional drains are needed.
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Call Now (305) 614-9705Frequently asked questions
How can I tell if my commercial roof drain is clogged?
Look for standing water that remains for more than two days after rain. Check the drain opening for visible debris like leaves or gravel. If the drain looks clear but water still pools, the clog may be deeper in the pipe. You might also see water stains on the ceiling below.
Can I clean the roof drain myself?
You can remove loose debris from the top of the drain with a gloved hand or a small scoop. For deeper clogs, you may need a plumber's snake or a pressure washer. If you are not comfortable on the roof, hire a professional to avoid injury and prevent damage to the roof membrane.
What should I do if water is backing up into the building?
If water is entering the building, first turn off any electrical equipment in the affected area and avoid standing in water. Then contact a water damage professional to remove the water and dry the space. Also, address the roof drain issue to stop the source of the water.