A roof drain lets rainwater off a flat roof, but it is more than a hole and a pipe. A water trap underneath keeps a shallow seal that stops sewer gas rising back up. Use the buttons to look inside and follow the rain from the roof to the outlet.
A roof drain is the fitting at the low point of a flat roof where rainwater leaves. The trap is the U-shaped bend in the pipe just below it. Together they do two jobs: take the rain away quickly, and hold a shallow pool of water that blocks the sewer line from the building.
From above it looks like a metal dome sitting on the roof. Everything useful is beneath it: the strainer that screens debris, the body that gathers the flow, and the trap whose permanent pool of water quietly protects the inside of the building from what is in the drain line.
A flat roof is laid to a gentle fall so water does not stand on it. The rain gathers and runs to the lowest point, where the drain sits, instead of pooling and finding its way into the building.
The domed grate on top lets water through but holds back leaves, twigs and rubbish. Its raised shape means debris tends to slide off, so the openings do not choke and the drain keeps working in a storm.
Under the strainer a bowl-shaped body collects the water and leads it into the leader pipe. It is shaped to funnel the flow smoothly downward rather than let it splash and back up.
The leader — the vertical pipe — takes the water down through the roof and the building to the trap below. It is the same idea as a downpipe: give the water a clear route downward.
The pipe dips into a U-shaped bend, and water always sits in the bottom of the U. That standing pool is the seal. It lets water push through, but it stands between the sewer line and the building, so smells and gas cannot rise back up the drain.
Once past the seal the water runs along the outlet to a storm drain or soakaway, well clear of the building, so the roof runoff is carried off instead of pooling near the foundations.
It holds a shallow pool of water in a U-shaped bend. That water seal blocks the air in the sewer line, so smells and gas cannot rise back up through the roof drain.
Only a little. The drain is sized so a normal downpour still empties faster than the roof can collect, while the stored water is always there to keep the seal.
The dome strainer lets water in but keeps leaves, twigs and rubbish out. Its raised shape means debris tends to slide off rather than sit on the openings and choke the drain.
If it sits empty for a long time the water seal is lost and sewer air can pass. On a roof drain this is rare, because each rain refills the trap, but the seal is the whole point of the bend.
Once it clears the trap the water flows along the outlet to a storm drain or soakaway, well away from the building, so the roof runoff does not pool around the foundations.