Draining Low-Slope Roofs

The small, 5-inch-by-5-inch scuppers in the parapet wall were inadequate to the task of draining this poorly sloped roof.

Properly flashed-in scuppers have several layers. The flashing itself can create a dam.

Receptors for the scuppers should be mounted as low as possible on the outside of the parapet so that water can't back up onto the roof if the receptors become obstructed.

Leaves can collect around the base of a drain, even one covered with a leaf basket. The leaves then keep water from draining off the roof.

Built-in gutters are not as good at draining water as they are at collecting leaves.

Drains are often positioned poorly, as in the corner of this gutter, where there is no room for a clamp ring and no room to solder.

Though this scupper was about 4 feet wide, it was only 4 inches tall and was quickly blocked by accumulating leaves.

A parapet wall can be installed on posts, leaving most of the roof edge open for drainage.

Because there is no complicated scupper flashing, parapet walls on posts are easier to detail.

As seen from the ground, the gutter obscures the open space between the parapet wall and the roof.

It's important to account for where the water goes once it drains off the roof. In this case, overflow was directed right into the basement. A small entryway roof was installed to keep the basement area dry.

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