Keys to Emergency Overflow

by: Steve Nelson, RRC

Improving drainage as part of a reroofing project most often means installing a tapered substrate material to create positive slope to drain. Another drainage enhancement that is often overlooked, but may be required by code, is the provision for emergency overflow.

The Uniform Building Code requires that overflow drains or scuppers be set with a flow elevation 2 inches above the primary drains or low point in the roof. The purpose of this requirement is to limit flooding of the roof to a depth of 2 inches (and a weight of 10 pounds per square foot), should the primary drainage system become clogged or restricted.

When drainage becomes blocked on a near level roof, the overflow can usually find its way to another drain or scupper, or merely flow over the perimeter of the roof, if not restricted by a raised edge. By adding 2 percent slope (1/4" per foot) to promote positive drainage, sumps and valleys can be created that have ridge elevations far above that of the primary drainage system. Water can no longer free flow between drains or over the roof edge. In this case, overflow drains or scuppers should be installed.

Overflow drains are required to have the same capacity as the primary drains. They must be plumbed independent of the primary system in order to remain functional should the storm sewer system, to which the primary drains are tied, exceed capacity and block drainage. The discharge for the overflow drains should be located near ground level in an area that is readily visible to maintenance staff. Flow from the pipe will then be noticed and recognized as an indication that the primary drains are not functioning properly and require attention.

Overflow for watersheds that are adjacent to a raised perimeter edge or parapet wall can be accommodated by constructing through-wall scuppers. The Uniform Building Code requires that the cross sectional area of the scuppers be three times that of the primary drains. A minimum scupper opening height of 4 inches is also required. Because of their location in relation to the tapered slope layout, installation of scuppers may require that the substrate surrounding the opening be lowered or sumped in order to establish the flow elevation at 2 inches above the low point in the roof.

Typically the scupper opening through the wall has a metal liner inserted that extends past the exterior face of the wall to form a drip edge. The liner is then flashed to the inside face of the wall and sealed at the exterior wall face to prevent moisture penetration. If aesthetics is an issue, discharge can be controlled with a conductor head and downspout. Care should be taken in locating scuppers and drain pipes so that the discharge does not create hazards, or interfere with pedestrian or vehicular access to the building.

Adding overflow drains and scuppers obviously increases the cost of reroofing. Considering that not having emergency overflow could result in flooding of the interior, or even structural failure, it is likely money well spent.