How Do You Size a Conventional Gravity Roof Drainage System?

Conventional gravity roof drainage is sized from the roof area and the local 100-year, 1-hour rainfall rate, using plumbing code tables to select roof drains, vertical leaders, and sloped horizontal piping. A separate overflow system — overflow drains or scuppers — sized for the same storm is required so a blocked primary drain doesn't pond the roof to structural failure. The math is a chain: rainfall rate times tributary area gives gallons per minute at each drain; the code tables convert that flow into a drain size and leader size; the horizontal collectors are sized from the accumulated flow at the code-minimum slope. Every link in that chain has to use the same storm — mixing a 100-year rainfall rate with pipe tables read for a different storm is a quiet way to undersize the whole system.

Primary drains fail. They clog with debris, they get paved over in reroofing, they freeze. The code knows this, which is why secondary drainage isn't optional. Overflow drains sit with their inlets a couple of inches above the primary inlets, so they only engage when water rises past the normal level; scuppers through the parapet do the same job by giving water a way off the roof edge. Either way, the overflow system is sized for the full design storm on its own. The structural side of this deserves respect. A foot of ponded water weighs about 5.2 pounds per square foot, and roofs have collapsed under ponding that started as a clogged drain on a roof designed for a lighter storm. The structural engineer designs for the ponding depth the overflow system allows — which is another reason the plumbing and structural designs have to agree on how the roof drains.

Gravity roof drainage is mature, code-driven engineering — which means the failures are almost always omissions, not mysteries. Run this checklist and the roof stays dry. I've never seen a well-drained roof fail, and I've seen plenty of poorly-drained ones.

• Use the local 100-year, 1-hour rainfall rate — verify it against the code maps for the project city
• Size drains, leaders, and horizontal piping from the code tables for the same design storm throughout
• Provide independent overflow drainage (overflow drains or scuppers) sized for the full storm
• Coordinate drain locations and ponding depths with the structural engineer
• Maintain minimum pipe slopes and verify the deep-pipe routing actually fits the structure