How to size roof drains and scuppers?

This question originated from one of my recent work experience:

A plumbing engineer was calling out 4”x20” parapet wall scuppers for our building with CMU exterior walls. I had always used 8”x16” parapet wall scuppers since this size fits the 8”x8”x16” CMU block module very well and the contractor does not need to cut CMU block.

So, I asked the plumbing engineer why he came up with the 4”x20” parapet wall scuppers. One thing led to another and after we look though all the code sections, I was able to work with him and change the parapet wall scuppers to 8”x16”. The following question documented the rough process of how we get to the solution.

If ARE exams do have a similar question, all tables will be available for you. I just want you to become familiar with the process and know how to use these tables so you can do well in the ARE exams, but most importantly, you can be a better architect who can coordinate well with your CLSEMP consultants.

Here is the question I came up with based on this experience:

How to size roof drains and scuppers?

You can do the following as an open book exam, and Google any reference you need:

An architect is designing a building with a roof with ¼” per foot slope in Palmdale, California. The roof area is 36,000 S.F., if the architect is using 4 roof drains and each roof drain are spaced evenly, how should the roof drains and overflow scuppers be sized? Show the process of your solution.

Scroll down to see the answer:

Answer:

1. Determine the rainfall data to use for Palmdale, California:

It is 3”/HR per Table D101.1 in UPC:

https://up.codes/viewer/nevada/upc-2018/chapter/D/sizing-storm-water-drainage-systems#D%20103.0

2. Determine the roof area that each roof drain will drain:

36,000/4=9,000 S.F.

3. Pick the roof drain size per 2018 IPC, TABLE 1103.1:

https://up.codes/viewer/nevada/upc-2018/chapter/11/storm-drainage#table_1103.1

4. Determine the scupper size:

Per 2018 IPC, 1106.5 Parapet wall scupper: Scupper openings shall be not less than 4 inches (102 mm) high and have a width equal to the circumference of the roof drain required for the area served, sized in accordance with Table 1103.1.

https://codes.iccsafe.org/content/IPC2018/chapter-11-storm-drainage#:~:text=Scupper%20openings%20shall%20be%20not,for%20the%20same%20roof%20area.

3. The roof drain is 5” diameter (D), so the circumference of the roof drain=πD =3.14x5=15.7” or about 16”.
4. Scupper openings shall be not less than 4 inches
5. So, the scupper size is 4”highx16” wide

Gang Chen, Author, AIA, LEED AP BD+C (GreenExamEducation.com)