2015 IPC 1110.4 Minimum number of rood drains

[Tim Mailloux]

IPC 1110.4 Minimum number of roof drains states that Not less than two roof drains shall be installed in roof areas 10,000 square feet (929 m2) or less and not less than four roof drains shall be installed in roofs over 10,000 square feet (929 m2) in area.

My question to the forum is weather or not secondary (emergency) roof drains per 1108 count towards the minimum number of roof drains? As an example for discussion, I have a 5000sf low slope membrane roof pitched to (1) primary roof drain, several feet away from the primary roof drain is an emergency over flow roof drain. Per section 1110.4 I am required a minimum of (2) roof drains as the roof area is below 10,000sf, is my design code compliant? Or do I need to provide (2) primary roof drains and (2) overflow drains?

 

[classicT]

They do not count. Chapter 11 references to roof drains and secondary roof drains independently.

However, 1110.4 only pertains to controlled flow roof drain systems. If not using a controlled flow system, then the minimum number does not apply. A single drain with a secondary "emergency" drain, if sufficiently sized per 1106, would be permitted.

 

[classicT]

As an example, given a 10,000sf roof and a rainfall rate of 1.5-inches/hr.

Equates to 9350gph = 156gpm

Per Table 1106.2, minimum storm drain pipe of 4-in (180gpm max capacity for vertical pipe, and 163gpm provided 1/4-in/ft on horizontal runs.
Per Table 1106.3, minimum vertical leader is 4-in (192gpm max capacity)

Based upon 1106, a single 4-inch roof drain could be used on a 10000sf building and meet the applicable requirements. In comparison, a controlled flow system with two drains would be required by 1110.4 for up to 10000sf, or four if over 10000sf.

 

[steveray]

❖ This section contains requirements for a storm drainage
system that is designed as a controlled-flow system.
Often such a system is used to maintain a small
level of water on the roof to help with cooling. Section
1107 also contains a requirement for emergency systems.
Controlled-flow roof drainage systems have
smaller diameter storm drainage pipes relying on the
roof to serve as a temporary storage or retention
basin. Rainwater collects on the roof while the vertical
conductors discharge at an assumed rate.
The principles of design for a controlled-flow roof
drainage system are the same as for a retention
basin or a dam for flood control. The water discharges
at a gradual rate and continues after the
storm has subsided.
Many engineers will size such a system for a maximum
period of storage, usually 24 hours.

 

[HForester]

"This section contains requirements for a storm drainage
system that is designed as a controlled-flow system."

For normal roof drain systems, you just need one primary roof drain and one secondary method for emergency overflow.