Opinion: Sizing Secondary Roof Drains

Date:

September 15, 2003

To:

Municipal Building Officials

From:

Thomas R. Joachim, State Building Official

Subject:

Sizing Secondary Roof Drains

Unlike the Uniform Building Code, the 2000 International Building Code (IBC) does not contain specific provisions for roof drainage; instead, it references the 2000 International Plumbing Code (IPC). Because Minnesota did not adopt the IPC, Section 1503.4 of the IBC titled "Roof drainage", was amended in Minnesota Rule part 1305.1503 to reference the Minnesota Plumbing Code for the design and installation of roof drainage systems. However, for designing secondary roof drainage, we reprinted provisions out of the 2000 IPC. Our intent was to mirror the scoping provisions contained in the plumbing companion to the IBC, the 2000 IPC.

One of the provisions we incorporated is located in 1305.1503.4.3.3, Sizing of secondary drains. It states in part, Secondary roof drain systems shall be based on the rainfall rate for which the primary system is sized by two. We have recently learned that the "by two" was removed from the first printing of the IPC by errata and no longer appears in subsequent printings. Again, it was our intent to mirror the provisions in the IPC, not exceed them. Had we actually adopted the IPC document, this "by two" would be gone by errata. But because we included the provisions by amendment, the "by two" remains. Therefore, this division will be granting a blanket "modification" (MR 1300.0110, Subp.12) to this provision for projects over which we have jurisdiction, thereby permitting secondary drains to be sized the same as that for the main roof drains, in accordance with the Minnesota Plumbing Code. Although our position is not binding on local government, we hope you will consider recognizing it as well.