IPC Gutter Sizing - 1106.6

[drawman]

I had a question about how to interpret IPC table 1106.6.

https://up.codes/viewer/pennsylvania/ipc-2018/chapter/11/storm-drainage#1106.6.

I can't for the life in me figure out why the table shows rectangular gutter sizes at slopes of 1/4" and 1/2" per foot, and semicircular gutters at 1/8" per foot. Are you allowed to convert a rectangular area to a semicircular area (and vice versa) to get the desired slope? I am trying to assess the allowable flow rates for a rectangular K-gutter sloped at 1/8" per foot.

 

[RLGA]

I believe the purpose is to limit the capacity of gutters at steeper slopes. If you calculate the capacity of a gutter using the Q=CIA method, that would work for rectangular gutters at level or near level slopes (e.g., 1/16" and 1/8"). However, at steeper slopes, the ability of the same size gutter to handle the calculated capacity is diminished as the gutter will very likely overflow at the lowest end; thus, the need to limit the flow rate.

For round gutters, the water is concentrated to the center of the flow line, so it flows more efficiently at lower slopes. This is why the circular gutters have limited capacities at the lower slopes.

If you calculate your gutter's flow rate using the Q=CIA method, then at a 1/8" slope, the maximum capacity would be the calculated flow rate.

I suggest asking your plumbing engineer if you know one.