Energy Codes and Roof Drains

What level of insulation should I have at a roof drain? The level of minimum ‘above-deck’ insulation level as prescribed by International Energy Conservation Code (IECC)? The answer is yes unless you are using tapered insulation.

When using the IECC, you can reference Table C402.1.3 to identify the minimum ‘above-deck’ insulation for your project’s Climate Zone [In 2018 IECC, CZ 2 or 3 is R-25, CZ 4 or 5 or 6 is R-30, etc.]. Under Roof Assembly exceptions, in section C402.2.2, there is a statement: “where tapered insulation is used with insulation entirely above deck, the R-Value, where the insulation thickness varies 1 inch or less from the minimum thickness of tapered insulation, shall comply with the R-value specified in Table C402.1.3”.

This is allowing the flat stock insulation, below the 1” of tapered, to be reduced by 1 inch for minimum R-Value compliance at a roof drain. We will use an example, a typical assembly for Climate Zone 2 or 3, requires an R-Value of 25, for roof insulation entirely above deck would consist of two layers of 2.2” Polyiso flat stock, for a total of 4.4”. [The energy code states you should use 2 layers of insulation with staggered joints, to minimize energy loss through the board gaps.]

But as we saw with the exception we want to taper or slope to the roof drain. A minimum 1/4” per foot slope (2% Slope) would allow a taper to start at the roof drain and grow to 4’-0” as it reached the 1” threshold. This is allowing for the creation of a sump area at a drain, or an area of 8’-0” by 8’-0”, which is sloping directly to the drain at 1/4” per foot. If you chose to design with a steeper slope, because it is a drain and you want more positive drainage, maybe you choose 1/2” per foot slope (4% slope) this would allow the distance away from the drain to be 2’ or an area of 4’-0” by 4’-0” around the drain.

Going back to our example, instead of using 2 layers of 2.2” Polyiso flat stock [4.4” Polyiso, R-25 design] we are designing with one layer of 2.4” Polyiso flat stock and one layer of a pre-fabricated target sump board (8’-0” by 8’-0”) which is a tapered Polyiso insulation which starts at the edge of the board at 2” thick and tapers down to the center at 1/4” per foot which has a final thickness of 1”, which meets the minimum R-value, while using the exception. Which means the Polyiso is a minimum of 3.4” at the drain, in lieu of 4.4”.

If we chose to use the 1/2” per foot sloped pre-fabricated target sump board (4’-0” x 4’-0”) it would start at the edge of the board at 3” thick and tapers down to the center at 1/2” per foot which has a final thickness of 1”, which meets the minimum R-value, while using the exception, as well. And again, means the Polyiso is a minimum of 3.4” at the drain, in lieu of 4.4”., just the target area or sump area is reduced from 8’-0” by 8’-0” to 4’-0” by 4’-0”.

Using the target sump in this example is also highlighting a rooftop labor saving product which saves the roof installer from cutting the sump out of tapered panels on the job site. The tapered insulation exception can also be employed when tapering at through-wall scuppers, as you would also design a sump area here to increase the flow of water off the roof.

For more information about tapered target sumps, refer to the sell sheet.
For more information about tapered insulation, refer to the technical data bulletin.