ROOF DRAIN PONDING
While most of the drain manufacturers have always produced plumbing products and have had very little roofing experience, it is understandable how a lot of the problems that exist with roof drains have come to be. Froet on the other hand,
has experience from the roofing side, so we developed our drain from the top side down and not the bottom side up. We feel we have a better understanding of what makes a good roof drain.
The integration of the roof drain into the roofing system has also been kind of a void area due to the integration of the Architect and the Engineer. It is an issue of where one stops and the other takes over. It has been a common problem
area for years. I deal with it almost every day. How is the drain being installed? How much insulation is at the drain? Is there a sump? Is the drain being installed flush with the roof surface or the roof deck? It sometimes takes a surprisingly
long time to get these answers.
One big issue is the drain being installed at the correct elevation in the roof.
As you can see in the above photo, there is severe ponding at the near drains, but if you look to the far roof area you will see no ponding at the drains. Now the interesting thing about this is that every condition is the same at all four
drains except for one.
Fully adhered EPDM roofing system
Roof slope and insulation thickness
Type of drain and location relative to roof
Date of installation
Location of trees
Near drains are flush with roof and far drains are sumped 1". Now you may wonder how that could produce such large ponding water areas on a roof with tapered insulation. Well the answer is quite simple; never let water begin to pond.
Let me explain. If a roof drain is installed flush with a roofing system, the drain ring itself will create a pond due
to the thickness of the ring and/or how it fits with the strainer and at what elevation water is allowed to flow into the drain. In this example, the drain used has no free area until 7/8" above the roof system. That will create a
15' diameter pond on a 1/8" slope roof and a 7' pond on a 1/4" slope roof.
But why is this pond much larger than that? It is due to the fact that water is sticky. What I mean by that is any leaves or debris that blows into the water will stay there. The leaves and debris that fall onto a dry EPDM roofing system
will freely blow across the roof and either off the roof, or into corner, or protected area from the wind. So debris that gets into the existing pond will then follow the flow of the water to the drain. This debris will then begin to slowly
block the drain strainer increasing the size of the pond, hence its ability to catch debris. In a sense, it will go exponential, rapidly increasing the size of the pond. This will continue until the drain is manually cleaned. We all know
how often maintenance happens on a roof. Now, if the drain is depressed/sumped in the roofing system (below the point where the drain being used free area starts) this will prevent water ponding, allowing the debris to blow free of the
drain. As the above picture shows!
Now this brings up an issue that is not covered under any of the codes or standards. Free area in close proximity to the roof, even if a drain has three times the free area required by the standard. If it does not start until 1” above
the roof and the drain is not sumped, all the free area in the world will not prevent that drain from becoming obstructed from debris. So this type of installation will require frequent maintenance to prevent the drain from eventually becoming
With the roofing background that I have, I understood the importance of this issue and that is why all the Froet drains have more free area in our integral drain ring gravel stop and under our strainers. So Froet roof drains technically
only need to be sumped 1/4 to prevent the formation of ponding water though a deeper sump for the roof drain can only help with roof drainage.
In regard to sumps for roof drains in the northern states, I have heard the concern that the R- factor must be maintained and the insulation cannot be sumped. Well while I understand the importance of R-factor today, creating a sump for
the roof drain will reduce the overall (average) R-factor for the roof area by nil. In fact if the roof insulation is tapered, it is usually designed around an average R-factor so the sumps can be included in that calculation. It may in
fact create an additional advantage by reducing the snow and/or ice around the roof drain to help with winter time roof drainage.
So to sum it all up, always sump a roof drain. It is good insurance against ponding and know the details of the drain you are specifying or approving for your project. Know where the free area begins on the specific model so that it can
be installed correctly and not lead to years of extensive maintenance.
See also: Roof Drain Basics