These roof drain installation instructions are the basic steps to follow if you are installing a cast iron roof drain. They do not apply to retrofit roof drains. The procedure shown relates to installing a Zurn model Z100 15" cast iron flat roof drain but applies to all flat roof drain sizes. Cast iron flat roof drains generally come in sizes: 8-1/2", 12" and 15 or 16" which refers to the bowl size.
This will generally be very obvious as debris settles in the lowest spots and the area will be discolored.
If the flat roof is dry and you really can’t tell, put some water on it and will it will gravitate to the low spot(s). You can also use a straight edge or a string. We've even used a tennis ball which rolls to the low spot.
If your joists are 16" apart, you can use an 8-1/2" diameter roof drain. If your joists are 24" apart or more you can use a 12" diameter or 15" diameter flat roof drain. I would not use a 12" diameter roof drain between joists that are 16" as the sump receiver or drain pan may not fit. You do not want to modify any structural members such as cutting the joists to get the roof drain to fit.
Next buy a roof drain pan, either a sump receiver or a custom fabricated roof drain sump pan from www.roofdrainpans.com. The sump receiver (link) does not have a true "sump" or lowered section and can cause further ponding, whereas the roof drain sump pan (link) does. Getting the roof drain as low in the flat roof as you can is VITAL for excellent flat roof drainage and to prevent further ponding.
Cut a hole in the flat roof between the rafters big enough to receive the roof drain pan or sump receiver.
Attach the roof drain to the roof drain pan. see the video or manf drawings.
Set the assembly on the flat roof with the "sump" section directly in the hole. Structural modifications should not be needed.
Secure the flanges on the roof drain sump pan to the joists with nails or screws on the two sides that straddle the roof rafters. (not supplied. Stainless steel screws work great)
a. It is always recommended to have professional contractors for those tasks. These roof drain installation instructions are general in nature and all applicable building codes should be followed.