1) Get the distance (how far apart) your roof joists/rafters are for each location where you want to install each roof drain.
2) On wood framed roofs, they are either 16" or 24" on center and on concrete and steel roofs, they are farther apart.
3) Look in the attic space unless it's open beam and measure. If you have no access, you will need to find blueprints or do a test hole. This is VITAL.
4) Generally speaking, you can use an 8-1/2" or 12" (bowl) diameter roof drain for where the joists are 16" on center.
5) You can use a 15" or 16" (bowl) diameter roof drain for joists 24" on center.
6) Items #5 and #6 apply when using a sump receiver.
ROOF DRAIN INSTALLATION TIP #5:
Should I use a roof drain sump pan or a sump receiver with my roof drain installation project?
ROOF DRAIN SUMP PAN: The purpose of the roof drain sump pan is to lower or "sump" the roof drain (as it is referred to in the industry) 2" below the roof level. Sumping a roof drain is always the preferred method as the sumped roof drain will prevent rainwater from ponding on the flat roof.
They are specifically engineered to fit perfectly between joists that are either 16" or 24" on center without a single modification.* (see the photos below)
Their matched roof drains complete the assembly and the entire unit fits between the joists.
You do not want to modify any structural members such as cutting the joists to get the roof drain to fit your flat roof drain installation!
* = model LCP sump pan requires one brace.
WHEN TO USE A ROOF DRAIN SUMP RECEIVER? It can be used where the roofing is thick. An example is an insulated roof that has perhaps 2" of insulation. The insulation is often tapered to the roof drains. In this case, the roof is raised, via the insulation, rather than lowering the roof drains. Other than that, sumping a roof drain is VITAL for excellent flat roof drainage and to prevent further ponding and roof damage.
If you will be installing a large 15" cast iron roof drain, watch this video. It shows exactly how to install the Zurn Z100 cast iron roof drain using model LSP roof drain sump pan. The roof drain installation was done on a two story commercial building with a wood framed roof. Likewise, this system is VERY FAST to install by nearly anyone in the trades with the added benefit of sumping the roof drain.
Use this basic info before you buy your roof drain...
DO I USE A BOTTOM OR SIDE OUTLET ROOF DRAIN? HOW TO CHOOSE...
1) Determine the vertical space from the roof to the ceiling (if there is a ceiling) per the drawing below for each roof drain location. Using the sketch below, you can determine whether you need to buy a side outlet roof drain or whether you can use a bottom outlet unit. VERY IMPORTANT to know this. You can also download the sketch. These are general dimensions. Always use spec sheets and make your own determination.
A division of All Tech Plumbing
If you will be installing a small cast iron roof drain, watch this video. It shows exactly how to install the Frank pattern 8-1/2" cast iron roof drain using model SSP roof drain sump pan. The roof drain installation was done on a one story building with a wood framed roof. This system is VERY FAST to install by nearly anyone in the trades with the added benefit of sumping the roof drain.
Whether you are using a sump receiver or a deck plate (same thing) with a Jay R. Smith, Wade, Josam or any other roof drain type for your roof drain installation, the procedure is almost exactly the same.
Use the techniques the pro's use by All Tech Plumbing
Roof Drain Pans Co 1-888 900-9145 (949) 297-8689
THIS is MUCH EASIER TO UNDERSTAND BY WATCHING THE VIDEOS.
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 roof drain 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).
IF YOU ALREADY KNOW WHERE YOUR LOW SPOT IS, YOU CAN SKIP THIS SECTION.
This will generally be very obvious as debris settles in the lowest spots and the area will be discolored. SEE HERE
Cast iron roof drains generally come in these sizes: 8-1/2", 12" and 15 or 16" which refers to the bowl size.
They are sold with a sump receiver or similar deck plate that cradles or receives the roof drain. The sump receiver and related pans do not lower or sump the roof drain. Rather it is left at deck level or higher.