Roof Drain Installation Tech 101​

3) Choosing your roof drain - space requirements


A roof drain sump receiver IS NOT the same as a roof drain sump pan. The sump receiver (link) does not does not lower the roof drain, rather, it "receives" or provides a stable base. A draw back is that it can leave the roof drain higher than the surrounding roof and cause further ponding. 


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. ​​


The deck plate is similar to the sump receiver except the roof drain bolts directly to the drain pan and eliminates the need for the underdeck clamp. 


The underdeck clamp is designed to anchor the roof drain to the roof by clamping it from the bottom. This effectively anchors the roof drain in place.  

They are available with an optional exterior or interior mounted overflow dams. This is 2" high to comply with the plumbing code.    


  • 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!

  • ​Using a roof drain sump pan is the recommended method by industry pro's and by one of the biggest roofing material manufacturers in the world.  

  • * = model LCP sump pan requires one brace.​​

Roof Drain Installation Videos

​​- How To Install a Roof Drain -

Model SSP

  • These simple roof drain installation instructions describe how to install a flat roof drain using a roof drain sump pan and a cast iron flat roof drain in the bottom outlet configuration. The same basic procedure applies if you prefer to use a sump receiver or a deck plate or a side outlet roof drain. 

  • These instructions DO NOT DO NOT apply to retrofit roof drains or scupper drains (sheet metal or otherwise)


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.   DO I USE A

1) Preparing For Your Flat Roof Drain Installation Project

Model SCP

Your preparations can be the most time consuming part of your roof drain installation project but the most important. Installing the incorrect materials is a waste of time and money. Luckily, the steps here should help you to make more informed decisions.  

Model LCP

4) Choosing your roof drain pan

​2) Finding the low spot(s) on your flat roof


Model LSP

 For easier navigation, this page is divided into sections. You  can skip to whatever section best suits your needs...

 1) Preparing for your roof drain installation project

 2) Find your low spots on your flat roof

 3) Choosing the correct roof drain

​     * large or small diameter roof drain?

 ​    * bottom or side outlet roof drain?
 4) Choosing the correct drain pan   ​   

      * sump receiver ?
      * underdeck clamp ?
      * deck plate ?
      * sump pan?
5) Overflow roof drains

 6) Basic roof drain installation procedure

 7) Roof drain installation videos 

Flat Roof drain installation...the basic procedure 

​​​​1) Cut a hole in​ the flat roof between the rafters big enough to receive the roof drain pan or sump receiver. 

2) Attach the roof drain to the roof drain pan. see the video or manf drawings.

3) Set the roof drain assembly on the flat roof with the "sump" section directly in the hole. Structural  modifications should not be needed.    

4) 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). 

Use the techniques the pro's use by All Tech Plumbing. Watching the videos is the fastest way to learn the basics.

A division of All Tech Plumbing

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 there is heavy standing rain water on the flat roof, get it off by whatever means but not all of it. Leave enough and push it around with a broom and it will gravitate to the low spot(s). 

Roof Drain Pans Co 1-888 900-9145  (949) 297-8689

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. ​