Welcome to the web page that shows you "How To Install A Roof Drain"

by ALL TECH Plumbing and Rooter

* Here are 8 basic steps you would take to successfully install roof drains on a any flat roof!

  • This is a universal procedure that will show you step by step how to install a deck mounted flat roof drain.
  • It applies particularly if you are installing or replacing roof drains on an existing flat roof but applies to new construction as well.
  • Scupper drains or slip on retrofit roof drains are not addressed in this article.
Zurn roof drain, JR Smith roof drain, Flat Roof Drain Installation. Plumber, Mission Viejo
Zurn roof drain, JR Smith roof drain, Flat Roof Drain Installation. Plumber, Mission Viejo
Properly sumped 15" flat roof drain

Follow this roof drain installation procedure regardless of the brand, size, or the material (cast iron, plastic etc) of the roof drain. There are slight variations, but these will be covered later. 

STEP 1. Find the low spot(s) on your flat roof:

If you have already found the low spots on your flat roof before arriving on this page or, in the case of new construction, they are outlined on your plans, you can skip to the next section. If not, please review the steps.

  1. This will generally be very obvious as debris settles in the lowest spots and the area will be discolored. Many people mark the low spots after a rain with a brick or stone or some such method so it can be identified after the rainwater evaporates.  
  2. If the flat roof is dry and you really can’t tell, put some water on it and and try and duplicate how the water gravitates 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.  
flat roof, standing rainwater

STEP 2: Due diligence is key on your roof drain installation project!

Gather this basic info before creating your material list... 

Like any construction project, it’s all about the planning. If planning goes well, the project is pulled off without a hitch. If not, time is wasted and TIME IS MONEY on any construction project. 

With that in mind, your fist task is to determine which direction and how far apart your roof joists/rafters are. On wood framed roof, they are either 16″ or 24″. On concrete and steel roofs, they are further apart. Look in the attic space unless it’s open beam ceiling and measure. If you have no access, you will need to find blueprints or do a test hole or make an educated guess. 

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Model LSP superior grade roof drain sump pan-standard showing a sumped 15" flat roof drain
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STEP 3. Using a bottom or side outlet roof drain. How To Choose...

Find Out The Vertical Distance. This is critical

 Measure the vertical distance between the roof (bottom side) and the ceiling if there is one.  There are just a few situations that you will encounter on this step and you can determine whether to use a bottom side outlet roof drain:

  1. ATTIC SPACE:   measure the distance between the ceiling and the roof.  Whether a drop ceiling or a hard lid (drywall), find a way to get that measurement.  You generally have enough room to use a bottom outlet drain. You will encounter this set up generally in a restaurant, apartment or condo etc. 
  2. OPEN BEAM: There is no ceiling  with unlimited space as in a warehouse, gymnasium or other open beam situations.  In this case you will need to take into consideration the beam size as you will most likely hang the pipe below the beams.  In this instance, you can use a bottom outlet roof drain.  
  3. CLOSED BEAM/LIMITED SPACE: Where you have minimum space between the roof and the ceiling as when the roof joist separates them.  Take your measurement.  This is the situation where you would use a side outlet roof drain. 
  4. Review these sketches. 


Select your roof drain that fits your space

    1. If your joists are 16″ apart and you have at least 13″ of vertical space, you can use a small diameter, (about 8-1/2″) bottom outlet roof drain
    2. If your joists are 24″ apart and you have 13″ of vertical space or more, you can use a 12″ diameter or 15″ diameter flat roof drain with a bottom outlet.  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. 
    1. If your joists are 16″ apart and you have minimum space such as 8″ of vertical space, you can use an 8-1/2″ diameter roof drain with a side outlet.
    2. If your joists are 24″ apart or more and you have minimum vertical space such as 8 or 10 inches, you can use a 12″ or 15″ side outlet roof drain

Step 5

What is the difference between a sump receiver, deck plate and roof drain sump pan...Which to choose?

Your choice in a roof sump pan is extremely important and is sometimes overlooked through lack of information. 

Using the wrong roof sump pan can have devastating effects. Consider this : using a sump receiver on a roof with a thin covering causes the roof drain to sit higher than the surrounding roof. This usually results in a huge pool of standing rainwater. This is the exact opposite effect that is being created by a roof drain. It can result in very expensive repairs or premature replacement to the roof and can add roof loads which exceed it’s engineered capability. Below are some usage notes… 

image of a roof drain sump receiver


A sump receiver is a roof drain accessory that is a flat plate that receives the roof drain. It requires an underdeck clamp to anchor the assembly to the roof.  

Common usage of this roof drain assembly is on thick roofs such as insulated roofs where the insulation is usually 2″ thick but can be as thin as 1 inch. The insulation is tapered to the roof drain and receiver.  

When purchasing, be sure to get the underdeck clamp.

zurn z100 Top set deck plate


A deck plate is often referred to as a TOP SET DECK PLATE by Zurn Mfg. however most roof drain brands have their version. 

It is similar to the sump receiver as your roof has to be thick enough. It eliminates the need for the underdeck clamp as the roof drain is bolted directly to the deck plate. The deck plate is then secured directly to the roof rafters or metal deck with screws. This makes the installation faster.  

roof drain pan, roof drain sump pan, flat roof drain


The roof drain sump pan lowers or “sumps” the roof drain 2″ below the roof surface. This effectively prevents rainwater from pooling on the flat roof. 

It can be used on all thicknesses of roofs, with or without insulation. Some major roof material manufacturers recommend this style sump pan.

Roof drain sump pans are not supplied with your roof drain but can be purchased on this website.


If you know the pipe size you would like to use, then you are almost done.  If you need to size your roof drain, please skip to the bottom of this page. 

Step 6...The Installation Procedure... The roof drain installation video below shows this process in action using a sump pan

  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 manufacturers drawings below) 
  3. Set the 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. Stainless steel screws work great.) That’s the basic process.     

BLOCKING YOUR ROOF DRAIN: What does it mean and should you do it.

Blocking your roof drain means installing blocks on the ends of the sump pan that are not over the joists. Often the plywood on the roof is thin and when someone steps between the joists, we have what is referred to as “deflection”. The plywood bends away from the pan and roof drain. This can and will cause leaks. Placing blocks will provide support and prevent this situation. Do this if you have excessive deflection. This photo demonstrates


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Step 7

Plumbing and roofing

Follow the steps shown in the video to drop the roof drain in the roof. Once done, you can connect to existing piping if you replacing the existing roof drain or install your new piping. While the video shows a small roof drain and pan being installed. the procedure is the same for larger roof drains. This can be seen in the video below and in other videos on our Youtube channel. 

There a many roofing videos showing how to waterproof roof drains on You Tube. Shortly, we will link those here.  

Step 8

Sizing your roof drain and piping

Generally speaking, any changes that are made to a roof drainage system are within the purview of an architect or engineer. Such should be consulted for changes, however the procedure shown can be used and submitted to your local Building and Safety Dept. for approval


  1. This is a huge subject to address as it comprises an entire chapter in the Uniform Plumbing Code. But in this article we are primarily adding new roof drains to an existing building or replacing existing roof drains, I will break it down to the basic tasks that apply: 
  2. The quickest and simplest procedure is to determine how big your ponding area is in square feet. If you have more than one, do the same for all ponding areas. The surface area may be much larger than the pond so take this into consideration. 
  3. Consult the rainfall chart in the Uniform Plumbing Code book. Once you find your City, find the maximum rainfall. 
  4. Next consult the sizing chart. This tells you the size of the roof drain and the horizontal and vertical pipe sizes.  Always consult the Uniform Plumbing Code and local building codes. Always pull a plumbing permit. 


This is when you have existing roof drains in place that need to be removed so as to install new drains. 

  1. Get the manufacturers brand name if possible.
  2. Measure the bowl size and the pipe size
  3. Determine if it is a bottom or side outlet roof drain. 
  4. Determine what type of piping is in place. 
  5. Generally, replace the existing roof drain with one that is comparable. If you have plans, use those and it is good practice to size the roof drains anyway as conditions may have changed since the property was built.
  6. Provide the local building dept with your findings for approval. Take pictures and dimensions of not only the roof drains but the ponding areas in question and the exterior of the building as well. Take the spec sheets for new proposed drains and piping with you.  
  7. Keep in mind that existing roof drains may not have been installed correctly and a sump pan may not have been used at all or a bona fide, code approved roof drain. We have found everything from shower drains to floor sinks installed on flat roofs as well as sheet metal scupper drains and flashings turned upside down. So you may have to start from scratch with the right drains. The good news is that the roof drain systems on this website have made it EASY to resolve the issue. 

 Free tech support. We’re located in California, USA.  Call and speak to a plumber at  (949) 297-8689  with any questions. Send us your plans and specs or sketches. 

Visit our You Tube channel to see all of our other roof drain installation videos. Go here…https://www.youtube.com/user/ROOFDRAINPANS   If this document has been of value, please like us or subscribe on YOU TUBE.  

roof drain detail
Sizing horizontal piping
roof drain details, pipe sizing chart
Sizing vertical piping


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tech support for roof drains sump pans

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