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!

Table of Contents

  • 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 if you are installing new roof drains on a new construction project. 
  • Scupper drains or slip on retrofit roof drains are not addressed in this article.
  • Use the table of contents to skip to the section of your choice or read through each step. If you already have your materials ready to go, feel free to jump to step 7. 
  • These steps are a general guideline. Evaluate your project requirements accordingly. 
JR Smith roof drain, Flat Roof Drain Installation
JR Smith roof drain, Flat Roof Drain Installation, LSP roof drain sump pan

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. 

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

Due diligence: your key to a successful 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, the following steps will provide the basic info you need to successfully plan your roof drain installation project:

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Model LSP superior grade roof drain sump pan-standard showing a sumped 15" flat roof drain
Zurn ZC100 cast iron 15" roof drain and model Z100-89 roof drain, LCP sump pan

Determine Your Vertical Spacing


  1. Measure the distance between the ceiling and the roof.  Whether it is a drop ceiling with ceiling tiles or a hard lid (drywall), find a way to get that measurement. 
  2. If you have at least 13″ of vertical space, you can use a bottom outlet roof drain.  
  3. If you have only about 8″ or 9″ you will need to use a side outlet roof rain. 


  1. In this case you can choose either type of roof drain. Some people like to keep the piping tight against the ceiling. This is fine but realize that your pipe run will not be below the joists which means you will need to run parallel to them. 
  2. If you use the bottom outlet, you will be below the joists and can run the piping in any direction. 
  3. NOTE: there are always exceptions to these rules. Use your best judgement.  
  4. Review these sketches. 

Download more pdf spec sheets. Click the link

Determine Your Horizontal Spacing

    1. Measure the distance between your roof joists. Common joist spacing is 16″ or 24″ on center (center to center). However that is not the dimension that we need.  We need to distance between the joists. In other word, inside to inside.  This is the space you will be using. 
    2. In some cases you have much more spacing such as with a concrete or metal roof. Not a problem, just find out what it is. 
    3. The spacing between joists that are 16″ oc is generally 14″ 
    4. The spacing between joists that are 24″ o.c. is generally 22″ 
    5. Now that you have this info, let’s go to the next step…

How To Choose Your Roof Drain

While the links below show certain drain manufacturers, the product recommendations shown apply to most major brands

For joists 16″ on center,  (14″ W x 13″ deep), you can use

For joists 16″ on center,  (14″ W x 8″ deep), you can use

For joists 24″ on center,  (22″ W x 13″ deep), you can use

 For joists 24″ on center,  (22″W x 8″-10″ deep), you can use

With concrete and steel decks, you will have enough space to use whatever fits the situation however bottom outlet roof drains are generally used

Always measure the the depression of your deck pan to confirm it fits between the joists.  

How To Choose Your Roof Drain Pan

Your roof sump pan choice is extremely important and it is sometimes an overlooked roof drain accessory through lack of information. The roof drain pan provides a stable base for the roof drain to rest on rather than just setting it on a plywood deck. Also the metal flanges on each side provide plenty of surface area for the roof material to bond to. This results in fewer roof leaks.. 

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. There are three general types of drain pans. 

  1. sump receiver
  2. the deck plate
  3. sump pan… 
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 does not require an underdeck clamp. Eliminating the underdeck clamp makes the installation faster.  

Like the sump receiver, the common usage is on insulated roofs The insulation can be tapered to the roof drain and receiver thus creating a sort of sump area. The deck plate is secured to the roof rather than an underdeck clamping mechanism. 

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


The roof drain sump pan lowers or “sumps” the roof drain 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. See which major manufacturer 

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. 

The Roof Drain Installation Procedure

Use this procedure when you are installing single or double roof drains

  1. The first step is to cut the roof open between the joists so that the sump will sit down inside it. Cut this dimension to the size of the sump on your drain pan.    
  2. The second step is to cut away the roof material to the perimeter dimension of the sump pan flange. Do this by setting the sump pan in the hole as in step 1.  Refer to the spec sheets above for all dims. 
  3. Draw around the perimeter of the sump pan leaving yourself a little extra room, maybe add an extra ½”.
  4. Cut away the roof material. 
  5. Attach the roof drain to the sump pan and set the assembly in the hole.
  6. Now, secure the sump pan to the roof by screwing it directly into the roof joists using the perimeter holes.  The holes will be directly over the joists.    
  7. That’s it. Now you can move forward with the waterproofing and plumbing work.      

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


JR Smith roof drain, JR Smith roof drain and overflow, Zurn roof drain, Zurn roof drain and overflow, Frank Pattern roof drain, Frank Pattern roof drain and overflow, roof drain detail, commercial roof drain, roof drain, roof drain and overflow, roof drain sump pan, sump pan, roof drain pan


Watch the installation process in action

The videos show a small roof drain and and large roof drains being installed.  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.  

Check out these installation projects by our valued customers...

See how customers just like you have used these products to eliminate standing rainwater and prevent flat roof damage.

How To Size Your Roof Drain and Piping


See All Products Here

tech support for roof drains sump pans

We Have Great Tech Support

  • Product questions?  We’re always happy to answer your questions.
  • Consulation service:  Send us your specs/plans/drawings. We’re plumbing contractors and will review the documents and will provide an estimate for this service.   
  • Visit our installation page, blog or FAQ pages.
  • Call and speak to our trained techs. 
  • Review our videos on our  YouTube channel.
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