What Is A Roof Drain?

The Plumbing Code Definition

OK, so what is a roof drain? Let’s take a look at the authority on the subject which is the uniform plumbing code. 

The Uniform Plumbing code reads as follows:

“ROOF DRAIN: A drain installed to receive water collecting on the surface of a roof and to discharge it into a leader, downspout or conductor.”

I would have been a bit more specific on the description. A type of drain (as there are many) that is installed on a flat roof that is designed to receive rainwater collecting on the surface of  a flat roof and to discharge it into a leader, downspout or conductor.  These three words are synonymous and simply mean the vertical piping. 

Ok, so the simplicity is that a roof drain prevents rainwater from ponding on any flat roof by draining it away to the ground.  Excellent. we’re glad it does. 

What are the different types of roof drains? How Do They Function?

There are two main categories:  Primary or main roof drains and Overflow or secondary roof drains. While there are many variations of these, they all fit into one of these categories. But before I address that issue I'll define these and their function. 

The Primary Roof Drain? How Does It Function? 

Jay R Smith 1010 Roof Drain
Jay R. Smith 1080 overflow roof drain


The primary roof drain is a single roof drain and it does all of the “heavy lifting” or most of the work in the roof drainage system. They are designed to be continuously in service and for this reason it is vital to prevent stoppages and make sure that they they are kept free of all debris.  

How The All of the primary roof drains are connected together on the same line and are not connected to any other part of the roof drainage system with few exceptions. They are tasked with draining the rainwater to the ground or another disposal location (like underground). This is called “conveyance” in the engineering circles and is an extremely important concept but we will address that in another blog.    

The overflow roof drain? How Does It Function? 

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The Overflow Roof Drain…

The overflow roof drain or as it is often referred to as the secondary roof drain, acts as a “backup” or a “failsafe” roof drain to the primary or main roof drain. It always has a 2″ water dam connected to it which looks like collar. This can be an external water dam that encircles the dome or it can be an internal water dam that is inside the dome. Both function the same These are pictured above. 

How The Overflow Roof Drain Functions… 

It allows the water to pond on the roof  a maximum of two inches before coming into service. This is a plumbing code requirement. The overflow or backup roof drains protect the flat roof from caving in under the weight of standing rainwater should the main drains become clogged with debris.  

What Are The Basic Components That Comprise A Roof Drain?

A side outlet roof drain and it's related components
  1. The roof drain bowl or body
  2. The dome or sometimes called the leaf grate
  3. The membrane clamp and gravel guard (same thing)
  4. Hardware; this includes nuts and bolts to secure the membrane clamp to the bowl or body and hardware to secure the roof drain bowl to the deck pan, also referred to as a deck plate.
  5. There are many options and accessories provided by the product manufacturers.  These are fully listed on the respective spec sheets. For quick reference for architects, engineers and contractors,  this website has a listing of many of the common roof drains with their spec sheets ready for easy download.     

We will break down these components and their function in the following paragraphs.

Frank Pattern dome strainer

What is the roof drain dome (leaf grate or strainer)? What is it's function?

Roof drains shall have domed strainers. Exception: roof drain strainers for use on sun decks, parking decks, and similar areas that are normally serviced and maintained, shall be permitted to be of the flat surface type. Such strainers shall be level with the deck. 

Dome strainers are included with the roof drain. Manufacturers provide a choice in materials generally as a special order. These can be made from aluminum, stainless steel, brass, ABS, PVC (plastic). Review their specs for the exact offering. 

Frank Pattern Membrane ClampRing

What is the roof drain membrane clamp/gravel guard? What is it's function?

This simple low tech device is a VITALLY important component in the roof drain assembly.  It could be said that it is the most important part. This is because the membrane clamp makes the water tight seal between the roof drain and the roof membrane or roof material. If this part isn’t used or incorrectly installed,  it will result in water intrusion into the building and subsequent property damage. 

NOTE: there are some very modern roofing materials that seal a roof and any component attached to it. This can render the membrane clamp unnecessary, however not installing it is not recommended as the dome attaches to it, and this part is equally necessary to have in place.  Always refer to the roof material manufacturer on this. 

What are flat roof drains made from?

UPC (uniform plumbing code 1102.1). Roof drains shall be constructed of aluminum, cast iron, copper alloy of not more than 15% zinc, leaded nickel bronze, stainless steel,ABS, PVC, polypropylene, polyethylene, or nylon and shall comply with ASME A112.3.1 or ASME A112.6.4.  

All of the roof drains on the market comply with these requirements. If you find one that doesn’t, don’t use it. 

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