google-site-verification: google634e99ea8d142f7a.html ',' '); } ?>
Norwalk CT 203 858 0080 / Pittsburgh PA 203 858 0080 / Longview TX 903 -291-0848
Select Page

Commercial Flat Roof Drains: Installation Position is Critical

Drains are the most important part of any commercial flat roof, however, it is critical that they be installed at the lowest point of a roof. When not positioned properly, water will pool, causing load bearing stress on the structure and the rubber membrane, which leads to leaks. Pooling water has a tendency to separate seams around skylights, vents, a/c units, etc.

Factors to Consider

  • Roof should slope toward the drain.
  • Drain and pipe must be large enough for any volume of water.
  • A drain strainer must be designed to allow small debris and water to flow freely to the drain.
  • The placement of the drain, in relation to other rooftop units, should be carefully determined.

Drain positioned at low part of roof but nor recessed therefor collecting debris

The video below demonstrates how to replace a drain on a commercial flat roof.

Drain Installation Video Transcription

Commercial Flat Roof Drain Installation – Video Transcription

Alright, this is another drain I want to give you a demonstration on, and this time, this roof was covered with a TPO roof and of course then they had to do some kind of detail around the drains. Drains are one of the most difficult parts of re-doing a roof or when you do a commercial roof. It has to be very well sealed and secured and you can tell by looking at this drain that um, what happened is you can see there are many layers.

There’s about five layers of TPO. They tried to secure this drain, and um, all of that means it’s difficult and then underneath this TPO roof, you also have a modified bitumen roof that has a drain in there. I’m gonna have to remove all of this, and then come to the copper insert that I, I assume there’s a copper insert underneath this and then I will redo my drain, but when you redo a roof, make sure you take everything out.

Preparation Before Installation

You cannot just go over somebody else’s work. It doesn’t work. In this case there are maybe ten different layers of roofing material that they’ve been over and over tried to patch it. You can see there’s an EPDM patch right on top of a TPO roof, the TPO roof on top of foam, the foam on top of the modified bitumen. This is too much and it’s built up and then the drain becomes higher than the rest of the water here and then you have water pooling around this drain because so much build up.

What I am going to do is remove all of this and then we gonna start ah, ah, connection to the drain. Alright, we’re removing the TPO there, you can see how wet it is. Underneath the TPO, there’s water there. All of those layers and attempts, still didn’t help solve the problem. Alright, so, redoing this roof and stripped off the TPO, and then we had to redo the drain, came to find out that the drain was messed up and which I thought was a copper drain underneath the other drain happened to be nothing. They messed up the drain and this was installed ah, as a retrofit, um, so I removed the drain and I ordered another drain to be installed. And what I’m going to demonstrate to you is how to install this drain and tie that into the roof.

Drain Assembly

That is what you are going to see now. This is fairly a pretty easy drain to install, uh and it fits right in to a three inch pipe. Let’s start. The drain is somewhere here. Alright, right there. Right here is the drain. I’m gonna put my, my expansion rubber in there. So you can see, me tightening up the bolts expanded this rubber right here. This will expand inside the pipe, in a three inch pipe and cause a tight seal, so water can never back up and then into your house, if you have a backup or a clogged drain.

That’s the reason you want expansion rubber like that make that tight seal. This is a very good system, um, and I recommend it very highly. Into the pipe with the expansion rubber there and then I get this ready where I’m gonna put that so I can flange there, up slightly, okay and I heat this up, take my adhesive, see this adhesive here smear that underneath there, just like that. I’m making a gasket goes down right there I get the screw gun. I already formed this so it will fit nice and tight. Nice. So, already that’s a seal and it will not leak it’s nice. Just doing that by itself it will almost not leak anymore. Okay, I tighten that expansion um rubber. Okay, that and okay, alright so I get the bolts complete lift this up.

Technique of attaching roofing material to the drain

This is where technique comes in. Most people who does torch roofing, they would heat this up with a torch and melt it to that metal. It’s a big mistake. This material melts and expands, the metal doesn’t expand and after awhile it becomes loose and you have a leak. That’s not how you do it. You put down our adhesive over the metal right there and we ready to and this was a pretty easy drain to install. I burned the plastic off so it will stick to this uh adhesive. Alright, there you go. Nice. Alright, make a good seal and well them together and this will never never, very good.

That’s this drain. And your final thing, you put your strainer on. This whole drain is a metal drain. There’s nothing plastic about this. What I took out here had plastic in it. Now, this is very good plastic, I realized it, but sometimes people put tar and all kinds of stuff in here and the plastic and the metal sometimes starts leaking because of the transition. I don’t say this drain is bad, I just say, I don’t like it. This strainer it starts cracking after awhile, it gets hard in the sun and they start cracking.

Plastic Drains versus Metal

Oh yeah, cracked broken already. I don’t like the plastic. If you’re gonna get a drain, get a metal drain, metal strainer like this. This is quality. The only thing that you get to do this roof once in twenty or thirty years. This roof is gonna last over thirty years, and you don’t want to replace strainers and drains and fix anything. This is the best way to do a drain, just get a metal drain and do it right from the first time, never have to be redone. Alright, that’s it for your drain

Types of Drainage Systems

Inner Drains –

Although drains are located on a flat roof’s surface, the inner drain pipes exist on the inside the building, and below the ground surface leading to a drainage system. This prevents the pipes from freezing during the extreme temperatures of winter.

Scuppers –

Scuppers are openings in parapet walls, of commercial flat roofs, from which water can drain. These are very effective, since water will flow freely through them, to a dedicated controlled area.

Outer Drains –

Outer drains begin on a commercial flat roof surface, that are attached to drain pipes, which lead down the outer wall of a building, to the ground surface. During extreme winter weather conditions, water in these leader pipes, can freeze and crack the pipes.

Gutters – 

Gutters are a type of drainage system that is used to divert water from a roof and away from a building’s walls and foundation. In extreme winter weather conditions, water has a tendency to freeze causing a build up of ice, in the troughs, which prevents drainage.

Commercial flat roof drain installation - This is an old cast iron drain and strainer. The strainer's holes and slots are too small, which caused small debris to build up and block water flow.
3D Video demonstrating the best drain and strainer system
Skylight flashing explained and corrected
Metal roof Repair - easy and permanently

Flat Roof Drain Basin – Video

A basin is a lowered area, recessed into a flat roof’s substrate, where water converges and drains, even when there is an accumulation of debris.

When Drain Basins are Needed

The images show drains that are properly located at the lowest part of the roof. The problem is, however, that an accumulation of debris is preventing water from draining. If these roofs had drain basins, water could have drained over the debris.

This flat roof slopes toward the center of the roof where the drain was installed, however, the installation of a basin would have prevented ponding water and the growth of plant life.

Although the drain is properly located, a drain basin is needed.
This drain have been blocked for a long time and plants started to grow.

When the Location, Basin, and Strainer Work Together… 

…this image shows an example of a drain that functions as a proper drainage system on a commercial flat roof.

When the Location, Basin, and Strainer Work Together... results in an effective drainage system.

A Cast Iron Basin Drain from the 1930s

This cast iron drain and strainer is from the 1930s. Notice the small “basin” around the strainer. This basin was a good idea, but could not handle the amount of debris accumulation, and the strainer holes and slots were too small and narrow.

A Cast Iron Basin Drain from the 1930s - basin is too small and the strainer holes and slots are too small an narrow.

Drains on Flat roofs

 We benefit on Google if you write a comment on Flat Roof Drains and how it is affecting your roof. 

Use “flat roof”  or “drains” as keywords in your comment. Thanks a lot.


Submit a Comment

Your email address will not be published. Required fields are marked *