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Flat Roof Drain Strainers – Watch Video

Flat Roof Drain Strainers – Watch Video

Flat Roof Drain Strainers are Important 

If appropriately designed, a strainer is supposed to keep larger objects from going down a drain, while allowing smaller debris through with the water flow. Most strainers, or domes, that cover a drain on flat roofs, are poorly designed because they block smaller debris and the strainer becomes clogged, preventing any water flow. Twigs, sea shells, and rocks should not be able to get through strainer holes or slots. These types of objects will cause blockage in drain pipes.

Drain Installation on a Flat Roof Video Transcription

Video Transcript of a Drain Strainer

This is an illustration of a roof here. The substrate of the roof, and when most roofers install a drain, they would install it on top of the substrate and the plywood here and then they will install the membrane right over the substrate like this. What happens is, this particular part of the drain area becomes higher than the rest of the roof.

So, when it rains water will fill up in this area, behind this flashing and cause some pooling, unnecessary pooling on the roof, especially when there’s skylights or stuff like this around. Debris will collect around the strainer, and make the water pool even more. This can go as much as 3 or 4 inches. I’ve seen a roof fill up with water like this.

Best solution is:

The best way is to take the substrate and cut through this plywood, where the beams are, and lower this section of the particular roof and make a basin where the drain is supposed to be.

Once the drain is in a basin like this, it’s around 4 or 5 inches lower than the roof itself. Then when it rain, only the basin gets filled with water. And even if debris collects around the strainer, it will allow the water to fill up in the basin, an not on the roof itself, and then flow over the debris into the strainer.

This is the correct way to have a drain on a roof. So, you collect debris in this basin, and it gives you enough time to come and clean up the basin of this drain area instead of having the whole roof flooded.


The best thing is to get your drains either lowered, or when they install a new roof have them lowered to a proper area like this.

Drain and Strainer Position Can Cause Ponding- Video

The video above shows how to correct the position of a drain and its strainer to prevent ponding.

Old Style Drain Strainers

There are many old style cast iron drain strainers found on roofs. These old cast iron strainers were not well-designed for smaller debris to flow through with water because the slots are too narrow.

Metal vs. Plastic Strainers

There are many companies that manufacture drains and strainers. Most plastic strainers end up cracking and breaking due to harmful UV exposure over a long period of time.

Metal strainers last longer because they endure the outside elements and are heavier, therefore, will tend to stay on a drain without having to be bolted down, which makes for easier cleaning.

This is a cast iron flat roof drain strainer. This was a badly designed drain strainer due to the small openings

Cast Iron Drain Strainers

These images are from roofs that still exist. The drain and strainer were not replaced when a new roof was installed because they are costly. Ponding or pooling water is likely to occur due to the poorly designed drain which will cause stress and possible leaks on the new roof.

Cast Iron flat roof drain strainers were not a good design

Plastic Drain Strainers

It is our experience that plastic strainers are the worst to install. They degrade quickly after only a few years due to constant harmful UV light exposure. With the force of water during rainstorms, we see how the brittle strainers shatter leaving the drain to become clogged with large debris.

Plastic flat roof drain strainers don't last and often are the cause for roof problems

Drain and Strainer Combination

The drain and strainer combination in the picture below is an example of a good drainage system. Slots on the strainer are big and wide enough for small debris to flow through. If debris collects in the catchment area, around the strainer, it is low enough, so water could flow over the top of the debris and down the drain.

Cast Iron strainer that's well designed

Custom Made Strainer

In the picture below, we have found that the best strainer for any flat roof is one shown; it is a 2 x 2 ft custom made metal strainer with big slots or openings to let fine debris through. This is ideal when there is a catchment area around the strainer. The strainer has a lower profile than a dome-shaped strainer, which will prevent a high debris build-up around the strainer.

This part of the roof was lowered so if debris collects around the strainer, water could still flow over its top and down the drain.

As you have read, a strainer, is an important part of a drain system. If well-designed, it will require only minimal regular maintenance.

Custom made flat roof drain strainer is by far the best

Conclusion to Drain Strainers

Often we are called to address large pools or ponded water that has collected on a flat roof because of improper drainage. These pools of water cause the roofing material to deteriorate prematurely, and cause leaks.

During the spring season, we have seen some flat roofs in such a state, that not only has moss grown in the damp areas, other types of plant life have thrived.

In the winter months, these ponded areas freeze and cause stress to the roof structure, which may affect the interior parts of the dwelling.

We have found a tried and true way to alleviate the ponding of water where there is no slope for proper drainage. We cut in channels that are angled toward the draining area, which can solve a flat roof drainage problem and preserve the life of the roof.

It is a combination of sloping a roof and the installation of a drain basin that will give the best flat roof drainage performance to last for decades.

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Flat Roof Drains: Location, Installation & Maintenance – Watch Video

Flat Roof Drains: Location, Installation & Maintenance

Flat roof drains require the proper location, installation and maintenance because, as a system, it is fundamental to the preservation of a building’s structure. We have seen many times that the location of a drain is installed at a higher level than the rest of the roof which causes it function improperly. Most, if not all, structural leaks begin with some kind of drainage issue, so it is important that flat roofs be initially designed with proper locations for drains.

When drain locations are determined, properly functioning, high quality drains should be installed to complete the system. Without a well-designed drainage system and regular maintenance, commercial flat roof membranes will be compromised and not last the 30 to 40 years they were meant to.

Flat Roof Drain Issues

  • Location – Drains SHOULD NOT be installed next to other rooftop objects like A/C units, vent pipes, or skylights.
  • Level – Drains SHOULD always be installed at the lowest level of the roof surface.
  • Strainers/Domes – Drain strainers SHOULD have holes or openings large enough for small debris to pass through.
  • Piping – Drain Pipes SHOULD  be large enough to handle the volume of water typical for rainstorms.
  • Trees – Branches SHOULD be pruned back to prevent hanging over any part of a flat roof.

Flat Roof Drain Maintenance

Flat roof drain maintenance is essential to the life of the roof and ultimately to the building’s structure. As seen in the video, the flow of rain water, to the drain, is restricted by a small accumulation of debris around the strainer which clogs it and prevents water from reaching the drain. This pile of wet debris left on the roof, without being removed during regular maintenance, will cause the deterioration of the seams and sealants of the roof.

When Drains are Installed Unnecessarily 

In the image below, the drain was installed to remove ponded water next to a gutter system. We determined that not only was this drain installed unnecessarily, it was placed too close to a wall. This roof was designed to drain water to a gutter system. The solution would have been to build up the area where water ponded or lower the edge of the roof at the gutter.

Flat Roof Drains - This drain was installed unnecessarily and too close to the wall.

Roofs are Compromised Without Maintenance

The image below is an example of a strainer’s slots being too narrow to allow small debris through. As a result, water pooled around the drain and compromised the seams and flashing of the roof at the wall, which caused leaks.

Flat Roof Drains - Roofs are Compromised Without Maintenance

Flat Roof Drain Level

The video below shows one of the most common problems we find with both commercial and residential flat roofs. A drain should always be installed lower than the rest of the roof. A good roofing mechanic will remove the existing plywood and lower the drain to include a square catchment area around the drain. This will help with the collecting of debris. We lowered this drain and added a double membrane for durability for snow shovel maintenance and standing precipitation.

Flat Roof Drain Basins

It is very important when a new flat roof is designed that a provision be made to install a lower roof drain basin. Because the basin, around the drain area, is much lower than the roof surface, the basin will likely collect debris that has washed from the roof into the basin, allowing room for water to rise above the debris. The basin area can be 3′ x 3′ and up to 5′ x 5′ wide.

Watch the video below which illustrates how a drain basin functions.

Image of a Functional Drain Basin 

When we installed this roof 20 years ago, we created a basin around the drain. You can see that debris collects around the basin catchment and around the strainer, however, water was still able to flow over the debris.

Flat Roof Drains - Drain with a catchment basin used to collect debris.

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Residential and Commercial Flat Roof

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How to repair roof leaks on a metal roof permanently
Flat Roof Drain Strainers
Caulking incorrectly applied causes leaks behind flashing

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Flat Roof Drains and Strainers on Commercial Roofs – Watch Video

Flat Roof Drains and Strainers on Commercial Roofs

Flat roof drains and strainers are the most important components of any commercial flat roof. They are responsible for properly draining any and all accumulated precipitation. To preserve the life of the roof, however, the debris that collects around them, must be removed on a regular basis. There are several ideas you can implement to improve the effectiveness of a drain system.

The video below shows how a large metal strainer is more effective than a smaller, dome shaped, plastic strainer.

Video Transcript - for Improving your Flat Roof Drainage System

Hi, my name is Erik with Commercial Roof USA.

Strainers are too Small 

A very common issue on flat roofs are the drains. Now, there’s many problems with drains but a very common one that we see all the time is having too small of a drain. They install these and then put on these tiny strainers that are like high but very, you know, small in circumference and the problem is especially in a area with a lot of trees around. Like this one is that just a few leaves clog it up.

As you can see when they wash up, up against it, it creates a high barrier so this roof, this particular one fills up quite deep. So like if you’re not up here every, every couple of days, every couple of weeks especially in the fall, these are constantly clogging up. So, the way we solve this problem is by, first of all, making the drain a little bit lower and we’ll show you that in just a minute, but replacing strainers like this these small strainers with much wider strainers cause like just a few leaves clogging up uh coming up here and clog this up.

Custom-Made Strainers

What we do is custom make a nice wide low strainer like this one. After we lower the drain this will be very good cause just a few leaves stay way out here. You have a lot more space for all the uh, for all the water to come in still. If they wash up over top, the water can still get through. So, small you know, small circular ones like that, they just don’t work.

Lowering the Roof

This is how we solve the issue. First of all, number one, lowering the drain slightly and number 2 putting a much wider, lower strainer on top. Alright I’ve lowered the drain here and um, this is the old strainer. If you remember how small it is just a few leaves and twigs and what not can block this up really quickly. So we’re getting rid of this and you can see how we lowered it a bit and we made it just the right shape for our custom drain here.

Water Flows Freely

Now, this fits in perfectly right here and when a leaf or leaves come in here and start blocking this up, the water then just flows over the leaves onto this, and this thing will block up much less than that. This will take weeks to block up where that can go for like a day. So, um, this is how you do, this is an effective strainer especially when you lower the drain like this and this fits in perfectly. This is the type of strainer you should have on your roof.

Improve Your Commercial Flat Roof’s Drainage System

Improve your commercial flat roof’s drainage system by replacing the small plastic or metal strainers with a customized strainer made from metal. Plastic strainers tend to deteriorate and break from harmful UV rays. Also, the holes and slots of both metal and plastic strainers are too small and narrow, preventing small debris from flowing through to reach the drain. If these types of strainers are not maintained regularly, the buildup of small debris around them will eventually block the flow of water, causing the area to pond. Ponded water can promote the growth of moss and other plant life. Also, leaks are likely to spring from these ponded areas.

Make a Custom Drain Strainer

Make a custom metal drain strainer to improve your drainage system and preserve the life of your commercial flat roof. In the video above, there is an example of the type of strainer we have made to improve water flow even when there is a build up of debris.

Flat Roof Drains and Strainers - This customized drain strainer improves drainage systems and the life of the roof.

Notice the cracks on this plastic strainer. Eventually, the strainer will develop more cracks, then break apart, and become ineffective.

Flat Roof Drains and Strainers - Notice the cracks on this plastic strainer. Eventually, the strainer will develop more cracks, then break apart, and become ineffective.

Lowering a Drain

Lowering a drain will improve drainage from your commercial flat roof. A professional roofer will remove a section of the drain pipe from below the roof, to shorten it. The drain and roof, will then, be reattached to the shortened drain pipe. This is, by far, the best way to remove pooling water from a drain area.

Considerations Before Lowering a Commercial Roof Drain

  • What is the current type of roofing membrane?
  • What other roofing membranes are under the current one?
  • What type of drain is installed? (plastic, copper, aluminum or cast iron.)
  • Are both a plumber and carpenter needed, or can a roofer do all the required work?

The cost of lowering a drain can be as low as $1200 and cost as much $2500.

This video shows how a drain can be lowered to improve drainage.

A Lowered Drain Will Work – Watch Video

Lowering a drain will work. We have lowered drains for over 20 years. A drain that we installed about 10 years ago, shows that even though debris was not removed on a regular basis, the drain still functioned as was intended.


The image below shows how debris can block a strainer. The muddy water pooled and weakened the rubber membrane seals which caused leaks.  

Debris around a strainer and drain will eventually clog drains up

Drain and strainers on a flat roof

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Gutters on Flat Roofs – Important Drainage Systems

Flat Roof Debris and Gutter Drainage

Flat roofs collect an abundance of debris that may not be removed regularly. During rainstorms, this debris gets washed into the gutter trough and clogs it, preventing proper drainage. Over time debris accumulates in the trough, causing it to overflow, which compromises the exterior walls and the foundation with water infiltration.

It is important to keep the roof free from debris at all times. Doing so, will allow water to flow freely to the tough, through the downspouts and leader pipes, and away from the building for proper drainage.


If there are branches that hang over your flat roof, we recommend that you remove the debris from the roof and the gutters as often as you rake leaves or mow your lawns or yards. The easiest way to remove debris from flat roofs and gutters is to use a hand held leaf blower.

Gutters on Flat Roofs and Drainage - Gutters accumulate debris very fast if tree branches hang over the roof.
Gutters and Flat Roofs - Important Drainage System - Gutters with built up of ice and icicles often break.

Flat Roof Edging and Gutters

Flat roofs should be pitched or sloped towards gutters for a positive water flow. Even when they are, we often see that the edge of the roof is slightly higher due to a tar build-up, from repeated repairs, preventing water from flowing into the gutter. This can cause ponding on the roof, which can lead to leaks. Gutters should also be flush with facia board to prevent water from flowing behind gutter troughs, if not, water will damage the walls or paint.

Flat Roof and Gutter Inspection for Maintenance

Follow these preventive flat roof and gutter inspection and maintenance procedures for a well maintained drainage system.

  • Inspect the entire gutter system; the quality of all seams, joints, and downspouts.
  • Check that gutter brackets and hangers are properly secured.
  • Make sure the gutter is properly angled to create a positive water flow.
  • Keep tree branches trimmed away from flat roofs and gutters to prevent damage from high winds.
  • After severe storms, inspect both the roof and gutters to remove debris.
  • Ensure that leader pipes are well positioned to funnel water away from the foundation.

Flat Roofs and Gutter problems

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Scuppers – A Flat Roof Drain System for Commercial Roofs – Watch video

What is a Scupper?

Scuppers are drain openings, placed through walls, that drain heavy water flow and debris away from commercial flat roofs.

Scupper Placement on Commercial Flat Roofs 

It is very important to install scuppers in the proper places on commercial flat roofs. There are several factors to consider:

  1. Find points on a roof where water will drain naturally.
  2. Make sure that run off from the scupper does not accumulate around a foundation or wall.
  3. Make scuppers wide enough to handle large amounts of water and debris.
  4. The scupper must be accessible for debris removal and maintenance.

An Incorrectly Installed Scupper

The image below shows an example of a scupper that was incorrectly installed on a flat roof. Water could not flow naturally from that position on the roof, which created ponded water that will eventually cause the rubber membrane to fail prematurely. If scuppers are the only way to drain water from a roof, they should always be positioned lower than the roof surface itself.

Scuppers Flat Roof Draining - An incorrectly installed scupper causes pooling water.

A Scupper Can Prevent Pooling Water

A scupper can be installed on almost any edge of a flat roof, when there is an issue of pooling water. Watch the video below.

Installing a Scupper on an EPDM Flat Roof

Watch the video below.  We are cutting away the edge of an EPDM rubber roof to lower it to install a scupper that will drain into a gutter. It is important to make the scupper wide enough to handle the amount of water drainage typical for each climate. A scupper that is too narrow will become easily clogged with leaf debris, causing the pooling of water.

Proper Placement and Size of a Scupper is Important

Proper positioning and size of a scupper is essential for water and debris drainage. The video below shows how we lowered and increased the size of an existing scupper, in a parapet wall. It was positioned too high, and it was too narrow which made it difficult to clean. We found that trapped tennis balls were restricting proper drainage.


Scuppers are drains for both commercial and residential flat roofs. They must be properly installed to accommodate the volume of water and debris that needs to flow off of a roof and away from its foundation.

Scuppers a flat roof drainage system

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Flat Roof Drains: Commercial Installation and Drainage – Watch Video

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

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