Flat Roof Drains – Which is the best drain design?

A Drain that is placed inside a basin makes for the most practical design

This Three-D video shows how an ideal flat roof drain should function.

This Three-D video shows how a flat roof drain should function if installed correctly. The best roof drain is normally one that sits in a basin. This video illustrates the perfect drain on a roof. There are times where a drain basin is not needed but as long as the drain is much lower than the roof surface itself, allowing debris to collect around the strainer and water can flow over the debris without causing a big pool around the drain.

The next video shows a quick way to tie an existing drain to a new roof.

We use Turbo Poly Seal from Bergstromm.com to make a transition from a rubber membrane to an existing cast iron drain. This is an excellent product that anyone can use. The trick is the preparation. The cleaner to the surface is, the best chance you have to be successful.

Using Turbo Poly Seal from Bergstromm.com to make a transition from a rubber roof to a cast iron drain

Flat Roof Drains, Scuppers, Gutters and Drainage Systems on Flat Roofs

One of the most overlooked items on a flat roof is the draining system. More attention should be given to the location of a drainage system to avoid future problems. Inner drains, gutters or scuppers each have their own function and deciding which system is best depends on the roof size, slope or no slope and cost.

The scupper through the wall is an emergency overflow drain incase the inner drain gets clogged or the strainer full of debris

Watch this Video on explaining flat roof drains.

No matter the type of drain system for a flat roof, as long as it functions correctly, this will extend the life of the roof.

When debris is allowed to collect around drains, scuppers or in gutters, then the roof is compromised. Pooling water has adverse effects on a flat roof. These are:

  1. Stress on the structure of the building
  2. Degradation of the roofing material over long periods of time
  3. The growth of moss and other plant material that will speed up the deterioration of the roofing membrane
  4. Freezing water during winters putting stress on the rubber material
  5. Leaks can start to develop from pooling water

There are three types of Drain Systems on a Flat Roof.

It is very critical to position drains for maximum effect. Most draining systems on flat roofs have a poor design or are not correctly installed. So often we find that Inner Drains are high on the roof causing water to pool around them. This pooling water can cause further issues with the rubber membrane collecting leaves, moss, and other debris that causes harm to rubber membranes.

The Pros and Cons of different draining systems.

I will try to explain each system and point out the possible areas that could be causing a leak or problem.

Water is any roofer’s nightmare. When water pools on a roof, it can cause problems if the roof is not up to standard. Here is a short list of issues that can happen due to pooling or ponding water.

When a roof section is underwater for long periods of time it can cause the following problems:

  • Overlapping seams that are not well sealed will separate and cause the roof to leakPlants are growing due to Ponding water. Periodically cleaning drains is essential.
  • Tar products (used to seal flashing) deteriorates over time
  • Caulking loses its adhesive properties
  • Corners of skylights, chimneys and vent pipes are the weakest areas of a roof and water can easily find a pinhole to leak in
  • Leaves and debris collects in the ponding area and causes plant growth
  • Mosquitoes breed
  • Ice form during winter causing stress on the roof
  • If the pond is more than two inches deep, it can cause unnecessary strain on the structure, and the wood rafters will bow over time making the pond deeper

Having sufficient drainage on a flat roof is vital. Most flat roofs suffer from poor drain locations or drain levels that are high. When designing a roof, it is essential to pay close attention to where the drains are. On many occasions, drains are too high, and water cannot flow off. Some pipes are too small to handle the volume of the water. The worst problem is that they don’t have well-designed strainers. Strainers that are part of the drains made by the manufacturers are inadequate.

Another problem is flat roofs are not well maintained and therefore fail prematurely.

Flat roof drains, scuppers are important - pooling water will cause plants to grow deteriorating the roof
The plant growth around this drain is due to a leaf filter that was never cleaned from all the debris
Flat Roof Drains and Strainers - Internal drains and how they are configured
Positioning inner drains are critical. Keep away from skylights

The Pros and Cons of Inner draining systems


  • Can be placed strategically on a roof
  • Does not freeze during winter
  • No gutters or downpipes needed – aesthetically pleasing
  • Drain pipes inside the ceiling and walls of the building – almost never have to be replaced
  • A custom filter can be made to prevent clogging – this will improve the draining system on a roof


  • The roof needs to be clean from all debris – strainers get blocked easily
  • If debris gets stuck in a drain pipe, it takes a professional to unclog it
  • When a strainer gets clogged with debris and leaves preventing water from draining off, it could lead to excessive water pooling and cause the roof to collapse (only on large commercial roofs)
  • More costly to install an inner drain system than a gutter or outside drainage like a scupper
Drain and Strainer Diagram. Flat roof drains should be cleaned often
A diagram of a roof with a drain installed
Ponding water due to a blocked drain. The water pooling is adding a lot of stress to the flat roof.
This image shows how much water can pool around a drain in just 20 minutes. The leaf filter was blocking the water from draining out

If there’s a leak and the Drain is a possible area, then check for the following.

  • Make sure that the roofing material is attached to the drain. One of the main issues on a roof happens when the roofing material separates from the pipe.
  • Check for possible tears or punctures that could have been caused by workmen during snow shoveling
  • If there has been a lot of repairs done around the drain area, it’s time to get a professional in to redo the drain flashing. Lots of tar and caulking is a good telltale.

The following pictures show real drain problems:

Little debris prevented water from draining off. Even so, the roof did not leak, and water could spill over the edge of the roof. The pooling water does put a lot of stress on the structure of the building.

The strainers of these two drains were clogged due to small pine needles that stopped the water from draining.
This images shows two drains and water is pooling around them. This happens because of small debris collecting around the leaf filter

 Drains are an essential part of a roof

One of the most critical factors of a roof is it ‘s draining system. If water cannot drain off in a reasonable time, then the chances increase for roof leaks.

When water is standing on a roof the following is possible:

  • Water leaking through a tiny crack or seam
  • Where the use of Tar, Roofing Cement or Caulking is, water will deteriorate the products and cause it to lose its bonding properties.
  • Standing water will freeze over winter and cause problems
  • Debris will accumulate and cause moss and other plants to grow
  • Mosquitoes will breed

Water pooling on TPO or EPDM membranes does not affect the longevity of the material. In the case of Modified Bitumen Membranes, it will deteriorate the rubber membrane.

Strainer or filter – the answer to a problem

Problem – See how little debris can clog a filter. Dirt is one of the main issues with internal drains.

Answer – A solution to this is to make a filter that is much bigger than this one, four feet in circumference, with larger slots so smaller debris can flow through. That way it will take longer to clog up, and little stuff will not be a problem.

Little debris can stop a lot of water draining off a roof putting stress on the roof.
This image is a close-up look of debris collecting around a leaf filter of a drain causing water to pool.

Cleaning a roof is essential on a regular basis. By removing the strainer or filter during heavy rains can help get rid of dirt and small leaves. Make sure no twigs or tennis balls washes down also.

Remove the strainer to wash the debris down the drain. Do not allow twigs and large objects to flow down the drain.
Small debris will not clog a drain. It is best to allow the small debris to flow down the drain.

Worst Strainer on planet earth!!

The worst strainer over a drain. This filter has too little slots to allow small debris through. The solution was to replace it with one that had larger slots or holes to allow smaller material through.

Who designed this strainer was not thinking – for the next 60 years the owner of this roof had to constantly have someone clean this strainer almost every day.

In the following images, you can see how little surface there is to attach the roofing membrane to the drain.

Cast iron drains on a flat roof are very common. When preparing an existing roof to receive a new roof, these drains are time-consuming. Removing old tar from the and cleaning the metal to attach the new rubber, is what takes time.

The worst strainer over a drain.This strainer has too little slots to allow small debris through.
Old Cast Iron Drain re-purposed on a new roof

The rubber membrane is attached to the drain. After cleaning and preparing the metal drain, the rubber roof gets connected to the metal ring. We use a unique technique to make sure the rubber will never separate from the pipe.

This is an existing drain that was prepared to attache a rubber membrane to it. Most of the time this is where a roof fail.
Removing tar and caulking making sure the drain is ready to receive the roofing membrane
Attaching a rubber membrane to the drain. We use a special technique to attache the rubber roof to the drain.
The membrane has tabs cut into it so it can fold into the drain and be attached to the drain wall

Making final finishing and removing excess material from the drain. Notice the roof slopes towards the drain-pipe. most roofing technicians fail to install roof-drains correctly. With a proper strainer over this drain, the roof will be maintenance free.

It is common for a roof membrane to separate from the drain. This is the first things to check on a roof if there is a leak.
This is what it looks like after attaching the membrane to the cast iron drain
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1 thought on “Flat Roof Drains – Which is the best drain design?”

  1. Wow that was odd. I just wrote an incredibly long comment but after I clicked submit my comment didn’t appear.
    Grrrr… well I’m not writing all that over again. Anyways,
    just wanted to say fantastic blog!

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