Flat Roofs Drains and Draining Systems

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

Inner Drains     Scuppers      Gutters

They all have their Pro’s and Cons. I will try to explain each system and point out the possible areas that could be causing a leak or problem.

Watch this Video on explaining an inner drain system.

No matter the type of drain system for a flat roof, as long as it is designed and functions properly, then the roof will last a long time.

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 problems that can happen due to pooling or ponding water.

When a roof section is submerged in water 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 leak

    Plants growing due to Ponding water. Drains should be cleaned periodically.

  • 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 on a roof and water can easily find a pinhole to leak in
  • Leaves and debris collects in the ponding area and cause plant growth
  • Mosquitoes breed
  • Ice form during winter causing stress on the roof
  • If the pond is deeper than two inches, it can cause unnecessary stress on the structure and the wood rafters will bow over time making the pond deeper

It is vital to have sufficient drainage on a flat roof. Most roofs suffer from bad drain locations or levels. When designing a roof, consideration should be given to where drains should be installed. On many occasions, the inner drains are too high and water can not flow to the drain. Some drains are too small to handle the volume of the water. The worst problem is that they don’t have well-designed strainers. The strainers that are supplied by the manufacturer’s of the drains are inadequate.

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

Plants growing due to Ponding water. Drains should be cleaned periodically.

Inner Drains (for scuppers or gutters click here)

This type of drain is mostly placed towards the center or areas where water needs to be drained from the roof.These drains are connected to pipes below the roof that will drain the water away from the building.Inner drains are good for large roofs. Some residential homes that have the commercial type of roofs will also have these drain systems.

The Pro’s and Cons of an Inner draining systems


  • Can be placed strategically on a roof
  • Will not freeze during winter
  • No gutters or downpipes needed – aesthetically pleasing
  • Pipes are protected inside the roof and walls – virtually never have to be replaced
  • A custom strainer can be made to prevent clogging – this will become the best draining system on a roof


  • Roof needs to be clean from all debris – strainers get blocked easily
  • If a drain pipe happens to get blocked, it takes a professional
  • 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 very large commercial roofs)
  • More costly to install an inner drain system than a gutter or outside drainage like a scupper


Inner drains and how they are configured
Drain and Strainer Diagram. Flat roof drains should be cleaned often
Ponding water due to a blocked drain. The water pooling is adding a lot of stress to the flat roof.

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

  • Make sure that the roofing material is properly attached to the drain. This is one of the main problems on a roof with a drain.
  • 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 tell tale.

The following pictures shows of 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. This does put a lot of stress on the structure of the building.

The strainers of these two drains had a little debris that stopped the water from draining.

 Drains are the most important part of a roof

One of the most important factors of a roof is it ‘s draining system. If water cannot drain off a roof 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 very small crack or seam
  • Where Tar, Roofing Cement or Caulking was used, 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 standing on TPO or EPDM membranes does not affect the longevity of the material. In the case of Modified Bitumen Membranes, it will deteriorate the material.

Strainer – answer to a problem

Problem – Look how little debris can clog a strainer. This is one of the main problems with internal drains.

Answer – A solution to this is to make a strainer 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 small stuff will not be a problem.

Little debris can stop a lot of water putting stress on the roof.

A roof should be cleaned regularly. The best way is to remove the strainer during rain so the debris can be washed down the drain.

Remove the strainer to wash the debris down the drain. Do not allow twigs and large objects to flow down the drain.

Worst Strainer on planet earth!!

The worst strainer over a drain.This strainer has too little slots to allow small debris through. The solution was to replace a the strainer that had bigger slots or holes to allow finer 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 an existing roof is prepared for a new roof, these drains are what takes time to prepare. Old roofing tar has to be removed and the metal must be very clean to attach the new rubber to it.

The worst strainer over a drain.This strainer has to little slots to allow small debris through.

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

Attaching a rubber membrane to the drain. We use a special technique to attache the rubber roof to the drain.
This is an existing drain that was prepared to attache a rubber membrane to it. Most of the times this where a roof fail.

Making final finishing and removing excessive material from the drain. Notice the roof slopes into the drain. Most of the roof drains roofers install are not installed properly. 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.