HomeSkills HubHow To Select The Correct Rainwater Outlet For A Flat Roof

How to select the correct rainwater outlet for a flat roof

  • Read time: 2 minutes
  • Date: 24 Jun 2021
  • Flat Roofing

The purpose of a flat roof outlet is to drain rainwater from a roof. Flat roof outlets are manufactured in numerous shapes, sizes, and materials. You will need to take into consideration that each of these factors will affect the performance of the outlet. Remember that all the water that passes over the roof will drain through this component, so your chosen flat roof outlet should be fit for purpose and last for the life expectancy of the roofing system.

There are different considerations depending on whether the outlet is for a new-build or a refurbishment.

Selecting outlets for a new-build project

The building designer needs to start by deciding how they want to drain the water. There are various options such as draining the water from the flat roof to a gutter with downpipes, vertical outlets that drain through the building, or parapet outlets (horizontal) which discharge through a pipe on either the outside or inside of the building.

Most drainage systems use the principle of gravity, although siphonic systems are an option.

Blue roofs are gaining popularity and they require a specific outlet design to restrict the amount of water that is discharged into sewerage and waterways during a storm.

Rainwater calculations

Once the method of draining the roof has been established a rainwater calculation should be carried out to establish the size of any gutters and downpipes or the size and quantity of any outlets and pipes. If parapet outlets are to be used, more outlets will be required than for a standard vertical outlet as the flow rate is much lower because the water drains horizontally. A rainwater calculation is affected by many factors including location, the size of the roof, its pitch, design-life of the building, type of outlet, type of leaf guard, depth of any sump, etc.

Further considerations for outlet selection:

  • Membrane:

    There are different ways of securing the flat roof membrane to the outlet. The two most common methods are to use a factory bonded membrane flange that has the same type of membrane as that used on the roof, or a clamping ring.

  • Pipe diameter

    The diameter of the outlet should be the same as the rainwater pipe to allow for a watertight connection. The roof outlet should be installed and fixed in place and the pipe should then be connected to the outlet.

  • Spigot length

    It is considered best practice to select an outlet with a spigot length that is greater than the build-up dimensions of either the flat roof system or the parapet wall it is passing through. This eliminates connections within the build-up and makes it simpler to attach any ongoing pipework to the outlet.

Selecting outlets for a refurbishment project

In refurbishment applications, the outlet positions and pipework are already installed and the outlet has to connect to the existing pipework. If the roof is original (hasn’t already been overlaid) it might be possible to leave the outlet in place, remove the leaf guard and insert a slightly smaller outlet through the existing outlet and into the existing pipe. It is important to achieve a watertight seal to prevent leakage if the water in the pipe backs up for any reason. The best way to achieve a watertight connection is to use a rubber ‘O’ ring or rib seal.

However, if the roof has previously been overlaid, the outlet should be removed and replaced. This is because a second, smaller outlet pipe will have already been inserted into the original outlet during that refurbishment. Installing a third pipe into the outlet will make the pipe diameter smaller still, which will reduce the flow rate and could result in water ponding on the roof.

The new outlet will need to sleeve and seal inside the existing pipework. Ideally, the spigot on the outlet should be long enough to sleeve into the pipe rather than the old outlet.