Everything You Need To Know About Roof Drains

flat roof drainage system

The team here at Elite has written extensively about the necessity of positive drainage for any roofing system and the dangers that can arise from ponding water. We know why positive drainage is important, but it’s a lot more challenging to put together the necessary components for a roof that will drain effectively. We’ve already covered tapered insulation; in this article we’re covering another important aspect of your positive drainage system, the drain itself. All of these elements work together to give your roof the best protection possible from water damage.

We’ve broken down the key components of roof drains, and what you should look out for.


How A Roof Drain Is Selected

Roof drains come in many different shapes, sizes, materials, and designations. Each of these is specifically designed to work within the larger mechanism of your positive drainage system. Before we select a drain for a roof we are working on, there are five things we consider:

1.  Materials

Drains typically come in three materials: Copper, aluminum, or cast iron. For most of our roofing projects, we recommend copper, which is strong and reliable, but there are use cases for each of these materials. We most often find cast iron drains used on new construction projects where they are supplied by the plumbing subcontractor.

2.  Baskets/Strainers

Strainers for drains are usually made of two materials; cast aluminum or plastic. When weather cycling is a concern, we recommend aluminum because of its ability to wear well through freeze/thaw cycles and prolonged exposure to the sun’s ultraviolet rays. Because our team works exclusively in Canada, our focus is always on materials that freeze and thaw effectively.

3.  Outlet Sizes on Roof Drain Connections

Diameters for drains range from two to ten inches.  The sizing differs based on mechanical engineering design criteria that consider such items as the amount of rainfall in inches per hour based on the geographic location of the building. Also, the steeper the slope of the roof, the greater the rate of flow into the roof drains.

The most common size of storm drains that we usually see in the Golden Horseshoe area are:

  • 3” and 4” for new construction, and
  • 2.25”, 3.25”, and 5.25” retrofit sizes on existing drains where inserts are required

4.  Drain Connections

It is our preference to replace existing drains during roof repair or replacement with the same size outlet for connection to the existing rainwater leaders.  This is called a clamp connection and is done by a licensed plumber.

In certain cases, we do not have access to the underside of the roof drains or where the roof is being recovered. In these situations, we install a retrofit drain inside the original drain complete with a proper backflow seal. While this method solves the issue of drain connections, it reduces the flow of water.

For example; a 4” drain is reduced 18.75% to 3.25” inside diameter and a 3” drain is reduced 25% to 2.25” inside diameter. This will affect the recommendations we give for drains, especially if the drainage has been insufficient.

5 . Length of Drains

The length of the drain is always examined to suit the structure in question. Longer drains are used when the goal is to pass through a covered soffit area and avoid inside connections, while more direct paths of drainage are suited to a shorter length.


Peripheral vs. Interior Drainage

There are two ways that drainage systems can be installed in any structure. This depends on the way the roof is built, the weather surrounding it, and how the building is laid out.

Peripheral or Exterior Drainage

A peripheral drainage system is true to its name. Peripheral systems allow drainage from elevated interior roof areas to peripheral low points. Water is eliminated through scuppers and rainwater leaders that are located on parapet walls outside of the building. The issue with this kind of drain is that the scuppers and leaders are not protected from the elements unless your roofer/electrician has specifically added these protections.

Because of this, temperatures during the winter months may cause the drainage pipes to freeze and distort. Peripheral drainage systems are known to be more vulnerable to distortion and deterioration from freeze and thaw cycles, even with flashings and/or heat tracing installed to protect the egress of water.

Interior Drainage

An interior drainage system collects water from elevated peripheral points into an interior drainage system. The water flows into a rainwater leader, which moves the water through the interior of the building. The leader is connected to exit points [such as the stormwater connections for the building] which eliminate the water. A strong advantage is because the drain pipes are located on the building’s interior they are constantly heated and do not freeze. Depending on the drain and rainwater leaders used, there is condensation on the underside of the drain body and water leaders.

This issue can be resolved through the use of pipe wrap insulation. The disadvantage of this method is that any failure or problem with the roof connection at the drain or the rainwater leader can be a source of leakage inside the building. This system is the gold standard for buildings that have to weather the Canadian winter. With winter damage being one of the biggest concerns, a drainage system that works year-round is a significant advantage.

How To Spot A Problem

Most of our clients usually spot a problem when they have ponding water on their roof. However, the problem can be spotted earlier, and be repaired before any damage occurs to your roof. In order for drains to work properly, they need to be cleaned and maintained regularly because the openings in the baskets are small and narrow. Even if there is no ponding water, slow drainage can cause corrosion and vegetation to breed in and around your roof drain(s), and leaks are likely to spring from these areas.
Drainage system health is one of the key components of maintaining your roof. Regular maintenance is always recommended. This is to make sure that your drainage system is working as effectively as possible, and that no water damage has occurred during the change of seasons. If you’re noticing ponding water on your roof, or are looking for more information regarding positive drainage systems. Get in touch with us! We’re happy to help. That’s the Elite Advantage.