How To Measure For Flat Roof Drain Replacement Parts

Over the life of your roof, your drains will be subjected to a lot of abuse—rain, sleet, snow, construction, excessive heat and cold, debris, etc.

measure-a-roof-drainThis can wear down, or even break your drain, causing leaks and water damage. This is especially an issue for large flat roof drains on commercial and residential buildings, where water has nowhere to go but inside the building.

Because of this, you need to regularly inspect your roof drains. Make sure to look for and replace any cracked, corroded or rusted drain parts.

Identify Your Drain’s Manufacturer and Size

To replace drain parts, you’ll need to identify the manufacturer to make sure the parts will match your drain system.

Most manufacturer drains come in distinct colors (e.g. Zurn - Blue; Wade - Red, Josam - Green), which can help you identify a specific drain needed. But what about in situations where you can’t tell the color, or the manufacturer’s drains don’t have a defined color?

In these situations, you’ll need to measure the drain ring.

The dimensions of the drain ring will help to identify a drain manufacturer and the size of the drain (e.g. 15-inch, 12-inch, 8-inch, etc.), at which point, buying the correct replacement dome, ring or bowl is easy.

How to Measure a Roof Drain Ring

When measuring a roof drain ring, there are three different options, two of which are useful, the third not so much.

Use Outside Diameter (OD)


Outside drain ring diameter is one of the most common, and accurate, measurements used to help find a replacement roof drain or part.

To measure the outside diameter, align the edge of your tape measure with the outside edge of the ring, and then measure across 180° to the opposite edge.

You may also consider flipping the drain ring over to avoid having to measure over the gravel stop.

Or Bolt-to-Bolt (3, 4 and 5 bolts)


The distance between the bolt slots on a roof drain ring is a common measurement used to help identify a proper replacement roof drain.

When measuring a roof drain ring’s bolts, your measurement should always be taken from the middle of the bolt slot (as pictured right) to the middle of the next or opposite bolt slot; never from the edges.

How the measurement is taken is contingent on the number of bolt slots:

  • 4-Bolt Drain Ring: Measure from one bolt slot to the bolt slot 180° directly across the ring.
  • 3- or 5-Bolt Drain Ring: Measure the distance to the middle of the next bolt slot going around the ring. Do not measure across the ring.

But Do not use: Inside Diameter (ID)

The third and final way is inside diameter, but this method should be avoided. Inside diameter is the distance across the ring between the gravel guard ridges.

Unfortunately, this measurement isn’t always consistent, and can lead to accidentally ordering the wrong drain or part.

Once you’ve got your measurements, you can easily order the part(s) you need online by using If you don’t find a match, or can’t verify the manufacturer, contact us and speak to our roof drain expert, who can get you what you need.

Have a Drain Question?

Have a question or in need of a specific roofing product or drain part? We have the resources to help you.

Reach out to the Drain Wizard.

Ask the Drain Wizard

Related Posts

How To Purchase A Replacement Flat Roof Drain
Aug 15, 2022
How To Purchase A Replacement Flat Roof Drain
Flat roof drains are often subjected to heavy weathering and regular wear and tear. As a result, their parts—including domes, rings, sump pans, and clamps—sometimes break, crack or rust over time,...
Pros and Cons of 3 Flat-Roofing Systems
Aug 15, 2022
Pros and Cons of 3 Flat-Roofing Systems
There are 3 main types of flat-roofing systems: built-up roof (BUR), modified bitumen and single-ply. Each system is designed to waterproof the roof and protect it from sun, rain, snow and more. This...
What Is A Retrofit Drain And How To Install One
Aug 15, 2022
What Is A Retrofit Drain And How To Install One
When reroofing or removing an existing drain, a replacement drain needed. This is where retrofit drains are used, as they replace the broken or corroded roof drains easily with almost no...
Background Pattern_Hy-Tech Logo_Gray

Find more resources

Whether you’re in search of roof drain specifications, product catalog, or anything in between, we have the resources to answer your questions.