How to properly measure roof drain parts

So, you need to do some work on a roof drain, maybe replace a broken dome or a busted clamp ring. Do you know how to measure those parts to make sure you can match up the parts you need? If you’re not sure, read on to learn what and where to measure drain parts to eliminate unnecessary, extra trips to the roof and to make identifying drain parts MUCH easier. Remember the #1 rule of the pros, “measure twice, then measure again”. Well, maybe that’s not quite it, but we can’t stress enough how important this point is when you’re working with drain parts.

Where to Measure Roof Drain Parts

The first thing I want to discuss are the pieces of the drain that should be measured when you need to replace broken or missing roof drain parts. There are two drain parts that should be measured if available.

  1. Clamp Ring – This ring clamps the drain body and the roofing material(s) securely together. The gravel stop/guard on the ring prevents roof surface aggregates from being washed into the drain while permitting total drainage through “V” notch weirs.
  2. Drain Dome, Basket or Strainer – Drain domes prevent leaves, garbage, animals & other debris from entering and/or blocking the drain.

The required measurements differ for each part, however it is important to get precise measurements and to measure exactly how the manufacturers list their sizes.

Clamping Rings – There are two measurements you will need to get for the clamp ring.

  • Outside Diameter: The most important measurement is from RIM to RIM. It’s pretty simple; just catch your tape measure on the rim, pull tight and measure 180° directly across to the outside edge of the opposite rim.

measure the clamp ring from rim to rim


  • Bolt Hole to Bolt Hole: Most clamp rings will have 3, 4, or 5 bolt holes. Ensure that your measurement(s) are from the center of one bolt hole to the center of the next bolt hole. For each size clamp ring you will need the following measurements:

  – 3 Bolt Hole Ring = 120º

measure drain clamp ring from bolt holt to bolt hole

  – 4 Bolt Hole Ring = 180º & 90º

measure 4 bolt clamp ring here

 – 5 Bolt Hole Ring = 72º
measure 5 bolt clamp ring here

Drain Dome, Basket or Strainer – There are also two measurements you will need to take for the dome.

  • Outside Diameter: Turn the dome upside down and catch the edge of your tape measure to the dome’s outside edge. Measure 180º across to the opposite outside edge. If there are locking tabs on your dome, do NOT include them in your measurement.
    measure roof drain domes from edge to edge
  • Height: From a flat surface, simply measure the base of the dome to the highest point. Do NOT include any standoff tabs on the bottom of the dome.
    measure roof drain domes to the highest point

What To Do Next

Once you get some or all of these dimensions, refer to our sizing charts to see if your dimensions match up with anything. There are a few other key drain part identifiers which we will discuss in future blog posts. If you’re not sure what you need, we’re always available to help you figure that out. Give us a call at 800-334-9815 and speak to a member of our team who can help answer any questions.

We hope you’ve found this post informative. For answers to even more questions, get our FREE eBook “Things You Need to Know About Roof Drains”.

Toni has been Protech's eCommerce Manager for over two years and writes our articles and posts to enable our customers to be more informed and educated on the products we sell.

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