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Published on Tue, 07/13/2021 - 10:03

How do you properly and accurately measure drain pipe size?

When specifying retrofit roof drains for your project, selecting the proper drain size is critical. Not only is this important for the roof’s overall performance, but it’s also important for the existing roof assembly and for keeping water out of the envelope.

Selecting the proper drain size requires an accurate measurement of the existing drain leader, which can be more complex than you might think. A common practice is using a tape measure to determine the inside diameter of the drain leader. For a more accurate measurement, use a Firm-Joint Inside Caliperimage of drain measuring with tape measure which can help to identify the right size drain for your project.

Even though a tape measure is by your side during each job and is great for most assessments, the bend in the tape can throw off your measurements enough to affect the project specifications. Instead, a firm-joint inside caliper, typically used to measure thickness, can also measure the distance between the two sides of a drainpipe and provide you that accurate inside diameter needed for precise drain measuring.

Note: OMG retrofit drains are sized based on ID drain leader measurement and have been undersized appropriately to fit from the factory. A 4” OMG retrofit drain is made to fit inside an existing 4” drain leader.

image of proper roof drain measuring

Drain Measuring with a Firm-Joint Inside Caliper

The following technique should be used to measure existing drains and ensure a correct fit on an OMG drain:

  1. Remove existing strainer dome to gain access to the drain leader. Check for any previous insert installations to measure the original plumbing system. Note: previous inserts must be removed before an OMG drain installation.
  2. Check for and remove obstructions inside the original drain, tailpiece, and elbow. Examples of obstructions include:
    1. Gravel
    2. Rust
    3. Bitumen
    4. Vegetation
    5. Caulking
    6. Glass
    7. Construction debris
  3. Insert caliper into drain leader and spread as wide as possible to measure the clean Inside Diameter (I.D.).image of how to measure a drain


  4. Remove caliper being careful not to close the legs. Once removed, measure the outside width of the caliper legs.image of caliper for drain measuring
  5. Record and double-check your measurement.
If you need additional support with drain measuring, contact us and we’ll be happy to help. Contact Us