Correct Backer Rod Sizes • Sizing Chart • All Joint Widths

By Jack Gray, Roof Online Editor • Last updated October 8, 2022

Table of Contents

Backer rod sizes: A box of 3-inch closed-cell backer rod lying on an EPDM roof.
A box of 3-inch closed-cell backer rod. Well, it used to be a box…this large-diameter backer rod was being installed under the expansion joint covers at the expansion joints between roof sections..

What is Backer Rod?

A backer rod is a cylindrical piece of foam material installed in a gap or joint between building materials to provide a substrate for building sealant.

The backer rod ensures that the sealant maintains a proper “hour glass” shape after application, and prevents three-sided adhesion, which is important for the sealant’s ability to maintain its bond while remaining flexible.

For more information on backer rod, see “Backer Rod Selection Guide” on the Tremco website.

Installing Backer Rod

Backer rod should be installed at a consistent depth along the length of the joint, and the depth at which the backer rod is installed should ensure that the depth of the final sealant application is half the width of the joint.

Ideally, you want to use one long piece of backer rod which will fill the entire joint by itself.

You don’t want to use smaller pieces of backer rod which may lead to gaps where the sealant isn’t supported or the sealant can ooze through and harden into a distorted shape.

To ensure that the backer rod is installed consistently at the proper depth, you may want to consider getting a good backer rod insertion tool. An insertion tool can also save you a whole lot of time if you have a lot of backer rod to install.

How to Size Backer Rod Correctly

Proper backer rod sizes are determined by the width of the joint and whether the backer rod is closed-cell or open-cell.

The backer rod sizes chart below gives the appropriate backer rod sizes for various building joint widths. This table draws on the published material data sheets from several manufacturers.

Closed-Cell Backer Rod Sizing

Standard practice for closed-cell backer rod is that with joints less than 3/4 inch (19mm) wide, the backer rod diameter should be 1/8 inch (3mm) larger than the joint width.

With joints 3/4 inch (19mm) or larger, the backer rod diameter should be around 25%-30% larger than the joint width.

You don’t have to be excessively precise with this. Backer rod is a relatively high tolerance material.

Open-Cell Backer Rod Sizing

Standard practice for open-cell backer rod is that the backer rod diameter should be 25% larger than the width of the joint.

Open-cell backer rod should not normally be used in exterior horizontal joints where water may pool, potentially penetrate the joint sealant, and be absorbed by the backer rod.

Larger Joints

Although backer rod is manufactured in diameters greater than two inches, sealant and backer rod is not the proper method for sealing building joints that are greater than two inches wide.

These larger-diameter backer rods have other uses, of course, such as blocking air and rain infiltration around poorly-fitting HVAC curb adapters and skylights, or stabilizing the loose membrane material at a low-profile expansion joint in a single-ply membrane roof.

Building joints larger than two inches should be sealed with specially-designed joint covers or expansion joint materials.


1. Some of the backer rod sizes recommended in the table may not be available as an actual product. If this is the case, the closest available size should work.

2. There is a relatively large amount of leeway with backer rod sizes, but it’s a good idea to try a small test piece first.

3. Don’t worry too much about backer rod quality or which manufacturer makes the best backer rod. Once your sealant has been installed and finishes curing, the backer rod has done its job. Long term backer rod performance isn’t really an issue.

4. You can find pretty much any size backer rod that you might need if you look through the backer rod products available on Amazon.

Table: Correct Backer Rod Sizes

Correct Backer Rod Sizes
Joint Width
in Inches and (Millimeters)
Closed-Cell Backer Rod
Recommended Diameter
in Inches and (Millimeters)
Open-Cell Backer Rod
Recommended Diameter
in Inches and (Millimeters)
3/16 inch (5 mm) 1/4 inch (8 mm) 1/4 inch (6 mm)
1/4 (6) 3/8 (9) 3/8 (8)
3/8 (10) 1/2 (13) 1/2 (13)
1/2 (13) 5/8 (16) 5/8 (16)
9/16 (14) 3/4 (17) 3/4 (17)
5/8 (16) 3/4 (19) 3/4 (20)
11/16 (17) 7/8 (20) 7/8 (21)
3/4 (19) 1 (25) 1 (24)
7/8 (22) 1-1/8 (29) 1-1/8 (28)
1 inch (25 mm) 1-3/8 inch (33 mm) 1-1/4 inch (31 mm)
1-1/8 (29) 1-1/2 (39) 1-3/8 (36)
1-1/4 (32) 1-5/8 (43) 1-5/8 (40)
1-3/8 (35) 1-7/8 (47) 1-3/4 (44)
1-1/2 (38) 2 (51) 1-7/8 (48)
1-5/8 (41) 2-1/8 (55) 2 (51)
1-3/4 (44) 2-3/8 (59) 2-1/4 (55)
1-7/8 (48) 2-1/2 (64) 2-3/8 (60)
2 inch (51 mm) 2-5/8 inch (68 mm) 2-1/2 inch (64 mm)
2-1/4 (57) 2-7/8 (74) 2-3/4 (71)
2-1/2 (64) 3-1/4 (83) 3-1/8 (80)
2-3/4 (70) 3-5/8 (91) 3-3/8 (87)
3 (76) 3-7/8 (98) 3-3/4 (95)
3-1/2 (89) 4-1/2 (115) 4-3/8 (111)
4 (102) 5-1/8 (132) 5 (128)

About the Author

Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.

Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.

He was awarded the Registered Roof Observer (RRO) professional credential in 2009.

He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University. Read full bio.

  1. Expansion Joints: Reference Page
  2. Thermal Expansion of Building Materials