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Correct Backer Rod Sizes

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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.

Backer rod should be installed at a consistent depth within the joint, and should ensure that the depth of the final sealant application is half the width of the joint. For more information on backer rod, see “Backer Rod Selection Guide” on the Tremco website

What is the Correct Backer Rod Size?

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 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.

A box of 3-inch closed-cell backer rod. Well, it used to be a box ...

A box of 3-inch closed-cell backer rod. Well, it used to be a boxThis backer rod was being installed within the expansion joint covers at the roof deck joints.

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 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 horizontal joints where water may pool, potentially penetrate the joint sealant, and be absorbed by the backer rod.

Notes on Our Sizing Chart

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.

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

Pictures of actual backer rod products can be seen on this Amazon search results page.*

You can also order backer rod products from the Home Depot.*

Table of Correct Backer Rod Sizes by Joint Width

Correct Backer Rod Sizing
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)