Quick Reference: Backer Rod Sizing Table
The following table provides appropriate backer rod sizes for various building joint widths. This table draws on the published material data sheets from several manufacturers.
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. Typically, backer rod should be installed at a consistant 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.
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, and with joints 3/4 inch (19mm) or larger, the backer rod diameter should be around 25%-30% larger than the joint width.
Standard practice for open-cell backer rod is that the backer rod diameter should be 25% larger than the width of the joint.
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*.