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What Size should the Backer Rods be for a Project?

April 27, 2021

Backer rods are found in several building joints, and because the latter can vary in width, it is important to carefully select the appropriate size of backer rod for its destination. Alcot Plastics Ltd. offers this product in a range of sizes, to ensure that we can provide the needed material to support the construction goals of our clients.

The following discusses backer rods and details surrounding their sizing:

Foam material in the shape of a cylinder is the appearance of a backer rod, and they are placed in a joint or gap between the sections of a building. These items serve as a substrate for building sealant. The aim is for backer rods to be situated at a constant depth within the gap, and the depth of the concluding sealant application should be half of the joint’s width.

Closed-cell backer rods have a standard practice attached to their use, which is broken down below.


  • For gaps less than 3/4 of an inch wide, the backer rod’s diameter would be 1/8 of an inch larger than the width of the gap.
  • For gaps 3/4 of an inch or greater, the backer rod’s diameter would be approximately 25 - 30% larger than the width of the gap.

A standard practice has also been established for open-cell backer rods. The guideline is that the diameter is ideally 25% larger than the joint’s width. It is important to note that this type of material should not be paired with horizontal joints, due to the possibility of water pooling in these areas. This can lead to water seeping through the joint sealant, which might result in the backer rod absorbing the liquid.


Exact precision is not required when selecting the size of backer rods for any project at hand. Using the closest available sizes is a good alternative if the specifications being sought out are not an option. Contact Alcot Plastics Ltd. today to discuss and acquire backer rods with the right dimensions for your building plans.