A complete guide to using backer rods
Backer rods may only be small, but they play a critical role in many construction projects. Their use in ensuring effective sealant performance and joint longevity is second to none. When used correctly, they help improve the durability of sealed joints, reduce material waste and support a professional, long-lasting finish.
In our guide, we’ll provide a comprehensive overview of everything you need to know about backer rods, including what they are, when and how to use them, why they’re important, and how to select the right type for your application.
What is a backer rod?
A backer rod is a piece of flexible, compressible foam that’s placed into joints or gaps before sealant is applied. This is to control the depth of the sealant, provide backing, allow the sealant to be tooled, and prevent three-sided adhesion.
Usually they’re installed into joints between substrates like concrete, masonry, metal, timber and cladding systems. You’ll often find them used in expansion joints, movement joints, façade joints and window & door frames. It works by occupying space within the joint and ensuring that sealant is applied to the correct thickness and shape, allowing it to accommodate movement.
Types of backer rod
There are two types of backer rod, both of which are usually made from non-absorbent polyethylene (PE) foam. For optimal sealant performance, it’s important that the correct one is used.
Closed cell
Closed-cell backer rods are made from non-absorbent polyethylene foam with a smooth outer skin. As the name suggests, the individual cells within the foam are sealed, preventing the absorption of water or air.
They’re the most commonly used type in construction and are suitable for most internal and external applications. Key characteristics include:
- Resistance to moisture absorption
- Good compressibility and recovery
- Compatibility with most sealant types
- Clean, smooth surface that allows easy insertion
Since they don’t absorb water, closed-cell backer rods are suited to external joints, façade applications and areas that are exposed to weathering (e.g. roof parapets, exterior window sills).
Open cell
Open-cell backer rods are made of polyurethane foam and have interconnected cells that allow air and moisture to pass through. They are softer and more flexible than closed-cell rods.
Typical applications include areas with irregular joint depths (e.g., internal control joints in concrete walls) or where gas permeability is required (such as drywall or partition joints). Common characteristics of open cell backer rods include:
- High flexibility and compressibility
- Air and moisture permeability
- Reduced risk of outgassing compared to closed-cell rods
- Good performance in varying joint widths
They’re often best for internal joints or where there’s a risk of sealant bubbling because of trapped air. However, because they can absorb moisture, they’re not recommended for external use or in permanently wet environments.
When to use a backer rod
Backer rods should be used whenever a joint is too deep to be filled effectively with sealant alone, or where joint movement needs to be accommodated without overstressing the sealant.
Typical situations where backer rods are required include:
- Expansion and movement joints in concrete or masonry
- Perimeter sealing around windows and doors
- Cladding and façade joints
- Precast concrete panel joints
- Floor and wall junctions
- Joints in civil engineering and infrastructure projects
They are important where joint depth exceeds the recommended sealant depth-to-width ratio. Without a backer rod, sealant could be applied too thickly, leading to increased stress, reduced elasticity, and eventually cracking or failure to adhere.
Backer rods are also used to prevent sealant from adhering to the base of the joint, ensuring that adhesion occurs only on the two opposing sides, allowing the sealant to stretch and compress as the joint moves.
How to use a backer rod
Correct installation of a backer rod is just as important as selecting the right product. In order to achieve top performance from your sealant, using a backer rod properly is essential. Here’s how to use backer rods:
- Prepare the joint
Ensure the joint is clean, dry and free from dust, debris, laitance and loose material. Any contaminants could impact adhesion and performance.
- Select the correct size
Choose a backer rod diameter that is slightly larger than the joint width so it can be compressed into place and remain secure.
- Insert the backer rod
Press the backer rod evenly into the joint using a blunt tool or roller. Avoid stretching, twisting or puncturing the rod during installation.
- Set the correct depth
Position the backer rod at the required depth to achieve the correct sealant profile, typically a concave finish with the correct depth-to-width ratio.
- Apply sealant
Apply the sealant over the backer rod and tool it to ensure full contact with the joint sides.
What size backer rod to use?
When you’re choosing a backer rod, it’s vital that it’s the right size so that it stays in place and works effectively. The diameter of the rod you select should be about 25-30% larger than the joint width. For example, if your joint width is 10mm, then a 12-13mm diameter backer rod would be required.
But why? Well, this oversizing means the rod can be compressed against the joint walls, preventing it from slipping. Using a rod that’s too small can cause movement during application, whilst one that is too large can be hard to install and may cause joint damage.
It’s important to note that you should always refer to the manufacturer's guidelines for the sealant you’re using because the recommended joint dimensions and sealant profiles can differ depending on the product.
Why are backer rods important?
Backer rods play a crucial role in joint design and sealant performance. While they may appear to be minor accessories, their impact on durability, efficiency, and overall quality is significant. Let’s look in more detail at why they’re such a beneficial tool.
Improved sealant performance
By controlling sealant depth and shape, backer rods help ensure the sealant performs as intended. Correct joint geometry allows the sealant to accommodate movement without excessive stress, reducing the risk of cracking or debonding.
Prevention of three-sided adhesion
One of the most important functions of a backer rod is to prevent three-sided adhesion. Sealant should only bond to the two sides of a joint. Adhesion to the base restricts movement and leads to premature failure.
Consistent, professional finish
Using a backer rod provides a firm backing for tooling the sealant, resulting in a cleaner, more consistent finish. This is great for use in visible joints and in architectural applications.
Compliance with standards and best practice
Some industry standards and manufacturer specifications require the use of backer rods in movement joints. Their use supports compliance with BS standards and best practice guidance for joint sealing.
Choosing the right backer rod
Yes, matching the backer rod with the joint width is crucial, but it’s not the only consideration. There are several factors to consider when you want to ensure compatibility and long-term performance. Here are a few other areas to think about when you’re choosing the backer rod for your project.
Joint environment
Consider whether the joint is internal or external, exposed or protected, and whether it will be exposed to moisture, temperature changes or chemical exposure. As we mentioned when we looked at the types of backer rods, closed-cell rods are generally preferred for external use, while open-cell rods are often best suited to controlled internal conditions.
Sealant compatibility
Always check that the backer rod is compatible with the chosen sealant. Some sealants might react with certain foam types (e.g., solvent-based polyurethane sealants), while others (e.g., silicone sealants) could require gas-permeable backing to prevent outgassing.
Joint movement requirements
Joints designed to accommodate significant movement require precise joint geometry. Choosing the correct backer rod size and level of compressibility helps ensure the sealant performs under changing conditions.
Manufacturer guidance and specifications
Make sure to read and follow recommendations provided by sealant and system manufacturers for the products you’re using, so that you can get specific guidance that will help you to complete your project effectively.
Choose the right backer rod for your project
With the right product in place, you can be confident your joints are correctly formed, movement is properly accommodated, and long-term performance is maximised. Whether you’re working on façade joints, window perimeters or movement joints in concrete, using a high-quality, non-absorbent backer rod helps ensure a professional, durable finish that meets best practice.
At Complete Construction Products, we supply non-absorbent polyethylene foam backer rods in a wide range of sizes, making it easy to select the correct diameter for your joining width and application. Our backer rods form part of our widerPolyethylene Joint Fillerrange, designed to support effective joint sealing across commercial, industrial and residential projects.
If you need technical advice, help choosing the right size, or support with a specific application, please don’t hesitate to contact us today!