Look inside a failed movement joint and you’ll often find the same thing: a solid plug of sealant, stuck to every surface it touched. No backing rod. The joint looked fine on handover and split within a season or two. Backing rod is a small detail, but it controls how a sealant joint behaves. Here is what it does, how it is sized and when bond-breaker tape is used instead.
What backing rod is
Backing rod is a round, flexible foam cord pushed into a joint before the sealant goes in. On most building joints it is closed-cell polyethylene. It comes in a range of diameters so it can be matched to the joint. Once it is in place, the sealant is applied over it and tooled.
The three jobs it does
- Controls depth. The rod sets how deep the sealant can go. Many sealant makers recommend a width-to-depth ratio of often around 2:1 for wider joints, as the manufacturer advises. Without a rod, the sealant fills the whole cavity and ends up far too deep.
- Stops three-sided adhesion. Sealant does not stick well to polyethylene foam, so it bonds only to the two sides of the joint. That lets it stretch and compress as the joint moves.
- Gives something to tool against. When the applicator presses the sealant into the joint, the rod provides a firm base, so the sealant is pushed onto the sides and takes a neat concave shape.
What goes wrong without it
A sealant bead bonded to the back of the joint as well as both sides can’t stretch properly. When the joint opens, the strain concentrates in one spot and the sealant tears, either through the middle or off one face. A bead that is too deep is also stiffer and takes longer to cure. The result is early cohesive or adhesive failure. Our guide to sealant failure types shows what each looks like.
Need the work done? Mastic Doctor covers South East England, London and all of Scotland, with a 12-month guarantee on our workmanship. Send details for a fixed quote.
Sizing and fitting
The rod should be a little larger than the joint width, so it sits firmly by compression and stays put. It is pushed in to a consistent depth, not stretched or twisted, and should not be punctured. A damaged closed-cell rod can release gas into fresh sealant and cause bubbles in the surface. Where a joint changes width along its length, more than one rod size may be needed. Full detail on proportions is in our joint design guide.
Bond-breaker tape for shallow joints
Some joints are too shallow to take a rod. A joint against a solid back, such as a frame sitting tight to a reveal, may only be a few millimetres deep. Here a strip of bond-breaker tape is laid in the base of the joint. It does the same job of stopping the sealant sticking to the back, so it bonds to two sides only.
Checking it on your site
For contractors and clerks of works, backing rod is worth checking before sealing starts, not after. Ask to see a short test length, or check a joint before it is filled. On resealing work, rod is normally replaced along with the old sealant. Our movement joint sealing work includes new rod as standard where the joint calls for it.
Quick answers
Is backing rod always needed?
Not always. It is used where the joint has depth behind it. In shallow joints, bond-breaker tape does the same job. In some fire-rated details, a different backing such as mineral wool is specified instead, and the tested detail must be followed.
Can old backing rod be left in when resealing?
It is usually better to replace it. Old rod may be damaged, compressed, wet or contaminated with sealant, and it may sit at the wrong depth for the new bead.
What size backing rod should be used?
Slightly larger than the joint width, so it grips by compression. The sealant manufacturer’s guidance and the project specification take priority.
Does backing rod affect how much sealant is used?
Yes. By setting the depth, rod stops the sealant filling the whole cavity. That saves material and gives a bead of the right proportions for the movement.
Related services
- Expansion and movement joint sealing
- Construction and new-build mastic
- Facade and window perimeter sealing
More advice
- Sealant Joint Design: Width, Depth and Backing
- Adhesive, Cohesive and Substrate Sealant Failure
- Why Expansion Joints Fail and How to Fix Them
- Sealing Precast Concrete and Cladding Joints
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.