Backer Rod: The £3 Foam Strip That Stops Sealant Cracking
UK guide to backer rod for window perimeters and movement joints: NHBC 6.7.3 rules, sizing, where to buy, and why your sealant cracks without it. From £2.39 per 10m roll.
The silicone around your new bifold doors looks perfect on handover day. Six months later, hairline cracks run the full length of the bead, water is wicking behind the frame, and the installer is back on site arguing that "thermal movement isn't a defect." It is a defect. The bead failed because nobody pushed a 13mm strip of foam into the gap before they sealed it. That foam costs less than a sandwich, takes ten minutes to install, and is the reason your installer's better-paid competitors don't get callbacks.
What it is and what it's for
Backer rod is a length of closed-cell polyethylene foam, shaped like a thin grey rope, that you push into a joint or gap before applying sealant. It's sold in coils of standard diameters (6mm to 50mm) at every common UK trade sealant supplier, and it does three jobs at once.
It controls the depth of the sealant bead, so the bead is the right thickness to flex without tearing. It prevents what the trade calls three-sided adhesion, where sealant bonds to all three faces of a joint (the two opposing surfaces and the bottom) and tears itself apart the first time the joint moves. And it fills the dead space at the back of a wide gap so you don't waste tubes of expensive sealant trying to fill it with silicone alone.
Most homeowners have never heard of backer rod. They've been told to seal a gap with silicone, they've gone to Screwfix or Toolstation or B&Q for the silicone, and none of those three carry backer rod on the shelf. Multiple UK forum threads document the same frustration: the product is mandated by warranty schemes for any joint over 6mm wide, but the mainstream DIY chains don't stock it. You buy it from specialist sealant suppliers (Sealants and Tools Direct, Sealant Supplies Ltd) or from Amazon, eBay, and online builders' merchants.
The product is governed by two UK technical standards. NHBC 6.7.3 requires "silicone sealants applied to a closed cell polyethylene foam backing strip" on window and door perimeter joints. BS 6213 covers selection of construction sealants generally and references backer rod use behind the bead. BS EN ISO 11600 classifies the sealant itself by movement capability and elasticity. None of those names appear on the product packaging at the merchant. They sit in the warranty inspector's clipboard, and that's where they bite if you skip the rod and the joint fails inside the warranty period.
The three-sided adhesion problem
The reason you need backer rod at all is geometry. A sealant bead is designed to stretch and compress as the joint widens and narrows with temperature. A typical low-modulus neutral-cure (LMN) silicone, which is what your window installer should be using on perimeter seals, stretches up to 25% before the bond fails.
For that flex to work, the sealant has to bond on two faces only, the opposing sides of the joint. If it also bonds to the back face (a third side), the bead is locked at the rear and can't stretch. The first hot day expands the masonry, the joint widens, and the bead tears down the middle.
A correctly tooled hourglass sealant bead can stretch by 50% before stress builds at the bond line.
The point of backer rod is to make the back of the joint slippery to the sealant so it cannot bond there.
Closed-cell polyethylene is chemically inert with the common sealant chemistries (silicone, polyurethane, polysulfide, MS polymer, hybrid). The sealant touches the rod but does not adhere to it. When the joint moves, the bead flexes between its two side bonds and the back face slides freely against the foam. That is the whole engineering trick.
Types: closed-cell, open-cell, and bicellular
There are three foam structures sold as backer rod. Most homeowners only need to know about the first.
| Type | Cell structure | Moisture | Oversize rule | Where it's used |
|---|---|---|---|---|
| Closed-cell polyethylene (PE) | Sealed cells, impermeable | Does not absorb water | 25% larger than gap | External window perimeters, wet locations, the UK default |
| Open-cell polyurethane (PU) | Porous cells, breathable | Absorbs water | 50% larger than gap (more compressible) | Internal movement joints only, where moisture-cure sealants need air access from behind |
| Bicellular (closed skin, open core) | Open core inside a closed skin | Skin is moisture-resistant | 25-30% larger than gap | Specialist use where outgassing under sun-warmed silicone is a known risk |
For UK domestic work the rule is simple: buy closed-cell polyethylene. The Sika Everbuild range (BACK10, BACK12, BACK15, BACK19, BACK25, BACK31) and the Sealoflex equivalent from Sealant Supplies Ltd are both closed-cell PE. The Cromar X1BACK range from independent merchants is the same product. CRL backer rod (sold through specialist glazing merchants) is a higher-grade closed-cell PE used on structural glazing where the sealant manufacturer's datasheet specifies it.
Open-cell PU (Soudal sells one) absorbs water like a sponge. On an external window perimeter that's a guarantee of frost damage and sealant failure within two winters. It exists for log-home chinking and certain specialist internal applications, neither of which apply to a UK extension build.
Bicellular rod was developed to solve a specific defect mechanism: closed-cell PE, when overcompressed or punctured, releases trapped air that bubbles up through the sealant on the next hot day. Bicellular has a closed outer skin (so it sheds water) but an open inner core (so trapped air vents safely). It costs more, and most homeowners will never need it. If a manufacturer datasheet specifies bicellular for the sealant you're buying, follow the datasheet.
How to size it: the 25% rule
Backer rod must be wider than the gap it goes into. The friction fit is what holds it in place while you load the sealant gun.
For closed-cell polyethylene, the rule is to buy a rod approximately 25% larger than the joint width. 25% This is the figure NHBC, Sika datasheets, and every credible installation guide give. Round up to the nearest standard size you can actually buy.
The standard UK retail sizes are 6mm, 10mm, 13mm, 15mm, 20mm, 25mm, 30mm, 40mm, and 50mm. Map your gap to a rod like this:
| Joint width | Rod to buy | Sealant depth (2:1 ratio) |
|---|---|---|
| 3-5mm | 6mm rod | No backer needed for sub-5mm; rod optional |
| 6-8mm | 10mm rod (BACK10) | 3-4mm sealant depth |
| 8-10mm | 13mm rod (BACK12) | 4-5mm sealant depth |
| 10-12mm | 15mm rod (BACK15) | 5-6mm sealant depth |
| 12-16mm | 20mm rod (BACK19) | 6-8mm sealant depth |
| 16-20mm | 25mm rod (BACK25) | 8-10mm sealant depth |
| 20-25mm | 30mm rod (BACK31) | 10-12mm sealant depth |
| Over 40mm | Joint cover or expansion profile, not backer rod | n/a |
The NHBC and LABC threshold for requiring backer rod is a joint width over 6mm. 6mm Below 6mm you can apply sealant directly. Above 6mm a backing rod is mandatory under those warranty schemes, and inspectors check.
For a sealant bead to behave correctly, the bead should be set to a 2:1 width-to-depth ratio. 2:1 A 12mm-wide joint needs a 6mm-deep sealant bead. The rod sits at the back of the joint, half its diameter compressed into the gap, leaving the right depth for tooling. The minimum sealant bead depth either standard accepts is 6mm. 6mm Below that the bead doesn't have the cross-section to flex and cracks within a season.
How to install it properly
The procedure is short. Doing it badly produces the same outcome as not using backer rod at all.
Clean the joint
The gap must be free of dust, debris, and old sealant before you start. A vacuum nozzle clears most of it. Methylated spirits on a clean cloth removes any remaining residue. Skip this and the sealant bonds to dust, not to the substrate, and peels off in a year.
Measure the gap
Run a measure or a steel rule across the joint at three or four points. Wide gaps are not uniform. Buy rod sized to the largest point you measure. The narrower sections will compress the rod a little tighter, which is fine; the wider sections need the rod to still friction-fit, which is the constraint.
Cut the rod to length
Use sharp scissors or a blunt utility knife. Don't stretch the rod when you measure it against the joint length. Stretching causes a stretched rod to recover (rebound) once installed, lifting it out of the gap behind your sealant bead.
Push it home with a blunt tool
A blunt putty knife, a wallpaper seam roller, or a dedicated jointing tool. Never a screwdriver, a Stanley knife edge, or scissor points. blunt only (putty knife or seam roller) The closed cells must stay sealed. Push the rod in to the depth you calculated for a 2:1 bead, half the joint width.
Confirm consistent depth
Run a small ruler or the depth gauge on a combination square along the joint to check the rod is at a uniform depth throughout. If it dips deeper in one spot, push that spot back out from behind (a thin needle works, as long as you don't puncture). If it sits proud, push it deeper.
Apply and tool the sealant
Load the cartridge into the sealant gun, cut the nozzle to a width slightly less than the joint, and lay a continuous bead. Tool to a hourglass profile with a wet finger or a Fugi tool. The wet tooling pushes the sealant against the side faces and produces the concave surface that lets the bead flex.
Tip
A wallpaper seam roller costs from £4 at any DIY store and is by far the easiest tool for backer rod insertion on long window perimeters. It rolls the rod into the joint at a consistent depth without risking a puncture, and it's quicker than a putty knife on runs over two metres.
Warning
A punctured backer rod is worse than no rod at all. Air trapped in the punctured cells escapes through the sealant on the first sun-warmed day and leaves crater bubbles in the cured bead. 25% If you've stabbed the rod with a screwdriver while pushing it in, cut that section out and replace it before you reach for the sealant gun.
Where you'd actually use it
The most frequent UK domestic uses are window and door perimeter sealing on extensions, expansion joints in render or masonry, and large gaps behind decorators' caulk at skirting and architrave junctions where the gap exceeds 5mm.
Window and door perimeters. This is the single biggest use case. New windows and bifold doors are fitted into structural openings with a deliberate clearance gap (typically 5-15mm) for tolerance. NHBC 6.7.3 requires the gap to be sealed externally with silicone over a closed-cell PE backer rod. On bespoke or non-standard window installs the gap can run wider; on one BuildHub thread an installer left a 30mm gap from double-counting tolerances, which is too wide for backer rod alone and needs frame packing first.
Expansion joints in masonry and render. Long runs of brickwork or rendered blockwork get vertical expansion joints every 6-8m to absorb thermal movement. The joint is sealed with movement-rated polysulfide or polyurethane sealant over a backer rod. Skip the rod on a movement joint and the sealant lasts a year before it tears.
Wide skirting and architrave gaps. Standard decorator's caulk sags and cracks on gaps wider than about 5mm. If you're caulking under a skirting board on an old, uneven floor where the gap runs to 8-10mm in places, push backer rod in first, then caulk over it. The rod stops the caulk slumping into the void and gives it the right depth to cure.
Patio and threshold details. External movement joints between paving slabs, between paving and a building, or at a door threshold all need backer rod under their sealant if the gap exceeds 6mm. UK paving sources confirm closed-cell PE as standard for these joints.
Bath rim re-seals (occasionally). If your bath has settled and pulled away from the wall by more than 5mm, you can push a small-diameter backer rod into the gap before applying a fresh silicone bead. The rod controls the bead depth, prevents three-sided adhesion to the wall and the bath rim, and stops the silicone slumping into the gap.
What it works with: sealant compatibility
Closed-cell PE backer rod is compatible with every common construction sealant chemistry: silicone, polyurethane, polysulfide, MS polymer, and hybrid. The sealant manufacturer's datasheet is the final authority. If the sheet specifies a particular rod type (CRL closed-cell, bicellular, or a primer), follow it.
For external window perimeters the sealant should be a low-modulus neutral-cure (LMN) silicone, typically Dow 796 or equivalent, which has a movement capability of around 25%. Standard general-purpose silicone fails on perimeter joints because it can't stretch enough.
Warning
Expanding foam is not a substitute for backer rod and sealant on an external joint. Expanding foam is insulation. It absorbs water, breaks down under UV, and on one documented build the expanding foam strips installed around roof windows in Oxfordshire deteriorated within two months and the leak recurred. The correct seal is foam insulation in the cavity, then a closed-cell PE backer rod nearer the outer face, then a tooled LMN silicone bead in front. Three layers, each doing one job.
Cost and where to buy
Backer rod is not stocked at Screwfix, Toolstation, or B&Q at the time of writing. You buy it from specialist sealant suppliers, online builders' merchants, or in bulk professional reels for whole-house jobs.
The headline cost spread for a 10m coil of any standard size in the Sika Everbuild range runs £2.39-£9.49. That's a small enough number that buying two or three sizes to cover variable joint widths on a single window install is sensible.
The most common DIY size is 10mm × 10m (Sika Everbuild BACK10), priced at £2.39-£4.20 across the specialist suppliers. If you're sealing the perimeter of one new window with a 6-8mm gap, this is the coil you buy.
For the slightly larger gaps that bifold doors, French doors, and bespoke window openings tend to throw up, the 13mm × 10m (BACK12) at £4.00-£4.50 and 15mm × 10m (BACK15) at £4.00-£4.50 are the next two sizes up. Buying both gives you flexibility on a job where you don't know the gap variance until you're on site.
For movement joints in masonry or render, or for the threshold detail under a sliding patio door, you'll want 20mm × 10m (BACK19) at £4.78-£5.11 or the 25mm size at £4.20-£9.49. The wider price spread on the 25mm size reflects significant variation between Sika Everbuild and the lower-cost Sealoflex brand.
For very wide structural movement joints the 30mm × 10m (BACK31) at £8.63 is the largest size most domestic builds will need.
If you're sealing the perimeters on a whole-house window install (8-12 windows plus doors) the maths flips. A bulk professional reel of 10mm × 1150m (Sikaseal 414) costs £65.99-£83.09, which works out at around 6-7p per metre versus 26p per metre on individual coils. Four-fold cost saving for the volume buyer.
Glazing-grade rod (CRL closed-cell) sits at a different price tier altogether. A 10mm × 30.5m roll costs £13.95 ex VAT, roughly five times the construction-grade equivalent. You only need this if a structural glazing manufacturer's datasheet specifies it.
The open-cell polyurethane variant from Soudal is priced as a 200-pack of 1m lengths at £66.88, which works out cheap per linear metre but only makes sense in volume and only for internal applications where moisture absorption isn't a problem.
External resource
Sealants and Tools Direct: Backer Rods
Full Sika Everbuild range stocked in 10m coils and bulk reels. Currently the most reliable UK retailer for individual sizes.
sealantsandtoolsdirect.co.uk
External resource
Sealant Supplies Ltd: Sealoflex Backer Rod
Lower-cost Sealoflex brand at uniform pricing across diameters. Alternative supplier when Sealants and Tools Direct is out of stock on a size.
sealantsupplies.co.uk
External resource
LABC Warranty: Gaps Around Windows, Sealants and Finishing Trims
The clearest UK technical statement of the 6mm threshold, the 2:1 width-to-depth rule, and the closed-cell PE requirement for external window perimeter joints.
labcwarranty.co.uk
Common mistakes
Skipping it on joints over 6mm. The most frequent defect in UK window-installer work. Sealant alone in a wide joint sags, gets the wrong depth ratio, and either three-sided-adheres to the masonry behind it or pulls a hollow as it cures. Either way it cracks within a season.
Stretching the rod when fitting it. A stretched rod recovers length over hours and pulls itself rearward into the joint, leaving a void behind your sealant bead. Cut the rod slightly longer than the joint, compress it gently into place, and let it relax.
Puncturing the closed cells. A screwdriver, a Stanley knife edge, or scissor points will all puncture a closed-cell rod. The trapped air outgases through the sealant on the next hot day. Use a blunt putty knife or a seam roller, every time.
Setting the rod too shallow. If the rod is sitting just below the surface, the sealant bead is too thin. It can't stretch and it cracks within months. The rod must sit deep enough that the sealant bead in front of it is at least 6mm deep at the centre.
Buying the wrong type for the job. Open-cell polyurethane on an external window perimeter is the textbook failure: rain hits the joint, the rod soaks up water, the water freezes, and the joint splits. For external use, every time, closed-cell PE.
Using backer rod where it isn't needed. On a sub-5mm joint the rod can compress so tight that it actually pushes back against the sealant during cure and produces a domed bead instead of a tooled hourglass. Below 5mm you don't need it.
Where you'll need this
Backer rod isn't currently linked to any tasks in the buildwiz tree, but the tasks where it belongs are clear:
- Windows and Doors - perimeter sealing of frame-to-masonry joints, the primary domestic use
- Bifold and Sliding Doors - large frame-perimeter joints and threshold details where movement-rated joints need backer rod with LMN silicone
Backer rod appears on any extension or renovation project where windows or doors meet masonry, where a render or blockwork run includes movement joints, or where a homeowner is dealing with old, uneven skirtings and gaps wider than caulk alone can handle. The product is small, the cost is trivial, and skipping it is the single most common cause of perimeter sealant failure in UK domestic work. Buy a 10mm and a 13mm coil before your installer arrives, hand them over if needed, and watch the bead survive its first summer.