Expansion Joints In Brickwork Explained
Expansion joints in brickwork are deliberate gaps built into masonry to allow the wall to move without cracking. Bricks, blocks and mortar expand and contract with changes in temperature and moisture, and if that movement has nowhere to go, the wall pays the price. You get stepped cracks, bulging courses, spalled brick faces and, eventually, water ingress. Understanding where these joints go, how wide they should be and how to maintain them will save you thousands in repair bills over the life of a property.
This article explains what expansion joints do, why modern brickwork needs them, where they should be positioned, and how to spot problems before they become structural. Whether you are extending a house, inspecting a recent build or maintaining an older property, the principles here apply.
Why Brickwork Moves In The First Place
Clay bricks do something that surprises a lot of homeowners. After they leave the kiln, they slowly absorb moisture from the air and expand over months and years. This is called irreversible moisture expansion, and it is permanent. On top of that, all masonry expands when warm and contracts when cold. This is reversible thermal movement, and it happens every single day.
A long run of unrelieved brickwork can move by several millimetres over its length. That may not sound like much, but masonry is brittle. It has very little ability to stretch or compress. When the accumulated movement exceeds what the mortar can tolerate, something has to give, and cracks appear at the weakest points, usually around openings, corners and changes in wall height.
Concrete blockwork behaves differently. It tends to shrink as it cures and dries out, which is why the joint strategy for a blockwork inner leaf differs from the clay brick outer leaf. The two materials pull in opposite directions, which is one reason cavity walls need careful detailing.
What An Expansion Joint Actually Is
An expansion joint, sometimes called a movement joint, is a continuous vertical break through the full thickness of a masonry leaf. Instead of being filled with rigid mortar, the gap is filled with a compressible material and sealed to keep water out.
The joint lets the two sections of wall either side move independently. When the brickwork expands, the compressible filler squashes. When it contracts, the filler recovers. The sealant on the face flexes with it and keeps the weather out.
A typical joint is around 10mm to 16mm wide for clay brickwork. The exact figure depends on the spacing between joints and the anticipated movement, which a structural engineer or the brick manufacturer will calculate. Wider spacing between joints means each joint has to accommodate more movement, so it needs to be wider.
The Components Of A Properly Formed Joint
- Compressible filler. Usually a closed cell foam or cellular polyethylene backer that squashes as the brickwork expands.
- Backing rod. A foam rod pushed into the joint to control the depth of the sealant and give it the correct profile.
- Sealant. A flexible polyurethane or low modulus silicone sealant that bonds to both brick faces and moves with the wall.
- Wall ties or slip ties. Special ties that hold the leaf stable across the joint while still allowing horizontal movement.
Get any one of these wrong and the joint fails. A common example is filling the gap with ordinary mortar during pointing, which effectively cancels the joint and reintroduces the cracking problem.
Expansion Joints Versus Contraction And Control Joints
The terminology causes confusion, so it is worth being precise.
Clay brick expands, so it needs expansion joints that can close up. Concrete and calcium silicate materials shrink, so they need control joints, sometimes called contraction joints, that can open up. The two are not interchangeable, and using the wrong detail can make cracking worse rather than better.
In a standard UK cavity wall, the clay brick outer leaf and the concrete block inner leaf move differently, so their joints do not have to line up. The engineer designs each leaf on its own merits.
Where Expansion Joints In Brickwork Should Be Positioned
The guidance most UK designers work to comes from the British Standard for the use of masonry, BS 5628 and its successor Eurocode 6 (BS EN 1996), along with brick manufacturer recommendations. As a rule of thumb, clay brickwork needs a vertical expansion joint roughly every 12 metres, with the first joint no more than half that distance from a corner or return.
That last point catches people out. Corners are stiff and cannot flex, so movement concentrates near them. Placing the first joint too far from a corner is a classic cause of stepped cracking a metre or two in from the return.
Joints are also needed at these locations.
- Where a wall changes height or thickness.
- At junctions between old and new masonry, such as where an extension meets the existing house.
- Near large openings where the pier of brickwork is slender.
- Above and below rows of openings that weaken a wall.
- At intervals along long parapet walls and freestanding garden walls, which are exposed on both faces and move more.
Parapets and freestanding walls deserve special mention. They are wetted and heated on both sides, so they move far more than a wall that has a warm interior behind it. Movement joints on parapets are often required at closer centres, sometimes as little as 6 metres.
Expansion Joints And Building Regulations
Movement control falls under the structural requirements of the Building Regulations, specifically Approved Document A. The regulations require that a building be constructed so that combined dead, imposed and wind loads do not cause deflection or deformation that impairs stability. Uncontrolled masonry movement clearly falls foul of that.
For most extension and new build work, your structural engineer will specify joint positions on the drawings, and building control will expect to see them. On a new build covered by an NHBC warranty, the NHBC Standards set out clear expectations for movement joint provision and spacing, and an inspector will check the detailing on site.
Retrofitting a movement joint into an older solid wall that has never had one is possible but should always be designed by an engineer. Cutting a continuous vertical slot through a loadbearing wall without proper support is dangerous.
How Older Buildings Coped Without Them
Walk around any Victorian terrace and you will not find a single expansion joint, yet the walls have stood for well over a century. Two things explain this.
First, older bricks had already done most of their moisture expansion long before they were laid, because they were often stored for months. Second, and more importantly, they were laid in lime mortar rather than cement.
Lime mortar is soft and forgiving. It accommodates small movements by flexing and even self healing hairline cracks over time. Cement mortar is hard and rigid, so movement that lime would have absorbed instead cracks the bricks. This is one of the biggest reasons repointing an old building in cement causes damage. If you are weighing up mortar types, our guide to Lime Mortar vs Cement Mortar for UK Brickwork How to Choose the Right Mix and Avoid Damp Damage explains the trade offs in detail.
The lesson is simple. Modern hard mortars need engineered movement joints. Traditional lime mortars managed movement through the joints themselves.
Signs Your Brickwork Has A Movement Problem
Not every crack is a movement crack, but some patterns are telltale.
- Stepped cracking following the mortar joints in a diagonal staircase pattern, often near corners or openings.
- Vertical cracks that run straight through bricks rather than around them, common when thermal expansion has nowhere to go.
- Spalled or crushed brick faces where two sections have pushed against each other.
- Cracked or debonded sealant in an existing joint that has failed and let water in.
- Bulging or bowing in long parapet walls.
A hairline crack that opens and closes slightly with the seasons is usually movement related. A crack that grows steadily and never recovers may point to subsidence, which is a different and more serious problem requiring investigation.
If cracking has let water into the mortar and the joints are crumbling, deal with it promptly. Our article on How to Repair Crumbling Mortar Joints in Brickwork Before Damp Sets In walks through the process.
Cost Of Forming And Maintaining Expansion Joints
Costs vary by region and access, but the figures below give a realistic UK picture for 2026. Prices assume ground level or easily scaffolded access. Working at height on parapets adds scaffolding costs.
| Task | Typical UK cost | Notes |
|---|---|---|
| Forming a movement joint in new brickwork | £15 to £30 per linear metre | Marginal cost during a build, mostly labour and filler |
| Supplying and fitting compressible filler and backing rod | £8 to £15 per linear metre | Materials plus fitting |
| Applying flexible sealant to face of joint | £10 to £20 per linear metre | Low modulus polyurethane or silicone |
| Raking out and resealing a failed existing joint | £25 to £45 per linear metre | Includes removal of old sealant and cleaning |
| Retrofitting a movement joint into a solid wall | £150 to £400 per metre plus engineer fees | Requires temporary support and design |
| Structural engineer report and joint design | £400 to £900 | For diagnosis and specification |
Resealing an existing joint is a maintenance task most homeowners overlook. Sealant does not last forever. A quality polyurethane sealant lasts around 15 to 20 years before it hardens and cracks. Silicone can last a little longer. Budgeting to reseal once a generation keeps water out and protects the whole wall.
How A Movement Joint Is Formed On Site
For anyone overseeing a build, here is what good practice looks like.
- The bricklayer leaves a continuous vertical gap at the designed position, keeping it plumb through every course.
- Compressible filler is built into the gap as the wall rises, not stuffed in afterwards.
- Slip ties or debonded ties are used across the joint so the leaf stays stable but can still move horizontally.
- Any cavity trays and weep holes are kept clear so the joint does not become a water trap.
- Once the mortar has cured, a backing rod is inserted to the correct depth.
- The face is primed if the sealant manufacturer requires it, then sealant is gunned in and tooled to a neat finish.
The finished joint should be a clean, consistent vertical line. On fair faced brickwork it is often positioned to align with a downpipe or a natural break so it is less obvious.
Common Mistakes To Avoid
Movement joints go wrong more often than they should. These are the failures I see most often on site and during surveys.
Filling the joint with mortar
The single most common error. A bricklayer or a well meaning repointer fills the gap with rigid mortar during finishing, which locks the wall solid again. Every joint must stay free of any hard material. If you are repointing near a joint, mask it off. Our guide to Pointing and Repointing Explained for Brick Homes Signs Mortar Types and Best Practice covers how to protect joints during this work.
Spacing joints too far apart
Skipping a joint to save money or because the elevation looks better without it stores up cracking for later. The 12 metre rule for clay brick is a maximum, not a target, and corners need a joint much closer.
Using the wrong sealant
General purpose builders silicone from a DIY shed does not have the movement capability for a structural joint. You need a low modulus sealant rated for the anticipated joint movement, typically 25 percent or more.
Ignoring the backing rod
Without a backing rod, sealant is applied too thick or bonds on three sides, which stops it flexing and causes early failure. The rod controls both depth and shape.
Blocking cavity trays and weep holes
A joint that traps water at the base of a cavity funnels moisture into the wall. Weep holes must stay clear.
Retrofitting without design
Cutting a slot through an existing loadbearing wall to add a joint, with no propping and no engineer, risks collapse. This is never a job to improvise.
Using the wrong mortar for the brick
A mortar that is too strong for the brick transfers stress into the units instead of the joints. Matching mortar to brick strength matters. Our piece on the Best Mortar Mix For Brickwork explains how to get the ratio right.
Maintenance Checklist For Homeowners
Run through this once a year, ideally in spring after winter has done its worst.
- Walk the elevations and locate any existing movement joints. They look like continuous vertical sealed lines.
- Press the sealant gently. It should be firm but flexible, not hard and brittle.
- Look for cracks, gaps or sealant that has pulled away from one edge.
- Check that no one has filled a joint with mortar during previous repairs.
- Inspect parapet and freestanding wall joints, which weather fastest.
- Look for stepped or vertical cracking near corners and openings, which may signal a missing or failed joint.
- Confirm weep holes at the base of the wall are clear.
If the sealant has hardened or cracked, book a reseal before water gets in. It is a cheap job that prevents expensive damage.
When To Call A Structural Engineer
Reach for professional advice in these situations.
- Cracks wider than about 5mm, or cracks that are clearly growing over weeks and months.
- Any crack accompanied by doors and windows sticking or floors sloping, which can indicate subsidence.
- Long walls or parapets with no visible joints and signs of movement.
- Any proposal to retrofit a joint into an existing wall.
A short engineer inspection costing a few hundred pounds can tell you whether you are dealing with harmless thermal movement or something that needs structural repair. That is money well spent before you commit to costly works.
Frequently Asked Questions
Do all brick houses need expansion joints?
No. Older properties built in lime mortar generally manage movement through the mortar itself and rarely have or need formal joints. Modern buildings laid in hard cement mortar do need them, and the requirement grows with the length of the wall. As a guide, clay brickwork over about 12 metres long, and any long parapet or freestanding wall, should have engineered movement joints. Short walls between openings may not need one at all.
Can I fill an expansion joint with mortar to tidy it up?
Never. Filling the joint with rigid mortar defeats its entire purpose and will cause the brickwork to crack as it expands. If a joint looks untidy because the sealant has failed, the correct fix is to rake out the old sealant, insert a fresh backing rod and apply new flexible sealant. This keeps the joint working while giving a clean finish.
How often do expansion joint sealants need replacing?
A good quality low modulus polyurethane sealant typically lasts 15 to 20 years, and some silicones last a little longer. Exposure matters. South facing walls and parapets that take more sun and rain degrade faster. Inspect the sealant every year and reseal when it hardens, cracks or pulls away from the brick. Resealing costs roughly £25 to £45 per linear metre and is far cheaper than repairing water damaged masonry.
What is the difference between an expansion joint and a control joint?
An expansion joint is designed to close up, allowing for materials that expand, such as clay brick. A control or contraction joint is designed to open up, allowing for materials that shrink, such as concrete blockwork and calcium silicate. They are not interchangeable. Using the wrong type for the material can make cracking worse rather than preventing it, which is why the two leaves of a cavity wall are often detailed separately.
Will adding a movement joint to my house need planning permission?
Forming or maintaining a movement joint is repair and maintenance work, so it does not normally need planning permission. However, any structural alteration to a loadbearing wall falls under the Building Regulations, and retrofitting a joint into an existing wall should be designed by a structural engineer and may need building control involvement. If your property is listed or in a conservation area, check with your local authority before altering the appearance of the brickwork.
Final Thoughts
Expansion joints are one of those details that stay invisible until they are missing or neglected. Get them right and a wall moves quietly through its daily and seasonal cycles for decades. Get them wrong, or leave the sealant to perish, and you invite cracking, spalling and damp that costs far more to put right than the joint ever cost to build.
The practical takeaways are straightforward. Modern hard mortared brickwork needs engineered joints at the correct spacing, especially near corners and on parapets. Never fill a joint with mortar. Inspect the sealant every year and reseal it before it fails. And when cracking looks structural rather than cosmetic, get a qualified engineer to look before you spend on repairs that may not solve the underlying cause.
If you need a bricklayer, a repointing specialist or a structural engineer to inspect movement in your brickwork, you can post a job on BookaBuilderUK and get free quotes from vetted local tradespeople near you.
