If you’ve noticed a vertical gap in a brick wall filled with something rubbery, or a line in a tiled floor that isn’t grout, you’ve seen a movement joint. They’re there on purpose. Without them, the building would crack in places nobody chose.
Movement joints only work if they can move. When the sealant in them goes hard, splits, comes away or gets filled with mortar, the joint stops doing its job and water gets in. This guide explains what movement and expansion joints are for, why they fail, how they should be sealed and how to look after them.
Movement joints and expansion joints: what’s the difference?
People use the terms loosely. Broadly, a movement joint is any deliberate gap designed to let parts of a building move independently. An expansion joint is a movement joint mainly there to take expansion, for example in long runs of brickwork or large floor areas. In everyday use they mean much the same thing: a gap, filled with a flexible material, that lets the building move without cracking.
Why buildings move
- Temperature. Materials expand when warm and contract when cold. A sunny wall moves more than a shaded one.
- Moisture. Many building materials swell a little when wet and shrink as they dry, and different materials do this by different amounts.
- Settlement. New buildings settle as they dry out and load is taken up.
- Different materials. Where brick meets render, a frame meets masonry, or a floor meets a wall, the two sides move by different amounts.
Movement joints are placed so this movement happens at the joint, not as a crack through the wall or floor.
Where you’ll find them
- Vertical joints in long runs of brickwork and blockwork.
- Joints in render and cladding, and between panels on façades.
- Around window and door frames, where frame meets wall.
- In concrete floors and slabs, in warehouses, car parks and commercial units.
- In large tiled floors, around the edges of tiled areas and over joints in the floor below.
- Where a floor meets a wall, a skirting or a threshold.
Why movement joints fail
- The wrong filler. Mortar, grout or rigid filler in a movement joint stops it moving. The wall or floor then cracks somewhere else.
- Three-sided bonding. If the sealant sticks to the back of the joint as well as both sides, it can’t stretch evenly and tears.
- Wrong depth. Sealant that is too thin tears; sealant that is too deep can’t flex properly and puts strain on the bond.
- Poor preparation. Dust, old sealant or damp surfaces stop the new sealant bonding.
- Age and weather. Sunlight, rain and frost harden and crack sealant over time.
- Traffic. In floors, wheels and feet wear out a sealant that wasn’t chosen for them.
How a movement joint should be sealed
- Cut out the old sealant and rake out any debris, mortar or vegetation.
- Clean the joint faces back to sound, dry material.
- Prime the faces where the sealant manufacturer calls for it.
- Fit backing rod. Closed-cell backing rod is pushed in to set the sealant depth and to stop the sealant bonding to the back of the joint.
- Apply a flexible sealant suited to the materials and the movement expected.
- Tool it so it presses firmly against both sides and has a neat, slightly concave finish.
The joint size is set by the sealant’s own data sheet. For example, Arbo’s technical data sheet for its PDC joint sealant gives a minimum depth of 6 mm on non-porous surfaces and 10 mm on porous ones, and refers to BS 8000 Part 16 for joint preparation, priming and sealant application. Other products set their own limits, which is why we choose the sealant for the joint rather than the other way round.
Need it done properly? Mr Mastic covers the South East, London and Scotland, with a one-year guarantee on every job. Call or email for a quick quote.
Comparison: what goes in which joint
| Joint | Typical sealant | What matters most |
|---|---|---|
| Brickwork movement joint | Low-modulus exterior sealant | Backing rod, correct depth, colour to suit brick |
| Render and cladding joints | Flexible exterior sealant | Bond to the render or panel edges; shedding water |
| Frame to masonry | Low-modulus silicone or hybrid; sand mastic on period windows | Suits both the frame and the wall |
| Concrete floor joint | Floor-joint sealant chosen for the traffic | Resistance to wheels and wear |
| Tiled floor movement joint | Flexible sealant in a colour to match the grout | Following the line of the joint below |
| Skirting to floor | Flexible sealant; decorators’ caulk only in dry rooms where it will be painted | Keeping mop water out of the gap in wet areas |
Tiled floors and movement joints
Large tiled floors, floors over underfloor heating and floors laid across joints in the base below all need movement joints. When they’re missing, grout cracks in a straight line and tiles can lift or crack. Where a floor needs one, grout along the right line can be cut out and replaced with flexible sealant in a matching colour. Around the edges of a tiled floor, the gap at the wall or skirting should also be flexible. Our floor joint sealing service covers skirting, tiled floor and concrete floor joints.
Signs a movement joint needs attention
- Sealant that has split down the middle or come away from one side.
- Hard, crazed sealant that no longer gives when pressed.
- Joints filled with mortar instead of sealant.
- Weeds, moss or debris growing in the joint.
- Cracks in the brickwork or render near the joint.
- Damp on the inside of the wall in line with the joint.
- In floors, crumbling joint edges or sealant worn away by traffic.
Maintenance and access
External movement joints are easy to overlook, especially high up. A look at them when gutters and windows are checked will catch most problems early. Ground-floor and low-level joints are straightforward to reseal. High-level joints need safe access, which should be planned before the work is priced.
Our external and façade joints service covers brickwork, render, cladding and façade joints. We cut out failed sealant, set the depth with backing rod and reseal with a flexible exterior sealant in a colour to suit the wall.
Scotland and the South East
The weather in different parts of the country is hard on external joints in different ways. The west of Scotland is wetter than the east, and exposed walls take a lot of wind-driven rain. Natural stone walls, such as sandstone and granite, need a sealant that bonds well to stone and won’t stain it. The South East is one of the drier parts of the UK, but sunny south- and west-facing walls still see a lot of temperature movement. In both, the principles are the same: the right sealant, the right depth, bonded on two sides only.
Quick answers
What is a movement joint?
A deliberate gap in brickwork, render, cladding, floors or tiling that lets the building expand and contract without cracking. It’s sealed with a flexible sealant, not mortar.
Can I fill a movement joint with mortar?
No. Mortar stops the joint moving, so the wall or floor cracks elsewhere.
What is backing rod for?
It sets the depth of the sealant and stops it bonding to the back of the joint, so the sealant can stretch between the two sides.
How often should movement joints be resealed?
When the sealant has split, hardened, come away or worn. Check them when you check gutters, windows and floors.
Is your movement joint work guaranteed?
Yes. Every Mr Mastic job comes with a one-year guarantee.
Related services
More advice
- Sealing Between Floors and Skirting Boards
- Scotland’s Weather and External Mastic
- Mastic on Granite and Sandstone Homes
- Mastic vs Silicone: What’s the Difference?
- 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.