Why Expansion Joints Fail and How to Fix Them

By Mastic Doctor · Updated 3 November 2026

Walk round almost any commercial building and you’ll find a failed movement joint somewhere. A brickwork joint with the sealant hanging out in strips. A car park deck joint packed with grit. A warehouse floor joint with chipped edges either side. Each one started as a small defect and grew.

This article explains what movement and expansion joints are for, the usual reasons the sealant in them fails, how to read the signs, and what a proper repair involves. It is aimed at facilities managers, estates teams and contractors who have to decide whether to patch, reseal or call in someone to look properly.

What movement joints are for

Buildings are never truly still. Materials expand in summer heat and contract in winter. Concrete shrinks as it cures. Brick and block take on and lose moisture. Floors deflect under load. Without somewhere to take up that movement, the structure cracks in random places.

Movement joints give it a planned place to go. You’ll find them in:

brickwork and blockwork walls
precast concrete and cladding panels
concrete slabs and screeds
car park decks, ramps and podiums
warehouse and retail floors

The sealant in the joint has one job: stay bonded to both sides while the gap opens and closes, and keep water, grit and debris out.

The three ways sealant fails

Look closely at a failed joint and it usually tells you what went wrong.

Failure type	What you see	Usual causes
Adhesive	Sealant has pulled cleanly away from one face	Poor preparation, no primer where needed, dust or contamination, three-sided adhesion
Cohesive	Sealant has split down the middle	Joint over-stretched, bead too thin, sealant movement class too low for the joint
Substrate	The edge of the brick, concrete or panel has broken away with sealant still stuck to it	Weak or friable substrate, or a sealant too stiff for the movement

On top of these, age and exposure take their toll. UV and weather cause chalking and hardening. A hardened bead can no longer stretch, so the next cold snap tears it open. Our guide to sealant failure types goes into more detail.

Design faults that set joints up to fail

Many failures trace back to the original joint rather than the sealant itself.

Joint too narrow for the movement expected, so even a good sealant is stretched beyond its limit.
No backing rod, so sealant fills the full depth and bonds to the back of the joint as well as the sides. It can’t stretch as designed and tears.
Bead too thin or too deep for the joint width, putting the wrong stresses on it.
Wrong product, such as a stiff sealant in a joint that moves a lot, or an internal caulk used outside.

If the joint geometry is wrong, resealing with the same layout will fail again. Our guide to joint width, depth and backing covers how width, depth and backing work together.

What happens if you leave it

A failed joint rarely stays a cosmetic problem. Once the seal opens:

rain gets into walls and cladding, leading to damp, stained finishes and internal leaks
grit and stones pack into the gap, so the joint can no longer close and edges crack
in floors, the joint arrises (the edges) spall under wheels and become a trip hazard
in car parks, water carrying de-icing salts reaches the deck and the steel inside it

The longer it runs, the more likely the repair moves from resealing to repairing the edges first, which is a bigger and more disruptive job.

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.

Car parks, ramps and trafficked joints

Joints in car park decks and ramps take a beating. Tyre loads roll over them all day. Winter brings de-icing salts and standing water. Turning vehicles scuff and shear the bead.

Sealants here need to handle traffic as well as movement, which is why polyurethane and other trafficable products are common. Joints also need to be clean and dry when sealed, which means planning around closures and weather. Phased work can be planned so parts of the deck stay in use. There is more on this in our guide to car park joint sealing.

Warehouse and industrial floor joints

In warehouses, the main damage comes from forklift and pallet-truck wheels. Saw-cut induced joints and construction joints are only a few millimetres wide, but an unsupported edge breaks up quickly under hard wheels.

A semi-rigid filler is common in trafficked industrial floor joints, because it supports the edges. Where movement is larger, a flexible sealant may be needed instead. The point is to use the right filler or sealant for the traffic and the movement, not one product for every joint. If edges are already broken, we’ll tell you if they need repairing first. See floor joint sealing and our guide to warehouse floor joints.

Questions to ask before resealing

Before ordering a straight like-for-like reseal, a few questions help decide the right scope:

Has this joint failed before, and how quickly?
Is the failure adhesive, cohesive or substrate?
Are the joint edges sound enough to seal against?
Is the existing sealant the right type for the movement and exposure?
Is there water damage behind the joint that needs dealing with?

The answers decide whether the job is a simple reseal or something more.

How a lasting repair is done

Good joint repair follows the same order every time:

Assess. Check the failure type, joint width and depth, substrate condition and likely movement. In older buildings, check the asbestos register before disturbing old mastic.
Cut out. Remove all the old sealant and backing. New sealant won’t bond reliably to old.
Prepare. Clean the faces back to sound, dry material with no dust or grease left on them.
Prime where the sealant manufacturer specifies it for that substrate.
Back. Insert closed-cell backing rod at the right depth, or bond-breaker tape in shallow joints.
Seal and tool. Apply a sealant with the right movement class and tool it onto both faces.

Skipping any of these is the most common reason a reseal fails within a short time.

Planning the work

For an FM or contractor, a few things make the job run smoothly. Have drawings or photos of the joints. Know the total length roughly. Flag any areas that must stay open. Tell the contractor your site hours and access windows so the work can be planned around them.

Mastic Doctor carries out expansion joint sealing on brickwork, cladding, concrete, car park and floor joints for commercial clients, with a 12-month guarantee on our workmanship. Send photos and a rough length through our quote form or contact us for a fixed quote.

Quick answers

Can you seal over an old movement joint?

No. New sealant does not bond reliably to old sealant. The old bead and backing come out, and the joint is cleaned back to sound, dry faces first.

Why has a new joint failed already?

Usually poor preparation, a missing primer, no backing rod causing three-sided adhesion, or a sealant with too low a movement class. The failure pattern points to the cause.

What sealant goes in a car park joint?

A trafficable product suited to the movement, often polyurethane. The joint has to be clean and dry, so closures and weather need planning.

Do broken floor joint edges need fixing first?

Often, yes. Sealant or filler needs firm edges to bond to and to support wheel loads. We’ll tell you if the edges need repairing before sealing.

How can we tell a movement joint from a crack?

Movement joints are straight, deliberate and usually at regular spacings, often filled with sealant. Cracks wander. If cracks appear near a joint, the joint may have locked up and stopped doing its job.

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More 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.