When a sealant joint fails, the instinct is to cut it out and redo it the same way. That often repeats the problem. The way a joint has failed points to the cause, and the cause decides the fix. This guide explains the main failure types in plain terms, so FMs, clerks of works and site managers can describe what they’re seeing and ask the right questions.
Adhesive failure: peeled off one face
The sealant has come away cleanly from one side of the joint, leaving that face bare. The bead itself is often still intact. Common causes:
- poor preparation, with dust, old sealant film, grease or moisture left on the face
- no primer where the manufacturer specifies one for that substrate
- contamination, such as release agents or cleaning residue
- three-sided adhesion, where the sealant also bonded to the back of the joint
The fix is proper preparation, the right primer and correct backing. See backing rod explained.
Cohesive failure: split through the middle
The sealant has stayed bonded to both faces but torn through its own body, often along the centre line. This usually means the sealant was asked to stretch more than it could. Causes include a joint too narrow for the movement, a bead too thin, or a product with the wrong movement class. Joint design is the usual answer: the width must suit the expected movement and the sealant’s capability. Our joint design guide explains.
Substrate failure: the material gave way
The sealant has held to the face, but the edge of the material itself has broken away. You see fragments of concrete, brick, render or stone stuck to the sealant. This happens where the substrate is weak or friable, or where the sealant is too stiff for a soft material. Repairing the edges or choosing a lower-modulus sealant may be needed. We’ll tell you if the edges need repairing first.
Substrate failure is more common on older or softer materials, such as weathered render, aged concrete edges and some natural stone. It can also follow a previous reseal where the faces were ground back too hard. In these cases the edge condition has to be dealt with as part of the job, not after it.
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.
Weathering and ageing
- UV and weathering: surface crazing and cracking on sunny elevations.
- Chalking: a powdery surface that rubs off on your finger.
- Hardening: the sealant has lost its flexibility and no longer moves with the joint.
These are signs of age and exposure. Sealant joints are maintenance items; their life depends on movement, UV and weather, cleaning, traffic and the product. Inspect regularly.
Mould in wet areas
Black or pink staining in washrooms and kitchens is mould growing into the sealant. It is not always a sign of bond failure, but it can’t usually be cleaned out. The sealant is replaced with an anti-mould sanitary silicone, and ventilation should be checked. See our washroom sealing service.
Reporting what you find
When you report a failed joint, a few photos tell a contractor a lot. Take one close-up showing which face the sealant has left, one wider shot showing where the joint is, and note the substrate and elevation. That is often enough to diagnose the cause and give a fixed quote for the treatment.
Quick answers
Can a failed joint be repaired in patches?
Small lengths can sometimes be cut out and replaced, but new sealant must bond to clean faces, not old sealant. If the cause is the joint design, patching won’t stop it happening again.
Which failure type is most common?
It varies by building and joint type. Adhesive failure linked to preparation, and cohesive failure linked to joint design, are both often seen on commercial jobs.
Does failure mean the wrong product was used?
Not always. Preparation, joint design and workmanship matter as much as the product. The failure type helps narrow down which one went wrong.
Can the same joint show more than one type of failure?
Yes. A joint may be peeling at one end and split in the middle at the other. Each part points to a cause, and a good reseal deals with all of them.
Related services
- Commercial sealant contractors
- Expansion and movement joint sealing
- Facade and window perimeter sealing
More advice
- Sealant Joint Design: Width, Depth and Backing
- Backing Rod Explained: Why It Matters
- Why Expansion Joints Fail and How to Fix Them
- How Long Does Commercial Sealant Last?
- 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.