Precast concrete and cladding panels are fixed to a frame with gaps between them. Those gaps are designed to let each panel move with heat, moisture and loading. The sealant in the joint keeps water and air out while that movement happens. Over time, it wears out. For owners, FMs and managing agents of panelled buildings, this guide explains why panel joints fail and how resealing is usually tackled.
What panel joints do
Panel joints have two jobs. They let panels move without cracking, and they form the weather seal between them. Some facades have a single seal at the face; others have a second line of defence behind. The joint width and the sealant’s movement capability must work together, and the depth is normally controlled with a backing rod so the sealant bonds to the two panel edges only.
Why they fail
- constant daily movement stretching an ageing bead
- UV and weathering on exposed elevations, leading to cracking, chalking and hardening
- joints too narrow for the movement, causing cohesive splits
- poor original preparation or no primer on porous concrete
- missing backing rod, giving three-sided adhesion
- weak or damaged panel edges, causing substrate failure
Our guide to sealant failure types helps you tell these apart.
Signs to look for
From the ground or a window, look for sealant that has pulled away from panel edges, splits along the bead, open gaps at corners, and staining or streaking below joints. Inside, water marks near panel joints after rain are a clue. On older buildings, panel joints may have been resealed before, sometimes over the top of old sealant, which hides problems.
Binoculars help from the ground. Photograph anything that looks wrong, with a note of the elevation and floor level, so the pattern of failure can be seen across the whole facade rather than one joint at a time.
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.
How a reseal works
The old sealant and backing are cut out completely. Panel edges are cleaned back to sound, dry faces. Where the sealant manufacturer specifies a primer for concrete or the cladding finish, it is applied. New backing rod sets the depth, often around 2:1 width to depth for wider joints, as the manufacturer advises. A sealant with the right movement class is installed and tooled to a neat concave finish. Colour can be matched to the panels. Our movement joint sealing page covers this.
Planning a programme
Panel joint work is usually done elevation by elevation. Access is the largest planning factor, so it makes sense to reseal everything reachable while access is in place. External work needs dry weather. On buildings built or refurbished before 2000, check the asbestos register before old sealant is disturbed. For cladding systems with their own manufacturer guidance, check that guidance before choosing a sealant. See also weather and sealant.
What to send when asking for a quote
- photos of typical joints, close up and from a distance
- elevation drawings or an estimate of joint lengths
- the age of the building and any previous resealing you know about
- what access exists, such as cradles, anchor points or ground access
- any leak reports, with locations
With that, we can give a fixed quote or say whether a site visit is needed first. Main contractors with new-build precast packages will find more on our construction mastic page.
Quick answers
Can new sealant be applied over the old panel joint?
No. Overlaying hides the failure and gives a poor bond. The old sealant and backing should be cut out and the faces prepared.
How do I know which sealant to use?
The movement, substrates, exposure and any system guidance decide it. Low-modulus silicone, PU and polysulphide are among the products used on facade joints, depending on the specification.
Do panel joints need regular inspection?
Yes. Sealant joints are maintenance items. Include them in a planned inspection cycle, and look again after storms or any reported leak.
Can joint colour be matched to the panels?
Usually. We’ll confirm the colours available for the specified product, and a short test length can be approved on site first.
Related services
- Expansion and movement joint sealing
- Facade and window perimeter sealing
- Coloured and colour-matched sealant
More advice
- Why Expansion Joints Fail and How to Fix Them
- Sealant Joint Design: Width, Depth and Backing
- Backing Rod Explained: Why It Matters
- Sealant and Curtain Walling: General Guidance
- 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.