“Seal all joints with suitable mastic” turns up in more specifications than it should. It leaves the choice of product to whoever is on site that day, and it gives the client nothing to check against. A tighter specification costs little to write and saves arguments at handover.
This guide is for main contractors, QSs, architects’ technicians and estates teams who write or review sealant clauses. It covers the questions to answer for each joint, the main sealant types, the standards worth naming, and the details that are often left out. It won’t replace a manufacturer’s technical advice, but it will help you ask for the right thing.
Start with the joint, not the product
Before choosing a sealant, describe the joint. For each joint type, note:
- Location: internal or external, wet or dry, trafficked or not.
- Substrates: brick, concrete, stone, aluminium, PVC-U, timber, glass, tile, steel.
- Movement: how much the joint is expected to open and close.
- Width and depth: the designed joint size and tolerance.
- Exposure: UV, rain, cleaning chemicals, food contact, foot or wheel traffic.
- Appearance: colour, paintability, finish.
- Special needs: fire rating, hygiene, mould resistance.
Once these are clear, the product choice usually narrows to one or two types.
The main sealant types
Generic types only; always check the specific product’s data sheet.
| Type | Typical uses | Notes |
|---|---|---|
| Low-modulus neutral-cure silicone | Facades, window perimeters, many movement joints | Good UV resistance; usually not paintable |
| Sanitary anti-mould silicone | Washrooms, kitchens, wet areas | Mould inhibitor added; for internal wet joints |
| Polyurethane (PU) | Floors, concrete, trafficked joints | Tough; paintable versions exist |
| Hybrid / MS polymer | Mixed substrates, general building joints | Bonds to many surfaces; often paintable |
| Polysulphide | Some facade and trafficked joints | Traditional choice in certain specifications |
| Fire-rated acrylic, silicone or intumescent | Fire-resisting walls and floors | Only to the manufacturer’s tested details |
| Hygienic / food-safe | Food preparation areas | Where food contact is possible |
| Acrylic decorators’ caulk | Internal, non-moving gaps before painting | Not for movement or wet joints |
One caution: acetoxy (vinegar-smelling) silicone can attack some metals, natural stone and mirror backings. Neutral-cure is the usual choice for building joints. For a closer look at three common types, see our guide to silicone, polyurethane and MS polymer.
Movement capability and standards to name
Sealants are classified by how much movement they can take. BS EN ISO 11600 classifies building sealants by movement capability. BS EN 15651 covers sealants for non-structural joints and is split into four parts:
- Part 1: facade elements
- Part 2: glazing
- Part 3: sanitary joints
- Part 4: pedestrian walkways
BS 6213 is the guide to selecting construction sealants, and is a sensible reference for the selection process itself.
Naming the relevant part and class in the specification means any proposed alternative can be checked like for like. There is a plain-English summary in our guide to BS EN 15651 explained.
Joint geometry and backing
A sealant can only perform if the joint is the right shape. The specification should state:
- joint width to suit the expected movement and the sealant’s movement capability
- sealant depth, often around 2:1 width to depth for wider joints, as the manufacturer advises
- closed-cell polyethylene backing rod to control depth, or bond-breaker tape in shallow joints
- a tooled, concave finish pressed onto both faces
The aim is two-sided adhesion. If the sealant bonds to the back of the joint as well, it can’t stretch properly and is likely to tear. Our guide to joint width, depth and backing explains the reasoning.
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.
Preparation and primers
Write preparation into the specification rather than assuming it. Typical clauses include:
- remove all existing sealant and backing; new sealant is not applied over old
- clean joint faces to sound, dry surfaces with no dust, grease or loose material
- prime where the sealant manufacturer specifies it for the substrate
- apply only within the product’s temperature range and to a dry joint
Cold, damp or frosty mornings can stop external work. Condensation on steel and glass is common in winter. The programme should allow for that, particularly on exposed sites. More in our guide to weather and sealant application.
Colour and appearance
Sealant is visible on most facades, shop fronts and interiors, so colour deserves a line in the specification. Sealants come in a wide range of standard colours, and matching to RAL references, cladding, stone, brick, frames or shop-fit finishes is commonly requested.
State the colour reference or the material to match, and say whether samples are needed for approval. Availability varies by product, so confirm the colour exists in the specified sealant before it is locked in. Our coloured sealant page covers how we handle this.
Things specifications often miss
- Which joints are fire-resisting and need a tested fire-rated product, and who has designed them.
- Food-safe sealant in kitchens where food contact is possible.
- Asbestos checks before old mastic is removed in older buildings.
- Compatibility between sealant and adjacent materials such as gaskets, coatings and natural stone.
- Whether the sealant will be painted over.
- The records the client wants at handover: product data, batch numbers, photos.
Alternatives, samples and approvals
Contractors often propose an alternative to the named product, usually for availability or cost reasons. That is reasonable, provided the alternative is checked properly. Ask for:
- the product data sheet, showing the same BS EN 15651 part and movement class
- confirmation the product suits the substrates, with any primer needed
- colour availability against the specified colour
- for fire-rated products, tested details matching the actual walls, floors and services
On larger packages, a sample panel or first-run area lets the design team approve colour, finish and tooling before work is rolled out. It is far easier to agree the standard once than to argue over it floor by floor at snagging. See our guide to sealant snagging before handover.
Working with the applicator
A good applicator will read the specification and raise anything that won’t work on site before starting. That might be a joint too narrow for the movement, a substrate the product isn’t approved for, or a colour that isn’t available.
Mastic Doctor works to the specification and the manufacturer’s guidance on construction mastic packages and maintenance work. If you’d like a second look at a sealant schedule before tender, send it through our quote form or contact us.
Quick answers
Which standard should a sealant specification name?
Usually the relevant part of BS EN 15651 for the joint type, with the movement class. BS 6213 is the guide to selecting sealants and BS EN ISO 11600 covers movement classification.
Is neutral-cure silicone better than acetoxy?
For most building joints, yes. Acetoxy silicone can attack some metals, stone and mirror backings, so neutral-cure is the usual choice.
What width-to-depth ratio should joints have?
Many manufacturers recommend around 2:1 for wider joints. Always follow the specific product’s guidance.
Can one sealant be used everywhere on a project?
Rarely. Facades, washrooms, floors, kitchens and fire-resisting walls each have different demands, so most projects need several products.
Should the specification say anything about weather?
Yes. Stating that sealant goes onto dry joints within the product’s temperature range protects quality and sets expectations for the programme.
Related services
- Construction and new-build mastic
- Facade and window perimeter sealing
- Coloured and colour-matched sealant
- Commercial sealant contractors
More advice
- BS EN 15651 Sealant Classes in Plain English
- Sealant Joint Design: Width, Depth and Backing
- Silicone, Polyurethane or MS Polymer Sealant?
- Weather and Sealant: When Work Can Go Ahead
- Sealant Snagging Before Handover
- 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.