Walk along almost any Victorian street in Glasgow or Edinburgh and look at the edges of the sash and case windows. The neat, sandy-coloured fillet between the timber frame and the stone is very often sand mastic. You’ll find it on period homes in London and the South East too.
It’s an old material, made by hand from burnt sand and an oil binder, and it behaves differently from modern sealants. On the right building it looks better and suits the masonry better than silicone does. This guide explains what sand mastic is made of, where it’s used, how it compares with modern sealants and how it should be applied, with notes for listed homes and Scottish buildings.
What sand mastic is
Sand mastic is a two-part material: sand that has been roasted (burnt), and an oil binder that is mixed in by hand. Traditional binders are linseed-oil based. For example, one supplier, Womersleys, describes the oil in its burnt sand mastic as double-boiled linseed oil with rosin driers.
Guidance published by East Ayrshire Council for its Mauchline conservation area scheme says burnt sand mastic has been used for at least 200 years to seal joints between timber window and door frames and masonry. It notes that older mixes included lead-based driers and crushed chalk, and that natural driers have replaced the lead. The same guidance says the mastic “hardens slowly but remains sufficiently flexible for the purpose”.
Where it’s used
- Around timber sash and case windows set in stone or brick openings.
- Around timber door frames in masonry.
- On traditional buildings in Scotland, where sash and case windows are common in older homes and tenements.
- On Georgian, Victorian and Edwardian homes in London and the South East.
- On listed buildings and in conservation areas, where like-for-like materials are often preferred.
It is a pointing material for the frame-to-masonry joint. It isn’t a repair for damaged stone or render; the East Ayrshire guidance says those faults should be repaired properly first.
Sand mastic vs modern sealants
| Sand mastic | Low-modulus silicone | Hybrid (MS polymer) sealant | |
|---|---|---|---|
| Look | Sandy, matt, reads as part of the masonry | Smooth, slightly glossy | Smooth; many can be painted |
| Behaviour | Hardens slowly; stays slightly flexible | Stays rubbery and flexible | Flexible |
| Best on | Timber frames in stone or brick on period buildings | Modern frames, moving joints | Frames and trims, including painted joinery |
| Painting | Published guidance says the mastic itself should not be overpainted | Generally can’t be painted | Often paintable (check the data sheet) |
| Applied | Mixed and pointed by hand | Sealant gun | Sealant gun |
There’s no single right answer for every window. On a timber sash in a sandstone wall, sand mastic is often the better choice. On a modern uPVC or aluminium replacement, a flexible exterior sealant usually is. Our guide to mastic vs silicone covers the modern options.
Signs your sand mastic needs renewing
- Mastic that has cracked, crumbled or fallen out of the joint.
- Gaps between the frame and the stone where you can see daylight or feel a draught.
- Damp on the inside reveal or around the window boards after rain.
- A previous repair in silicone or filler that looks out of place and has come away.
- Redecorating planned, which is a good time to renew the mastic first.
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.
How sand mastic is applied
The East Ayrshire guidance and suppliers’ instructions describe a similar process. In outline:
- Rake out old, loose mastic and clean the joint.
- Pack wide gaps. The guidance suggests packing wide gaps with well-haired lime mortar that has been allowed to harden first, keeping the joint slightly recessed.
- Prepare the timber. Suppliers’ guidance says the timber frame should be primed and undercoated before the mastic goes on.
- Mix the mastic. Burnt sand and oil are mixed by hand, using as little oil as possible, and left to stand before remixing. Only as much as can be used is mixed.
- Prime the joint. A light coat of the oil is brushed onto the frame edge and masonry along the joint line, without over-oiling.
- Point the joint. The mastic is pressed firmly into the angle between frame and masonry, working from the bottom up, and shaped into a neat fillet at around 45 degrees.
- Finish. An oiled trowel is drawn along to form a neat, regular fillet, and the timber edges are wiped clean. At the sill, the mastic is kept behind the drip so it doesn’t bridge it.
It’s skilled handwork, and the finish depends on the person doing it. Our sand mastic service renews it around sash and case windows and doors in stone and brick.
Painting and aftercare
Advice on painting is worth knowing before you plan decorating. Both the East Ayrshire guidance and Womersleys’ product guidance say the mastic itself should not be overpainted. That’s why the timber frame is primed and undercoated before the mastic is applied, so the joinery is protected and the mastic is left as a natural, sandy finish.
Sand mastic hardens slowly after it is applied. Avoid knocking or cleaning the new fillet hard while it firms up, and ask your applicator when it’s safe to carry on with any final decoration of the timber.
Listed buildings and conservation areas
If your home is listed or in a conservation area, check with your local planning authority before changing windows or how they look. In Scotland, Historic Environment Scotland’s consents and permissions pages explain when consent might be needed, and its guide on reducing heat loss from sash windows recommends repairing and draught-proofing existing windows rather than replacing them.
Renewing failed sand mastic like for like is routine maintenance on many period buildings, but it’s always worth checking rather than assuming.
Scotland: sandstone, granite and tenements
Sand mastic is particularly common in Scotland. Glasgow’s tenements are largely red and blond sandstone, Aberdeen is known as the Granite City, and Edinburgh’s Old and New Towns are a UNESCO World Heritage Site. Many of those windows were finished with sand mastic, and renewing it keeps the look consistent along a street.
In tenements, windows belong to the individual flats, while some repairs to the building are shared and arranged through a property factor. If several owners want their window mastic renewed, doing it together can be practical. We work across all of Scotland, as well as London and the South East. Our guide to mastic on granite and sandstone has more on stone.
Older mastic and safety
Some older mastics and sealants can contain asbestos. The HSE notes that asbestos can be found in any building built before 2000. Traditional sand mastic is sand and oil, but over the years other materials may have been used to patch windows. If you are unsure what an old filler is, have it checked before it is disturbed.
Quick answers
What is sand mastic made of?
Burnt (roasted) sand mixed by hand with an oil binder, traditionally based on linseed oil.
Is sand mastic better than silicone for sash windows?
On timber sash and case windows in stone or brick, it’s often the better-looking and more appropriate choice. On modern frames, a flexible sealant usually suits better.
Can sand mastic be painted?
Published guidance for burnt sand mastic says the mastic itself should not be overpainted. The timber is primed and undercoated before the mastic goes on.
Do I need consent to renew sand mastic on a listed building?
Check with your local planning authority. Like-for-like repairs are often straightforward, but it’s worth confirming first.
Do you do sand mastic in Scotland and England?
Yes. We work across all of Scotland, London and the South East.
Related services
More advice
- Mastic on Granite and Sandstone Homes
- Window Mastic: When to Replace It and How
- Scotland’s Weather and External Mastic
- 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.