Window and door mastic is easy to ignore. It sits around the outside of the frame, often high up, and it fails slowly. By the time most people notice it, there’s a draught at the frame edge, a damp patch on the reveal or a stain on the wall under the sill.
Replacing it is not complicated, but it is easy to do badly. The old mastic has to come out, deep gaps need backing, and the sealant has to suit both the frame and the wall it meets. This guide explains the signs, the process, the choice of sealant for uPVC, aluminium, timber and period windows, and a few things to watch for in older buildings and in Scotland’s weather.
What window mastic does
When a window or door is fitted, there’s a gap between the frame and the opening in the wall. The fixings hold the frame in place, and the gap is filled and sealed so that weather can’t get in. The external sealant is the first line of defence against rain and wind.
Inside, there’s often a second joint between the frame and the plaster. That one is about draughts and a tidy finish, rather than rain. Both can fail, and they can be renewed separately or together.
Signs your window mastic needs replacing
- Cracks, splits or gaps in the sealant around the frame.
- Mastic that has gone hard, brittle or crazed, or has shrunk away from the wall.
- Draughts around the frame when the window is shut. MyJobQuote’s resealing guide lists this as a common sign.
- Damp on the reveal, the window board or the wall under the sill.
- Black mould on the internal frame joint.
- Old mastic that has fallen out completely in places.
- Gaps left after new windows were fitted, or untidy sealant that was never finished properly.
When it isn’t the mastic
Not every window problem is a sealant problem. Mastic won’t fix:
- Misting between the panes. That’s a failed double-glazed unit, which needs a glazier.
- Draughts through the opening part of the window. Usually worn gaskets, hinges or handles.
- Rotten timber. Soft or crumbling frames need joinery repair before resealing.
- Water through the wall itself. Cracked render, failed pointing or a blocked gutter can all put water on the inside face of the wall near a window.
A good applicator will tell you if what they find isn’t a sealant job.
How window mastic is replaced
- Check each frame. Agree which windows and doors need doing, inside, outside or both.
- Cut out the old mastic. All of it, back to the frame and the wall.
- Clean back to sound surfaces. Loose render, dust and old sealant residue are removed so the new sealant bonds.
- Fit backing rod in deep gaps. Closed-cell backing rod sets the depth of the sealant and stops it bonding to the back of the joint, so it can stretch and recover as the frame moves.
- Apply the sealant. A weatherproof, flexible sealant suited to the frame and the wall, in a colour to suit.
- Tool it neatly so it’s pressed into both sides of the joint and sheds water.
Our window and door mastic service follows these steps on uPVC, aluminium, timber and composite frames.
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.
Which sealant suits which frame
| Frame and wall | Usual choice | Notes |
|---|---|---|
| uPVC frame in brick or render | Low-modulus silicone or hybrid exterior sealant | White, brown, anthracite and other colours to match the frame |
| Aluminium frame | Neutral-cure low-modulus silicone or hybrid sealant | Avoid acid-curing sealants that can attack some metals and finishes |
| Painted timber frame | Paintable hybrid sealant, or sealant applied after painting | Timber needs to be sound and dry |
| Timber sash and case in stone or brick | Traditional sand mastic | Often the right finish on period and listed homes |
| Frame in natural stone | Neutral-cure sealant suited to stone, or sand mastic | Some sealants can stain porous stone |
| Internal frame to plaster | Paintable sealant or decorators’ caulk in dry rooms | Flexible silicone where the window is in a bathroom |
Older buildings: asbestos and listed homes
Some older mastics and sealants can contain asbestos. The Health and Safety Executive notes that asbestos can be found in any building built before 2000. If your building is from before then and the old mastic is hard, grey or putty-like, have it checked before it is cut out. Our guide on asbestos in old mastic explains what to do.
If your home is listed or in a conservation area, check with your local planning authority before changing the look of the windows. Like-for-like sand mastic on period windows is often the preferred finish; see our guide to sand mastic for sash and case windows.
Scotland: weather, stone and harling
Window mastic in Scotland has a tough job. The west of Scotland in particular is wet, and exposed coastal and upland homes take wind-driven rain. Many homes are sandstone (red and blond sandstone tenements in Glasgow, for example), granite in Aberdeen, or harled. Sash and case windows are common in older homes.
That means choosing a sealant that bonds well to stone and harling, setting the right depth with backing rod in wide gaps, and making sure water is shed away from the frame, not held against it. On traditional sash and case windows, sand mastic is often the right answer. We work across Scotland as well as London and the South East.
Doing it yourself
Ground-floor windows with a single, clean bead to replace are a reasonable DIY job if you’re careful. Things to watch:
- Work on a dry day, and don’t seal onto damp masonry.
- Remove all of the old mastic, not just the loose bits.
- Don’t fill deep gaps with sealant alone; use backing rod.
- Use a sealant made for exterior frames, not bathroom silicone.
- Don’t work from a ladder you aren’t comfortable on. Upstairs windows are a job for someone with the right access.
For landlords and managing agents
Failed window mastic shows up as damp complaints, draughts and mould on reveals, which tenants often blame on condensation. Checking the external joints when you check gutters and the roofline helps catch problems early. In Scotland, private rented homes must meet the Repairing Standard, which includes being wind and watertight.
Quick answers
How do I know if my window mastic has failed?
Look for cracks, gaps, hardened or shrunken mastic, draughts around the frame, and damp on the reveal or under the sill.
Can you seal over old window mastic?
No. The old mastic should be cut out so the new sealant bonds to the frame and the wall.
Is silicone OK around uPVC windows?
A low-modulus exterior silicone or hybrid sealant suits most uPVC frames. Bathroom silicone isn’t the right product outside.
Will new mastic stop draughts?
If the draught is coming around the frame, yes. If it’s through the opening window or worn gaskets, mastic won’t fix it.
Do you cover Scotland?
Yes. We work across all of Scotland, as well as London and the South East.
Related services
More advice
- Sand Mastic for Sash and Case Windows
- Scotland’s Weather and External Mastic
- Could Old Mastic Contain Asbestos?
- 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.