Window and door mastic: when to replace it

Signs the frame mastic has failed

• The bead has cracked, split or pulled away from the brick, render or frame.
• It has gone hard and brittle, or chalky to the touch.
• You can see daylight or feel a draught at the edge of the frame inside.
• Damp patches or mould on the reveal or the plaster beside the window.
• Gaps at the bottom corners, where water collects on the sill.

Inside and outside joints do different jobs

The external joint keeps the weather out and has to cope with sun, frost and the frame expanding and contracting. It needs a sealant made for external movement joints, usually a low-modulus silicone or an MS polymer. The internal joint, between the frame and the plaster or window board, is mainly about draughts and a neat finish, and is often a paintable acrylic or hybrid.

Frame materials

• uPVC: the most common. The sealant must bond to plastic and stay flexible, because uPVC moves noticeably with temperature.
• Timber: the frame should be sound and painted before sealing. Rot behind the bead needs repairing first.
• Aluminium: often needs a primer, and the manufacturer may specify the sealant.
• Patio, bi-fold and sliding doors: long joints with a lot of movement at the threshold, where a good backing and the right width matter most.

What a proper reseal involves

1. Cutting out all the old mastic, not sealing over it.
2. Cleaning the joint and removing loose mortar or render at the edge.
3. Fitting a backing rod or foam where the gap is deep, so the bead is the right depth.
4. Priming where the surface needs it.
5. Applying a continuous bead and tooling it so it bonds to both sides.

Older buildings: check for asbestos

Window mastic in buildings from before 2000 can contain asbestos. If you are not sure, have it tested before anyone cuts it out, and tick the possible-asbestos box when you post. See asbestos in old mastic and sealant.

Get it priced

Post a window and door job with the number of windows and doors, which floors they are on and a photo of the worst one. Read more about window and door mastic.

Quick answers

Can I put new mastic over the old?

No. It will not bond properly and usually fails quickly. The old bead has to come out.

Do new windows need mastic?

Usually yes: most installers seal the perimeter once the frame is fixed. On new-build sites this is often done by a plot mastic applicator.