Traditional Perimeter Weathering
Sand mastic replacement around sash window cases
Renewing failed perimeter mastic helps close the weather joint between a timber window case and surrounding masonry while preserving the traditional appearance of the opening.
Sand mastic replacement in Stirling
The junction around a traditional window case is exposed to wind-driven rain and small movements between different materials. When old mastic cracks, separates or falls away, water can enter behind the case and remain hidden. This may contribute to damp internal finishes and decay in vulnerable external timber.
Sand mastic is valued on traditional Scottish buildings because its textured appearance sits naturally beside stone and lime-based finishes. Successful replacement is not simply a matter of covering the old joint. Loose material must be removed, the background examined and the junction prepared so the new seal has suitable depth, adhesion and weathering.
Signs that the perimeter needs attention
Cracked or Missing Mastic
Visible gaps, fractured edges and detached sections allow water and air to reach the junction behind the case.
Damp at Window Reveals
Moisture marks can have several causes, but a defective external perimeter joint should be included in the investigation.
Soft External Timber
Persistent wetting near case edges or sills may contribute to localised decay requiring separate timber repair.
Inappropriate Hard Sealants
Rigid or visually intrusive previous repairs may crack, trap moisture or sit poorly against traditional masonry.
How we renew the joint
- Inspection: the perimeter, sill, masonry and adjacent paint are checked to identify related defects.
- Careful removal: failed mastic and loose contaminants are taken out without damaging arrises or sound timber.
- Background preparation: joint depth and support are corrected where necessary so the new material is not applied as a thin surface smear.
- New sand mastic: compatible material is installed and finished to form a neat weathering joint.
- Associated repairs: any decayed timber, open putty or defective coatings are addressed as separate but connected issues.
A complete weatherproofing strategy
Perimeter mastic protects the junction around the case; it does not seal the moving sashes. Excessive air leakage through the window itself is treated through draught proofing. Failed glazing putty requires glass and putty repair, while exposed timber relies on a well-maintained external paint system. Considering these elements together prevents one isolated repair from concealing a wider maintenance problem.
Frequently asked questions
Can new mastic be applied over the old material?
Loose, cracked or contaminated material should be removed. Covering a failed joint usually produces a short-lived and untidy result.
Will sand mastic repair rotten timber?
No. It is a perimeter weather seal, not a structural filler. Decayed timber must be investigated and repaired appropriately first.
Is every gap around a window filled in the same way?
No. Joint width, depth, substrate, exposure and the historic detail all matter. We inspect the opening before recommending a material and finish.
Protect the junction around your windows
Send us photographs or arrange a survey if the perimeter mastic is cracked, missing or allowing water around the case.
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