Scotland’s weather tests external mastic hard. Rain is driven against walls by strong winds, joints get wet and stay wet, and in colder spells water in small cracks freezes and thaws again and again. The west of Scotland is wetter than the east, but every part of the country sees weather that finds weak joints. This guide explains what that does to mastic and how to choose and fit sealant that copes.
What the weather does to mastic
Wind-driven rain is pushed sideways into the joint between frame and wall. A small gap that would stay dry in still rain can let water in during a storm.
Freeze–thaw works on any crack. Water gets in, freezes, expands and widens the gap. Repeated through a winter, a hairline crack can become an open joint.
Sunlight and age also matter. Over time, mastic hardens, shrinks and loses its grip on the frame or the stone.
Signs your external mastic has failed
- Cracks along the bead or at the corners of the frame
- Mastic that has pulled away from the stone or the frame
- Hard, brittle mastic that crumbles when pressed
- Draughts around closed windows
- Damp patches on the inside reveals or below the sill
Our window mastic guide goes through these in more detail.
Stone, harling and the right product
Many Scottish homes have sandstone or granite walls, or harled (roughcast) finishes. These are uneven and porous, so the joint is rarely a neat, even gap.
Product choice matters. Low-modulus silicones are designed for joints that move, but not every silicone is suitable for natural stone. Adshead Ratcliffe’s data sheet for Arbosil LM, for example, lists PVCu frames and brickwork among its uses but says it is not recommended for use against natural stone (Arbosil LM technical data). On stone, use a sealant the manufacturer lists as suitable for it. See our guide to mastic on granite and sandstone.
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.
Sash and case windows
Sash and case windows are common in older Scottish homes, and many are set in stone openings. Traditionally, the joint between the case and the stone was pointed with burnt sand mastic, a linseed-based mix that suits traditional buildings. Our sand mastic service covers this work.
Getting the joint right
Whatever the product, the joint needs to be prepared properly. Old mastic should be cut out, the surfaces cleaned and dried, and a primer used where the manufacturer calls for one. In deeper joints, a backing rod controls the depth so the sealant can stretch and recover. Weather matters on the day too. Sealant should go onto dry surfaces, within the temperature range on the data sheet.
We cover all of Scotland for window, door and stone mastic. Every job carries our one-year guarantee.
Doors, sills and other weak spots
Windows get most of the attention, but other joints suffer too. Door frames, especially on exposed elevations, take the full force of wind-driven rain. Sills collect water and hold it against the joint beneath. Joints between a newer extension and the original house, or between render and stone, can open as the two move differently.
A walk around the outside of the house after a winter, looking at every frame and joint, is a good habit. Small cracks are far easier to deal with than water that has been getting in for months. Our external mastic service covers movement joints and joints between different materials.
Quick answers
Can mastic be replaced in winter?
It can, on dry days within the temperature range the manufacturer gives. Wet or frosty surfaces need to wait.
Is silicone better than sand mastic on a sash window?
Not always. On traditional stone buildings, sand mastic is often the better match. On modern frames, a suitable silicone is usual.
Can I seal over cracked mastic?
It rarely lasts. The old mastic should be cut out so the new sealant bonds to clean frame and stone.
Related services
More advice
- Window Mastic: When to Replace It and How
- Sand Mastic for Sash and Case Windows
- Mastic on Granite and Sandstone Homes
- 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.