# Outside lining

An outside lining (also known as an external reveal liner or box frame extension) is a timber component fitted to the exterior of a sash window frame to bridge the gap between the window box and the outer wall surface-especially in older properties with thick brick or stone walls.

It ensures a clean, weather-tight transition while maintaining the window’s alignment and historic proportions. In conservation work, outside linings are carefully matched to original profiles and finished with breathable microporous paint to prevent moisture trapping and support long-term durability.