Cracks, movement and structure
Almost every masonry house in Britain has cracks. A small number of them matter. The difference is measurable, and you can do most of the measuring yourself before spending a penny.
Width first, then pattern, then movement
Engineers grade cracks on the BRE Digest 251 scale, and categories 0 to 2 — anything up to about 5mm — are cosmetic or easily filled. What escalates a crack is the pattern and the behaviour: diagonal, tapered, present on both faces of the wall in the same place, near an opening, with doors that have stopped closing. A wide crack that stopped moving in 1987 is a very different proposition from a fine one that is opening now.
Monitoring is free and it is what an engineer will ask for
Mark each end of the crack with a pencil and the date, photograph it with a ruler alongside, measure at three points, and repeat monthly for six months across a change of season. Cracks that open in summer and close in winter are usually clay shrinkage, often with a tree involved. Cracks that only get wider need a structural engineer. Turning up with six months of evidence makes that report cheaper and more useful.
Underpinning is the last resort, not the first
Confirmed subsidence far more often ends in tree management, drain repair and monitoring than in underpinning. Establish the cause before anyone prices a solution — and if you are buying, check with an insurer that cover is available at a normal premium before you exchange. A property with a subsidence claim in its history carries that history forever.
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