Why cement pointing damages historic stone walls

If there is one single thing that has done more damage to historic walls in Somerset over the past sixty years than any other, it is Portland cement. From the 1960s onwards cement was sold as the universal mortar — stronger, faster, cheaper, no special skills needed. On a modern brick cavity wall it works perfectly well. On a soft red sandstone cottage in Wellington, a lias rubble wall outside Glastonbury, or a cob barn near Crediton, it is a slow disaster.

The simple physics of it

A traditional masonry wall is a system. Lime mortar is deliberately softer than the stone or brick around it. Rain that gets into the wall — and it always does — moves through the mortar joints and evaporates back out through them. The mortar takes the wear; the stone is protected. When the mortar eventually fails, you rake it out and replace it. The stone never gets touched.

Cement reverses the system. Portland cement is harder than soft historic stone, brittle rather than slightly flexible, and effectively waterproof. Water that gets into the wall — through the top, through cracks, through windward elevations — can no longer leave through the joint. It is funnelled into the only soft path it can find, which is the body of the stone itself. There it sits, freezes in winter, expands, and lifts the face of the stone away in flakes. Year after year.

What the damage looks like

On red sandstone around Wellington and the Blackdowns we see deep spalling of the stone face, often two or three centimetres deep, exactly bordering the cement joints. On lias rubble walls around Glastonbury and Wells we see whole stones lifting out as the cement-and-stone interface fails in frost. On cob, sealed under cement render, we see otherwise sound seventeenth-century walls effectively melting from inside.

The crueller detail is that the cement joints themselves usually look fine. Hard, sharp, intact. It is the historic fabric they were meant to protect that is being destroyed. Owners often think the wall is "weathering" or "tired". It is being slowly killed by the wrong mortar.

Why disc-cutting makes it worse

When cement does eventually need to come out, the temptation is to cut it with a disc cutter or grinder. Don't. A 115mm grinder leaves a perfectly square 4mm slot exactly the height of the disc — wider than many original joints — and any contact between the blade and the surrounding stone leaves a permanent scar. We remove cement with quill picks, lump hammers and small chisels, by hand, in lifts. It is slower. It is the only way that does not damage what is left of the original fabric.

The right mortar for a Somerset wall

For most of the historic walls we work on, the right mortar is a hot-mixed lime — quicklime slaked on site directly in the sand and aggregate, used hot, and beaten in. For walls needing slightly more initial set we use a natural hydraulic lime, usually NHL 2 or NHL 3.5. The mix is matched to a surviving sample of the original wherever one can be found, by sieving for aggregate grading and matching colour against a panel of mortars.

If you have a building with cement pointing and you are worried about what is happening behind it, the simplest thing is to send a photograph. We will tell you honestly whether it is urgent, whether it can wait, or whether — occasionally — it is doing no harm at all.