Repointing granite: a short guide for owners

Most of the houses we work on in the north of Portugal are built of granite: thick walls of rough stone laid in lime mortar, often a century old or more. Owners usually call us when something is wrong inside, a damp patch, a smell in the bedroom, plaster that will not stay on. Very often the cause is not the stone at all. It is the joint.

The wall breathes through its joints

Granite is dense and barely absorbs water. Lime mortar is the opposite: soft, porous and slightly flexible. In a traditional wall the mortar is the path by which moisture leaves the building. Rain wets the face, the joints take it in, and wind and sun draw it out again. The wall is never completely dry, and it does not need to be.

Cement is stronger than the wall around it, and that is exactly the problem.

From the 1960s onwards many of these walls were repointed with cement mortar. It looked tidy and it lasted, but it closed the pores. Water that gets behind a cement joint can no longer escape outwards, so it travels inwards and appears as damp on the plaster. In winter it freezes behind the hard joint and pushes the stone face off. We see both problems every year.

Fig. 02 — Granite walls and a new timber roof. AI-generated retrofit study.

What to ask your builder

Before the scaffold goes up, ask four questions. First, what mortar will be used? The answer should be a natural hydraulic lime, NHL 2 or NHL 3.5, with a washed sand that matches the colour of the original. Second, how deep will the old joints be raked out? Twenty to thirty millimetres is usual, by hand, never with a grinder that chips the arrises. Third, when will the work happen? Lime needs mild, damp weather to cure; avoid frost and full summer sun. Fourth, how will the new mortar be protected while it sets? Hessian and a fine water spray for a week is normal practice.

A small test panel first

We always ask for a test panel of one square metre on a wall nobody looks at. Let it cure for a month, look at it after rain and agree on it together. It costs a day. It saves arguments, and it gives the house another hundred years.