Marés stone: how to maintain it on the island

It has held up the island's architecture for centuries and still works. What destroys it in a few years is not time but the wrong product.

What it is and why it decays

A calcareous sandstone of cemented grains, with a high porosity that gives it its warm colour and its workability. That same porosity lets in water carrying salts, which crystallise inside the pores, exert pressure and break the stone apart from within. In coastal locations the supply of sea salt accelerates that process considerably.

The most expensive mistake

Painting or sealing with impermeable plastic products. Marés has to breathe: close the surface and any moisture entering by another route becomes trapped, and the disintegration happens directly behind the coating, producing detachment in sheets. Repairing a facade treated that way costs 2 to 4 times what maintaining it correctly from the start would have done.

Correct maintenance

Gentle cleaning with low pressure water and a brush, never abrasive blasting. Repointing with lime mortar, never with cement, which is more rigid and introduces salts. A breathable silicate based water repellent applied to dry stone and renewed every 5 to 10 years according to exposure to sea and wind. And control of the moisture sources: gutters, sills and ventilation at the base of the wall.

In properties less than 500 metres from the sea, renewal falls at the shorter end of that range.

Indoors

An exposed marés wall inside follows the same logic: a breathable, colourless product, or simply nothing where there is no damp problem. In splash zones a stain resistant treatment specific to porous stone is worth applying. And for the dust it naturally sheds, the answer is a consolidant applied by a professional, not a varnish, which would create exactly the problem described above.