Brick, stone and render: how exterior materials age

Brick, stone and render age at different rates because each takes in, holds and releases water in its own way. Water drives most of the decay in exterior walls, through freezing, through dissolving and carrying salts, and through moisture trapped behind coatings that cannot let it escape. Knowing which material a wall is made from, what mortar or finish it was built with and how water reaches it will explain most of what you see on an older facade.

The same knowledge decides how a wall should be repaired. A repair suited to one material can harm another, so identifying each part of the wall comes before any work begins.

How brick weathers

Fired clay brick varies widely in porosity. Historic handmade bricks were often fired at lower temperatures than modern units and can hold more water. In freezing weather, water that has soaked into the pores expands as it turns to ice, and the face of the brick can flake away in thin layers. This process, called spalling, is most common where water sits on the surface, such as on sills, copings and the lower courses below a leaking downspout.

Efflorescence is a white, powdery deposit left when water carries dissolved salts to the surface and evaporates. It is usually cosmetic and often fades as the wall dries, but a deposit that keeps returning shows that moisture is still moving through the wall. Brush it off dry, then trace the water source before worrying about the surface. A spalling or heavily efflorescing brick is showing where water enters, which is more useful than the stain itself.

Mortar decides how the wall moves water

Earlier walls were usually bedded in lime mortar, which is softer and more vapor-permeable than the Portland cement mortar used in many later repairs. Many early-twentieth-century walls were laid in lime-cement mixes, which sit between the two. Conservation practice generally holds that mortar should be softer than the masonry around it, so that moisture and movement are taken up by the joints, which can be repaired, rather than by the brick faces, which cannot. When hard cement mortar is used in repointing, moisture gets trapped inside the brick and the faces decay quickly.

A joint that powders under a screwdriver tip, or sits visibly recessed behind the brick face, is ready for attention. Repointing starts with raking out the failing mortar to a depth of two to two and a half times the width of the joint. Power grinders can chip the edges of the bricks, so hand tools are the usual choice. The new mortar should be applied in layers and matched in color, sand and tooling to the original, which is why a specialist mason is the safest choice for a historic facade.

Stone weathers according to its type

Sandstone is granular and porous, and its surface can erode grain by grain, especially where rain runs in channels down the face. Some sandstones split along their bedding planes, which is why stone laid on its natural bed, the way it lay in the quarry, weathers more evenly than stone set on edge. Look for flaking layers parallel to the face, since they often show that a stone was laid the wrong way or has suffered repeated wetting and drying.

Limestone dissolves slowly in acidic water. In sheltered places, where rain does not wash the surface, air pollution can react with the stone and form a dark, crusty layer of gypsum. These black crusts were common on older buildings in polluted areas and can hide the stone beneath. Removing them is a job for a conservation professional, because acid cleaners will etch the stone and attack the material underneath the crust.

Granite is dense and resists weathering well, so its problems usually lie elsewhere: in cracks, in mortar beds and in the joints between blocks, where water collects and freezes. A granite wall with failing joints needs the same attention as a brick wall with failing mortar.

Render and the finishes that suit older walls

Render is a mortar coat, usually called stucco in the United States, and it is applied over masonry or over a wood frame. Over a wood frame, it is held on wood lath or metal lath nailed to the studs. Historic renders were usually lime-based, with sand and sometimes animal hair or other fiber mixed in for reinforcement. They flex slightly with the wall and let moisture pass through. Cement-rich renders are harder and far less permeable. Applied over soft brick or stone, they can trap moisture beneath and pull the surface away from the wall when water pushes outward. Over a wood frame, a hard stucco that traps water can rot the lath and framing behind it, which is why the water-resistive barrier behind the lath matters.

Check render by tapping gently with a knuckle or a coin along a section of wall. A solid, even sound means the coat is still bonded. A dull or hollow sound suggests the render has come away from the wall beneath. Hairline cracks are common in lime render and often reflect normal movement, but cracks that widen, bulges that grow or missing sections call for a specialist assessment. Keep any loose patch dry until it has been assessed.

Paint is the other common decision. Impermeable paints and sealers can trap moisture inside brick and render, causing blistering and spalling beneath the coating. Breathable finishes suit historic walls better. Mineral paints and limewash were traditional choices, and limewash wears gradually and can be renewed without sealing the wall. Check a modern masonry paint's data on vapor permeability before choosing it.

Cleaning without damage

Pressure washing and abrasive blasting remove the harder fired skin from brick, which protected the softer body beneath, and they wash the detail out of carved stone. Use low water pressure, a soft brush and clean water wherever cleaning is needed. Test any method on a small, inconspicuous area first, and watch the result for several days, since some damage shows only after the surface has dried.

Acid cleaners harm lime mortar and many stones, and their residues can leave salts that add to efflorescence. Sealing a wall with a film-forming product is rarely the answer, because it traps moisture inside the wall. An older wall rewards patience: fix the water route first, repoint with a mortar matched to the original and no harder than the masonry, keep the finish breathable and clean only what needs cleaning.