Why Your Interior Wall Is Damp, and Why the Fix Is Outside

A tenant reports a damp patch on a bedroom wall. Somebody paints it. Six months later it is back, slightly bigger, and now the paint is lifting. This is one of the most common calls we get, and the answer is almost always the same: the water is coming through the wall from outside, and nothing you do on the inside will change that.

Brick is not waterproof, and it was never meant to be

A solid masonry wall is designed to absorb some water and then release it. What keeps the interior dry is the wall's ability to dry out faster than it takes water on. Every failure mode below is really the same failure: something has tipped that balance.

The four things we find most often

1. Eroded mortar joints

Mortar is softer than brick, deliberately, so that it takes the movement and the weather rather than the masonry units. On a wall that has not been pointed in decades, the joints erode back behind the face of the brick, and every recessed joint becomes a small shelf that holds water against the wall. This is what repointing fixes: cutting the joints out to a proper depth and filling them with mortar that matches the original in colour, profile and, importantly, hardness.

Using a mortar that is too hard is a classic and expensive mistake. If the mortar is stronger than the brick, the wall stops moving at the joints and starts moving through the brick faces instead, and you get spalling: brick faces popping off after a freeze.

2. A rusting lintel

If there is a horizontal crack in the brickwork directly above a window, look at the steel lintel carrying that opening. Steel expands as it rusts, by a lot, and the expansion lifts the masonry above it. That crack is not cosmetic and pointing over it will not hold.

3. The parapet and the coping

On flat-roofed buildings the parapet is exposed on both faces and takes weather from every angle, and the coping stones on top of it are the building's umbrella. A cracked or displaced coping puts water directly into the top of the wall, where it travels down inside the masonry and appears on an interior wall a floor or two below. Anyone diagnosing a damp wall who has not looked at the roof edge has not finished looking.

4. Drainage

A gutter that overflows, or a downspout discharging against the base of a wall, will soak masonry continuously every time it rains. This is the cheapest problem on this list to fix and the one most often left alone.

How to read your own wall

• White powdery deposits on the brick: efflorescence, which is salt left behind by water moving through the masonry and evaporating. It is not the problem, it is the receipt.
• A horizontal crack above an opening: probably a lintel.
• Joints you can push a finger into: pointing is overdue.
• Brick faces flaking or popping off: freeze damage, often after the wall was pointed with mortar that was too hard.
• A parapet that is not plumb when you sight along it from the sidewalk: that is structural, and it is the one to act on soonest.

Access is usually the bigger half of the job

Worth knowing before you get quotes: on anything above the second floor, the scaffolding or suspended rig often costs more than the masonry. That is why it is worth doing a whole elevation at once rather than the one patch you noticed, and why two quotes can be wildly different if one of them quietly left the access out.

We repoint, rebuild and repair masonry across all five boroughs and we price the repair and the access together, in writing. Send us a photo of the wall and we will tell you what we think you are looking at.