Where water actually gets into a building

Both approaches to water ingress work. What differs is what they cost you, and the cost is what this sets out.

Junctions are the weak points

Roofs, walls and windows shed water well; the places where they meet each other are where detailing has to work. Flashings around chimneys, at abutments where a roof meets a wall, and in valleys between roof slopes concentrate large volumes of water into small details.

Failed or displaced flashing is a common cause of internal staining that gets misdiagnosed as rising damp. Any penetration through a roof or wall — a soil vent pipe, a satellite bracket, an extractor outlet — is a detail that can fail.

Rainwater goods do the heavy lifting

A roof delivers an enormous volume of water to a small number of gutters and downpipes, so a blockage overflows onto the wall below. Repeated saturation of masonry keeps it wet enough to allow damp internally and accelerates frost damage. Downpipes discharging onto the ground rather than into a gully soak the base of the wall continuously.

A leaking joint in a gutter is usually a small repair and, left alone, becomes a wall repair.

Ground levels bridge the defences

Older buildings rely on a damp-proof course, and newer ones on a similar barrier, which only works if the external ground stays below it. Successive layers of paving, decking, raised beds or gravel over decades commonly bury that barrier.

For most people, once bridged, moisture travels around the barrier and appears internally regardless of any injected treatment. Lowering the external level is often the actual fix and is far cheaper than a chemical treatment that addresses a different mechanism.

Breathability in older construction

Solid-wall buildings built with lime mortar and permeable finishes are designed to absorb and release moisture. Applying cement render, plastic paint or impermeable internal finishes traps moisture in the wall, which then emerges elsewhere or damages the fabric. This is a common cause of persistent damp in old buildings that have been modernised with the wrong materials.

Repairs to older construction should generally use breathable materials, which is a matter for someone experienced with the construction type.

Water from inside

Condensation is water generated indoors by cooking, washing and breathing, condensing on cold surfaces. It is the most common damp mechanism in homes and the cheapest to address, through ventilation, heating and insulation of cold spots.

Leaking waste pipes, failed shower seals and overflowing appliances produce localised damp that is easily mistaken for something structural. Checking the simple internal causes before commissioning treatment saves a great deal of money.

None of this is a substitute for talking to a clinician if something feels wrong.

Diagnose before you buy a cure

Each mechanism looks similar on a wall and costs a completely different amount to remedy. Independent diagnosis from someone who does not sell the treatment separates the finding from the sales pitch. Fixing the cause and then repairing the damage is the correct order, and reversing it wastes the repair.

Put simply, this is general information; a qualified surveyor should assess any specific damp problem.

Side by side

The takeaway

Follow the water to its entry point before anyone quotes for a cure.

Small and repeatable beats ambitious and abandoned, almost every time.