Moisture is the single biggest threat to masonry. Brick, stone and block are porous, so water finds its way in from the ground, through driving rain, or as condensation, and once it is inside it causes salt damage, crumbling mortar, mould and structural decay. Learning how to control moisture in masonry is less about sealing the wall shut and more about managing where the water comes from and letting the wall dry out.
This guide explains what causes damp in masonry, how to control each type, and the one common mistake that makes the problem worse.
What causes moisture in masonry?
Masonry gets damp from three directions: up from the ground (rising damp), in through the face (penetrating damp), and out of the air (condensation). Leaks and poor drainage feed all three. Controlling moisture starts with working out which one you are dealing with, because the fix for each is different, and treating the wrong cause wastes money or makes the problem worse.
| Type of damp | Where it comes from | Tell-tale signs |
| Rising damp | Groundwater wicking up through porous masonry | A tide mark low on the wall, white salt deposits, crumbling mortar |
| Penetrating damp | Rain pushed through walls, cracks or failed pointing | Damp patches that worsen after rain, often around windows or higher up |
| Condensation | Warm, moist indoor air meeting cold masonry | Surface moisture and mould in corners, worse in winter and wet rooms |
| Leaks and poor drainage | Blocked gutters, downpipes, plumbing, pooling water | Localised damp near the source, staining, overflow marks |
| Bridged damp-proof course | Render, paths or garden beds carrying moisture past the DPC | Rising-damp signs where ground or render sits above the DPC line |
Because masonry is porous, it readily absorbs and holds water, which is why the same material that looks solid can stay wet for weeks. Understanding how porous the stone or masonry is tells you how vulnerable a wall is in the first place.
How to control each type of damp
1. Rising damp
Rising damp is groundwater climbing through the wall by capillary action, usually where the damp-proof course (DPC) is missing, failed or bridged. To control it, make sure a working DPC is in place, and keep soil, paths and garden beds below the DPC line, generally at least 150mm below it (confirm against current building requirements). Then improve drainage so water is not pooling against the base of the wall.
Where the DPC has failed, a chemical DPC (an injected damp-proofing cream) or a new physical DPC restores the barrier.
2. Penetrating (lateral) damp
Penetrating damp is rain or ground water pushed through the wall, through cracks, failed pointing, or where external ground sits above the internal floor. To control it, repoint failed mortar joints, seal cracks, and fit or repair flashings around windows, doors and where walls meet the roof. Lower any ground or render that is bridging the wall. On exposed walls, a breathable water-repellent can help shed rain while still letting the wall dry, but it is not a substitute for fixing the defect.
3. Condensation
Condensation forms when warm, moist indoor air meets cold masonry, and it is worst in poorly ventilated bathrooms, kitchens and laundries. To control it, improve ventilation with exhaust fans and vents, keep air moving, and reduce indoor moisture at the source (cover pots when cooking, vent the dryer outside). Where you can, insulate cold surfaces so they do not drop below the temperature at which moisture condenses.
Get the drainage and ground levels right
Most masonry damp is a drainage problem in disguise. Keep water away from the wall and most damp never starts in the first place.
- Keep ground, paving and garden beds below the damp-proof course. Soil banked up against a wall is one of the most common causes of rising and penetrating damp.
- Slope the ground away from the wall so surface water runs off rather than pooling against the base.
- Keep gutters and downpipes clear and discharging well clear of the footings. A blocked downpipe can soak a wall in a single storm.
- Keep weep holes clear in cavity and brick-veneer walls so the cavity can drain and ventilate. Do not render over them or bury them under paving.
- Drain behind retaining walls. Use free-draining gravel backfill, an ag drain at the base, weep holes through the wall, and a waterproof membrane on the retained face to relieve water pressure. Our stone retaining walls guide covers this in detail.
Drainage matters more in some locations than others. Walls in coastal and humid climates carry a heavier moisture load, so the drainage and ventilation have to work harder.
Why sealing masonry can trap moisture
Never try to seal your way out of a moisture problem. An impermeable coating traps moisture inside the wall, where it concentrates salts, blisters paint and spalls the surface, and it often just pushes the damp somewhere else. This is the most common mistake people make with damp masonry, and it usually costs more to undo than the original problem.
Masonry needs to breathe. Use vapour-permeable products: breathable masonry sealers, and mineral or lime-based renders and paints on solid and older walls, rather than plastic membrane paints or hard cement render over damp solid masonry. The order matters: fix the moisture source first, let the wall dry, then protect it with a breathable finish. The same breathability principle drives the right approach to caring for sandstone walls.
How to deal with efflorescence and salt damp
Efflorescence, the white powdery salt that appears on masonry, is a symptom of moisture moving through the wall, not the disease itself. Treating the salt without treating the moisture just lets it return.
- Brush off efflorescence dry with a stiff non-metal brush. Avoid washing it, which can dissolve the salts and carry them back into the stone.
- Find and fix the moisture source, whether that is a bridged DPC, high ground or a leak. The salt will keep coming back until the water stops.
- For salt damp, let the wall dry fully after fixing the source, then replace salt-affected mortar and render with salt-resistant or lime-based materials.
Where the damage has gone further, our guide to repairing and restoring walls covers the steps to bring the masonry back.
When to call a professional
Surface damp and drainage problems you can often handle yourself. Rising damp, structural cracking and DPC failure need a professional.
Call someone in when you see a persistent tide mark and crumbling mortar low on the wall (the signature of rising or salt damp), when damp returns after you have fixed the drainage, when cracking looks structural, or when the job needs a chemical or physical DPC installed. A professional can diagnose the actual source, which is often more than one, and specify the right fix rather than a guess. Our guide to hiring the right stonemason covers what to ask.
Final word
Controlling moisture in masonry comes down to three habits: find the source, manage the water, and let the wall breathe. Keep the ground and drainage right, keep the damp-proof course clear, and protect the wall with breathable materials rather than sealing it shut, and you avoid the salt damage, mould and decay that damp causes. When the damp is rising, structural or stubborn, bring in a professional before it gets expensive.
A regular stone maintenance routine catches most moisture problems early, while they are still cheap to fix.
Frequently asked questions
1. What is the main cause of damp in masonry walls?
The most common causes are rising damp from groundwater, penetrating damp from rain getting through the wall, and condensation from indoor moisture. Many damp problems trace back to poor drainage or a damp-proof course that is missing or bridged.
2. What is a damp-proof course (DPC)?
A DPC is a waterproof barrier built into a wall near its base to stop groundwater rising through the masonry. If it is missing, damaged, or bridged by render, paths or garden beds, rising damp follows.
3. Should you seal masonry to stop damp?
Not with an impermeable sealer. Sealing the surface traps moisture inside the wall and usually makes salt damage and spalling worse. Fix the moisture source first, then protect the wall with a breathable, vapour-permeable product.
4. How do you stop rising damp in a brick or stone wall?
Restore a working damp-proof course, often by chemical injection, lower any ground, paving or render that is bridging it, and improve drainage so water is not sitting against the base of the wall.
