Last reviewed 16 Sept 2026 · 9 min read
Dampness in buildings
Dampness is the presence of unwanted moisture in walls, floors or roofs. Keeping buildings dry is essential for durability, health and comfort.
Causes and sources
| Source | Mechanism |
|---|---|
| Rising damp (ground moisture) | Water from the ground rises by capillary action through porous foundations and walls where there is no or a defective damp-proof course |
| Rain penetration | Driving rain through porous bricks, cracks, defective pointing and plaster; splashing of rain at the base of walls; poor sills and copings without throating |
| Roof leakage | Defective roof coverings, inadequate slope, blocked rainwater outlets, cracks at parapet–roof junctions, poor flashing |
| Condensation | Warm moist indoor air condensing on cold surfaces (cold walls, windows, uninsulated roofs) — kitchens, bathrooms, poorly ventilated rooms |
| Leaking services | Water supply and drainage pipes, sanitary fittings, overflowing tanks |
| Construction moisture | Water used in construction trapped in walls and floors (drying out takes time) |
| Groundwater pressure | Basements below the water table |
| Faulty site conditions | Buildings on low-lying, badly drained sites; high water table; improper orientation (walls facing prevailing rain) |
| Hygroscopic salts | Salts in masonry absorb moisture from the air |
Effects of dampness
- Efflorescence, peeling and blistering of plaster and paint, stains and discolouration.
- Decay of timber (dry rot, wet rot), warping of wooden floors and frames.
- Corrosion of steel reinforcement and fixtures; deterioration of masonry and mortar.
- Growth of mould, algae and termites — unhealthy living conditions (respiratory problems, allergies).
- Damage to electrical installations, furnishings and floor coverings.
- Reduced thermal insulation; unpleasant appearance and smell; reduced life of the building.
Damp-proof course (DPC)
A DPC is a layer of impervious material built into walls and floors to prevent the passage of moisture.
Requirements of a good DPC material
Impervious to water; durable (as long as the building); strong to carry loads without being squeezed out; flexible enough to accommodate small movements without cracking; resistant to decay and chemicals; not easily damaged during construction; dimensionally stable; economical; free from harmful deleterious materials.
Materials
| Category | Material | Remarks |
|---|---|---|
| Flexible | Hot bitumen, bituminous felts (hessian- or fibre-based), plastic sheets (LDPE, PVC), metal sheets (lead, copper, aluminium) | Accommodate movements; lead and copper are durable but costly; aluminium needs protection from alkaline mortar; bitumen felts should be lapped (commonly about 100 mm) |
| Semi-rigid | Mastic asphalt | Impervious, durable, laid hot in layers; may squeeze out under heavy loads in hot climates |
| Rigid | Dense cement concrete (commonly 1 : 1.5 : 3, about 40–50 mm thick) with waterproofing compound, often painted with hot bitumen; cement mortar (1 : 3) with waterproofing compound; stones (slate, dense granite) laid in mortar; engineering/dense bricks in cement mortar | Common in India (cement concrete DPC); rigid — cracks if the building settles |
| Coatings | Bitumen and polymer-based coatings | Used with other treatments |
Position and principles of laying DPC
- The horizontal DPC in walls is laid at plinth level, generally at least about 150 mm above the finished ground level to be above splashing, and below the lowest timber (floor joists) and above the highest ground.
- The DPC must cover the full thickness of the wall (excluding the cavity in cavity walls).
- It should be laid on a level, smooth mortar bed; joints in sheet materials lapped and sealed.
- It should be continuous — carried across door openings and connected to the floor damp-proof membrane; where levels change, vertical DPC links horizontal DPCs.
- It must not be pierced by fixings, pipes or plaster bridging (plaster should not bridge over the DPC externally).
- In cavity walls, DPCs and cavity trays direct water outward above openings and at the base (with weep holes).
- Provide DPC below sills, copings and parapets, and at junctions of roofs and walls.
Methods of damp prevention
| Method | Description |
|---|---|
| Membrane damp proofing | Layers of impervious DPC materials in walls and floors (horizontal and vertical) |
| Integral damp proofing | Waterproofing admixtures added to concrete/mortar: pore fillers (chalk, talc, fuller's earth), water repellents (soaps, stearates, oils), crystalline admixtures; plus dense, low w/c concrete |
| Surface treatment | Water-repellent coatings on external surfaces — silicones, silanes/siloxanes, sodium silicate, bitumen coatings, cement-based paints; keep surfaces breathable where possible |
| Cavity walls | Two leaves separated by a cavity (commonly about 50–75 mm); moisture cannot cross the cavity; wall ties, cavity trays and weep holes |
| Guniting (shotcreting) | Pneumatic application of rich cement mortar on exposed surfaces — dense impervious layer |
| Pressure grouting | Injection of cement grout into cracks, voids and joints of masonry and foundations |
| Site drainage | Grading ground away from the building, plinth protection/apron, surface and subsoil drains (French drains) around foundations, lowering water table |
| Chemical DPC injection | For existing walls — silicone or siliconate solutions/creams injected into drilled holes to form a water-repellent barrier |
| Electro-osmotic DPC | Electrical system to reverse capillary rise (limited use) |
| Good design details | Adequate roof overhangs, drip moulds and throating on sills, copings and chajjas, weathered projections, proper flashing, rainwater pipes |
Damp proofing of floors and basements
Ground floors
- Damp-proof membrane (DPM) — e.g. polythene sheet or bitumen layer below the floor bed (over a blinding layer), turned up and connected to the wall DPC.
- Sub-base of compacted coarse sand, gravel or brick ballast (capillary break) under the floor concrete; floor raised above ground.
Basements — tanking
Tanking is the continuous waterproofing of basement walls and floors against groundwater.
| Method | Description |
|---|---|
| External tanking | Waterproofing membrane (mastic asphalt, bitumen sheets, polymer membranes, bentonite) applied on the outside of walls and below the floor slab, protected by a brick/block protection wall — preferred, since water pressure presses the membrane against the structure |
| Internal tanking | Membrane on the inside face, held in place by a loading coat (internal wall and heavy floor screed) designed to resist water pressure — used when outside access is not possible |
| Integral waterproof (watertight) concrete | Dense concrete with low permeability, waterproofing admixtures, water bars (waterstops) at joints and crack control |
| Drained cavity systems | Internal cavity membranes collect water that penetrates and lead it to sumps for pumping |
| Crystalline and injection treatments | For repairs and joints |
Hydrostatic uplift on basement floors is resisted by the weight of the slab (and structure) or by tension piles/anchors.