Dampness is one of the most common and most damaging problems in buildings. The usual source is capillary attraction, which pulls sub-soil water upward through foundations and masonry walls. Water also enters through faulty sills, copings, and workmanship defects, so builders introduce a layer of water repellent material called a damp proof course, or DPC, which acts as a barrier against the capillary rise of water. Choosing the right membrane starts with the same logic used across construction, where the properties and applications of building materials decide what survives contact with moisture, load, and time.
This article reviews the materials commonly specified for damp proofing, the requirements they must meet, and the site practices that make them work. Minimum thicknesses, overlaps, and unit weights give a practical baseline for site estimating and checking.
What Is Dampness and How Moisture Enters Walls
Dampness is the presence of unwanted moisture inside the fabric of a building. Rising damp appears when ground water moves upward through the pores and joints of masonry by capillary action, and the height of the rise depends on the pore structure of the brick or stone and on the salts dissolved in the water. Falling damp comes from rain driven against walls through defective sills, copings, and parapets. Condensation forms when warm, humid indoor air meets cold surfaces.
The Role of the Damp Proof Course
A damp proof course is a continuous layer of impervious material built into the wall above ground level, normally 150 mm or more above the finished ground surface. It interrupts the capillary path so moisture cannot climb past it. The targets that guide modern building materials apply to the membrane as well: it must be easy to place, durable in contact with wet mortar, and able to absorb the movements of the structure.
How Water Reaches the Wall
The common routes are:
- Capillary rise of sub-soil water through foundation masonry and walls
- Rain penetration through faulty sills, copings, and bad workmanship
- Leakage from damaged gutters, downpipes, and underground drains
- Condensation on cold wall surfaces in poorly ventilated rooms
- Splash-back from ground surfaces that keeps the base of the wall permanently wet
Requirements of an Ideal DPC Material
An ideal damp proofing material has to satisfy several conditions at once, which is why no single product suits every wall. The standard requirements are:
- The material must be durable and dimensionally stable over the life of the building.
- It must be reasonably cheap.
- It must be free from sulfates, chlorides, and nitrates, which attack mortar and masonry.
- It must stay steady and resist movement under its own weight.
- It must be perfectly impervious to water and water vapour.
- It must carry the loads coming on it without crushing or creeping.
- It must be flexible enough to adjust to structural movements without fracture.
Hot Bitumen, Mastic Asphalt, and Bituminous Felts
Bitumen-based products are the most widely used damp proofing materials. They form a continuous film across the full wall width and tolerate minor movement. Moisture control also starts below grade, because the ground beside the footing decides how much water ever reaches the wall; the six types of backfill materials used in construction show how much drainage depends on what is placed around the foundation.
Hot Bitumen
Hot bitumen is a flexible material applied on a bed of concrete or mortar, usually in two coats. It should be applied with a minimum thickness of 3 mm so the film stays continuous and free of pinholes. Because it is heated and spread while fluid, it bonds well to the bedding and seals small irregularities in the surface.
Mastic Asphalt
Mastic asphalt is a semi-rigid material that forms an excellent impervious layer for damp proofing. It is durable, completely waterproof, and laid to a uniform thickness over large areas. The trade-off is stiffness: it can withstand only very slight distortion, and it is liable to squeeze out in very hot climates or under very heavy pressure. It suits flat areas and tanking work where structural movement is limited.
Bituminous Felts
Bituminous felts are flexible, easy to lay, and available in rolls of normal wall width. They are laid on a layer of cement mortar, with an overlap of 100 mm at the joints and a full overlap at all corners. The laps may be sealed with bitumen when necessary. The felt accommodates slight movements, but it is liable to squeeze out under heavy pressure and offers little resistance to sliding. Rolls should be unrolled carefully, especially in cold weather, because stiff felt cracks when forced.
How the Flexible Options Compare
| Material | Form | Typical thickness | Main limitation |
|---|---|---|---|
| Hot bitumen | Hot-applied coating on mortar or concrete bed | Minimum 3 mm | Needs clean, dry bedding |
| Mastic asphalt | Semi-rigid laid slab | 20 to 25 mm in common practice | Squeezes out under heat or heavy pressure |
| Bituminous felt | Rolled membrane on cement mortar | Single layer with 100 mm laps | Little resistance to sliding |
| Polyethylene sheet | Rolled membrane on mortar bed | 0.25 to 0.5 mm | Easily punctured by sharp bedding |
Metal Sheets, Combined Membranes, and Stone
Where walls carry heavy loads or where sliding forces are a risk, sheet materials offer a different set of properties. Metal and stone membranes are stiffer and more expensive than bitumen products, and they are specified for particular conditions.
Lead, Copper, and Aluminum Sheets
Sheets of lead, copper, and aluminum can be used as membranes for damp proofing. Lead is the most common because it is flexible and can be dressed to complex shapes without fracture. It is impervious to moisture, resists ordinary atmospheric corrosion, and does not squeeze out under ordinary pressure. Lead also resists sliding action, which matters when the membrane is bedded between masonry courses.
Minimum Lead Sheet Weight
The thickness of lead sheets is specified by weight rather than by dimension. The sheet should weigh not less than 200 N/m2, which works out to roughly 20 kg/m2, so the membrane stays thick enough to dress over irregularities without tearing.
Combination of Sheets and Felts
Combining sheets and felts layers the strengths of both materials: the sheet provides the impervious barrier, and the felt cushions it from the masonry and absorbs minor movement. The same logic that explains why lightweight infilling materials are used for voids in raft foundations applies here: separate components share the jobs of carrying load and excluding water.
Stones
Dense natural stone can serve as a damp proof course in thick masonry walls where a full-width membrane is impractical. Slabs of granite, slate, or other impervious stone are bedded in mortar with staggered joints so no continuous crack lines up across the wall. Stone DPCs carry load well and last for centuries, but they are heavy, expensive, and slow to install, so they are reserved for special cases.
Bricks, Mortar, Concrete, and Plastic Sheets
For ordinary buildings, the most economical damp proofing materials are often already on site: bricks, mortar, and concrete. Their performance depends heavily on workmanship, and damp masonry design, materials, construction methods, and quality assurance decide whether a low-cost DPC actually keeps water out.
Bricks
Well-burnt, dense bricks laid in rich cement mortar can act as a damp proof course in low-rise walls. The first course above ground is built with the densest bricks available, set in mortar with a waterproofing admixture, and the full wall thickness is filled solid. Because brick remains porous, this method works only where the water table is low and the capillary rise is short.
Choosing Bricks for a DPC
Suitable bricks have low water absorption, typically below 10 percent, with no cracks or laminations. Engineering bricks and other dense pressed units meet this test, while soft, unevenly burnt bricks do not.
Mortar
A rich cement mortar, usually 1:3 cement to sand, can be used as a horizontal damp proof course where moisture loads are modest. Waterproofing compounds are added to the mix to reduce capillary suction. A mortar DPC is cheap and simple, but it is only as good as its compaction, and it must be laid in one continuous operation to avoid cold joints.
Cement Concrete
Cement concrete DPCs are cast in place over the wall with a mix near 1:2:4. A thickness of 40 to 50 mm is common, and light mesh reinforcement is often added where the course crosses openings. Concrete DPCs are rigid, so they suit walls that are not expected to move and must be keyed to the masonry to prevent sliding.
Plastic Sheets
Polyethylene and other plastic sheets are the modern low-cost option. They are laid on a mortar bed with joints lapped at least 100 mm and sealed with adhesive or tape. Plastic is completely impervious but thin and easily punctured by sharp stones in the bedding, so the mortar bed must be smooth and free of debris.
Selecting and Installing a Damp Proof Course
Selection Criteria
The choice between these materials comes down to a handful of site conditions. Check each one before specifying:
- The load on the wall and whether the structure is expected to settle or move
- The severity of the moisture source, from a dry subsoil to a high water table
- The wall thickness and whether a full-width membrane is practical
- The climate, since heat softens bitumen and cold makes felts brittle
- The budget and the skill of the crew placing the material
Installation Sequence
A damp proof course fails most often because of bad placement, not bad material. The sequence that gets it right is:
- Prepare the footing so the top of the foundation is level and clean.
- Brush the surface and remove mortar droppings and sharp projections.
- Lay a uniform bed of mortar or concrete to receive the membrane.
- Place the DPC across the full wall width, including cavity trays in hollow walls.
- Lap joints by at least 100 mm and seal the laps with bitumen or adhesive.
- Bed the next course immediately so the membrane is held in place.
- Inspect exposed edges before backfilling and patch any damage.
Performance Upgrades and Interior Finishes
Buildings with demanding moisture environments often go beyond traditional membranes, and advanced construction materials such as fiber reinforced polymers and cross laminated timber bring different loads, movement, and detailing that the DPC must work with.
The damp proof course is only the first line of defence. Interior surfaces still need protection wherever humidity runs high, and the same care applied to installing drywall in damp areas keeps finishes sound long after the membrane is buried. Specify the material against the moisture load, place it to the minimum thickness or overlap, and protect it during construction, and the wall stays dry for the life of the building.