Dampness is one of the most common causes of premature failure in buildings, and it arrives by three routes: rising damp from the ground, penetrating damp through walls and roofs, and condensation from interior humidity. Stopping it requires a barrier, and the most common barrier is a damp proof course, or DPC. Before choosing a treatment, it pays to understand waterproofing versus damp proofing, because the two terms are often used interchangeably even though they protect a building in different ways. This article covers what a DPC must do, the general principles of damp-proofing, the five standard methods, and how to select materials for a given site.
How Dampness Enters a Building and What a DPC Must Do
A damp proof course is a layer of water-repellent material interposed between the source of moisture and the structure. It stops water from rising out of the sub-soil and climbing the walls, and it can run horizontally or vertically depending on where the water comes from. A typical horizontal DPC about 2.5 cm thick sits in the wall above ground level, while vertical sections protect wall-floor junctions.
Dampness damage shows up as efflorescence on brickwork, peeling plaster, rotting skirting boards, and a musty smell that no amount of cleaning removes. Catching it early matters, because salts carried by rising damp keep damaging finishes long after the visible stain dries.
The Five Requirements of a Good DPC
- It must be totally impervious and must not allow the travel of dampness.
- It must be durable and keep the building damp-proof permanently.
- It must be economical and easy to lay.
- It must resist the loads to which it is subjected.
- It must accommodate some structural movement without fracture.
Material choice and detailing come together in damp masonry design, where the DPC is only one element of a moisture strategy that includes copings, flashings, and drainage around the base of the wall.
DPC Level in Construction
The DPC is placed above the finished ground line, typically at least 150 mm above external ground and 100 mm above internal floor level, so that splash and surface runoff cannot bypass it. In cavity walls the DPC bridges the cavity at openings, and weep holes let trapped water escape.
General Principles of Damp-Proofing
Every damp-proofing method follows the same set of rules, and the rules matter more than the material.
Placement Rules That Apply to Every Method
- The damp proof course may be horizontal or vertical, matching the direction of moisture travel.
- A horizontal DPC should cover the full thickness of the wall.
- At junctions and corners the horizontal DPC should be laid continuous, with no gaps.
- The DPC should be laid as one continuous run, so the protection forms an unbroken barrier.
For a worked comparison of each technique, the write-up on methods of damp proofing in building walks through the same five options with sketches and material notes.
Choosing DPC Materials
The effective DPC material depends on the climatic and atmospheric conditions of the area, the site, and the nature of the structure. The materials below cover the range from flexible membranes to rigid courses.
| Material | Form | Typical use | Notes |
|---|---|---|---|
| Bituminous felt | Flexible sheet | Walls and floors | Cheap, must be lapped and sealed at joints |
| Mastic asphalt | Hot-applied layer | Floors and tanking | Continuous, no joints, needs protection from damage |
| Plastic or PVC sheet | Flexible membrane | Walls and under slabs | Durable, resists most soil conditions |
| Cement concrete with waterproofing compound | Rigid course | Walls and floors | Strong, load-bearing, laid in mortar |
| Copper or lead sheet | Metal membrane | Critical junctions | Very durable, expensive |
| Slates, stones, and bricks painted with bitumen | Rigid course | Walls | Local materials, bedded in mortar |
Two coats of a good bituminous felt with sealed laps handle most residential walls. For basements and below-ground rooms, mastic asphalt or a PVC membrane with welded joints is the more reliable choice, because a single pinhole in a flexible sheet becomes a permanent leak once the wall is backfilled.
The Five Standard Methods of Damp-Proofing
Five methods are generally adopted to prevent dampness in a structure, and they are often combined in a single building.
- Membrane damp-proofing: a layer of water-repellent material, the DPC itself, placed between the source of dampness and the structure.
- Integral damp-proofing: waterproofing compounds added to the concrete mix to increase its impermeability.
- Surface treatment: water-repellent coatings applied to the exposed faces of walls.
- Guniting: a cement mortar layer sprayed onto the surface at high velocity.
- Cavity wall construction: a hollow wall that stops moisture crossing the gap.
Membrane Damp-Proofing
The membrane may be bituminous felt, mastic asphalt, silicone, epoxy, polymers, plastics, or cement concrete, and it is placed horizontally or vertically depending on the source of dampness. It is the most common method because it works on walls and floors alike and can be inspected before it is covered.
The membrane must extend through the full thickness of the wall and lap at corners and junctions, because a break anywhere in the line gives damp a direct path. At openings such as doors and windows, the DPC turns up or down to shed water past the frame.
Integral Damp-Proofing
Waterproofing compounds come in powder and liquid form. Compounds made from clay, sand, or lime fill the voids in concrete and make it waterproof by densifying the paste. Other compounds, such as alkaline silicates, aluminum sulfates, and calcium chlorides, react chemically in the mix to form insoluble products that block the capillary pores. Integral treatment suits foundations, basements, and water-retaining structures where a surface layer would be damaged.
Surface Treatment, Guniting, and Grouting
Surface treatment applies a water-repellent coating to the exposed face of a wall. It works for light exposure but fails where water pressure builds up. Guniting sprays a dense mortar onto the wall at high velocity, repairing cracks and adding a protective shell. Pressure grouting, also called cementation, injects grout into cracks and voids to seal them from within.
Once the structure is dry, the same logic carries inside, where moisture protection behind interior finishes like drywall keeps damp from returning through the finished surface and damaging the decoration.
Cavity Walls and Special Cases
How Cavity Wall Construction Stops Damp
A cavity wall has two leaves separated by a continuous air gap. Moisture that penetrates the outer leaf hits the gap and runs down to the base, where weep holes drain it out, so the inner leaf stays dry. The gap must stay clean and bridged only by wall ties designed to shed water.
Retrofitting cavity wall insulation must preserve that gap, because a filled or bridged cavity becomes a path for damp rather than a barrier.
Signs that a cavity wall is failing include damp patches that appear after heavy rain, often at floor level or around openings. The usual causes are mortar droppings bridging the cavity, missing weep holes, or insulation installed without regard for the gap.
Pressure Grouting and Cementation
Where cracks already admit water, pressure grouting forces a cement or chemical grout into the voids under pressure, sealing the wall from inside. It suits existing buildings where opening the wall is not practical, and it is often paired with surface treatment on the exposed face.
Selecting the Right DPC for Your Project
Selection Criteria
The choice of effective DPC materials requires a judicious selection based on the climatic and atmospheric conditions of the area and site, the nature of the structure, the loads on the course, and the cost. Flexible membranes suit movement-prone buildings, rigid courses suit load-bearing walls, and metal courses are reserved for critical junctions where failure would be expensive to fix.
Thickness and Positioning Details
Typical thicknesses run from 2.5 cm for cement concrete courses to a few millimeters for plastic sheet. The horizontal course sits at plinth level above ground, vertical courses protect wall-floor junctions, and all laps are sealed in the direction of water travel. The practical details of damp proof course construction, from material preparation to lapping at corners, decide whether the barrier works on site.
Costs vary widely with the method. A bituminous felt DPC is the least expensive per meter of wall, integral treatments add a few percent to the concrete bill, and tanking a basement with mastic asphalt costs several times as much. The premium pays for redundancy in the places where a leak is hardest to fix.
No DPC survives a wet foundation, so foundation drainage and damp proofing are planned as a pair: the drainage keeps water away from the structure, and the DPC stops whatever remains from climbing the walls. A building that controls all three routes of dampness keeps its structure sound, its interior healthier, and its maintenance costs lower over the full service life.