- Project plans
- Project activities
- Legislation and standards
- Industry context
- Specialist wikis
Last edited 24 Mar 2021
Damp proof membrane DPM
Damp in buildings can cause a number of serious problems, such as:
- Damp patches.
- Mould growth, which is a cause of respiratory allergies.
- Mildew, salts, staining and ‘tide marks’.
- Damage to surface finishes.
- Corrosion and decay of the building fabric.
- Slip hazards.
- Frost damage.
- Poor performance of insulation.
- Damage to equipment, or electrical failure.
- Surface condensation.
- Interstitial condensation (condensation within the fabric of a building's construction, either on the surfaces of components that make up the fabric, or sometimes within the components themselves).
- Penetrating damp.
- Rising damp.
- Resist the passage of ground moisture to the upper surface of the floor.
- Not be damaged by moisture from the ground.
- Not be damaged by groundwater.
- Be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation.
- Should not promote surface condensation or mould growth.
A damp-proof membrane (DPM) is a membrane material applied to prevent moisture transmission. Typically, a DPM is a polyethylene sheet laid under a concrete slab to prevent the concrete from gaining moisture through capillary action.
The approved document suggests that a ground-supported floor will meet these requirements if the ground is covered with dense concrete laid on a hardcore bed and a DPM is provided. It suggests that the damp proof membrane may be above or below the concrete, and continuous with the damp proof courses (DPC) in walls, piers, and so on.
If the ground could contain water soluble sulphates, or there is any risk that sulphate or other deleterious matter could contaminate the hardcore, the membrane should be placed at the base of the concrete slab.
The approved document proposes that:
- A membrane below the concrete could be formed with a sheet of polyethylene, which should be at least 300μm thick (1200 gauge) with sealed joints and laid on a bed of material that will not damage the sheet.
- A membrane laid above the concrete may be either polyethylene sheet as described above (but without the bedding material) or three coats of cold applied bitumen solution or similar moisture and water vapour resisting material. It should be protected by either a screed or a floor finish, unless the membrane is pitchmastic or similar material which will also serve as a floor finish.
A timber floor finish laid directly on concrete may be bedded in a material which may also serve as a damp-proof membrane. Timber fillets laid in the concrete as a fixing for a floor finish should be treated with an effective preservative unless they are above the damp-proof membrane.
 Related articles on Designing Buildings Wiki
- Basement waterproofing.
- Breather membrane.
- Capillary action.
- Cavity tray.
- Chemical injected DPC.
- Damp proof course.
- Damp proofing.
- Defects in brickwork
- Defects in stonework.
- Does damp proofing work?
- Dew point.
- Interstitial condensation.
- Penetrating damp.
- Rising damp.
- Vapour barrier.
Featured articles and news
Organisation releases mobile app for its members.
BSRIA explores US share of 2020 VRF market.
New fire safety requirement comes into force.
Different types of bridges are meant to move.
A logical approach to handling the internal voice of self doubt.
First fashionable in the US, decorative metal has become globally desirable.
Helping communities preserve and enhance historic environments.
Creating comfortable climates despite extreme temperatures.
Study examines how adjustable arrangements can succeed.
Government announces plans to improve accessibility.
Resource addresses pandemic-related NEC4 contract issues.
Incorporating EDI into the provision of fair access.