Last edited 11 Sep 2019

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.

The most common causes of persistent damp in buildings are:

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.

Approved document C, Site preparation and resistance to contaminants and moisture, requires that (where appropriate) floors next to the ground should:

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.

In order to resist degradation, insulation placed below the damp proof membrane should have low water absorption. If necessary the insulant should be resistant to contaminants in the ground.

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.