Last edited 16 Jun 2018

Damp-proof course DPC

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:

Rising damp is caused by capillary action drawing moisture up through the porous elements of a building’s fabric. Rising damp, and some penetrating damp, can be caused by faults to, or the absence of a damp-proof course (DPC) or damp-proof membrane (DPM).

A damp-proof course is a barrier, usually formed by a membrane built into the walls of a property, typically 150 mm above ground level, to prevent damp rising through the walls. Historically, damp-proof courses may have been formed using bitumen, slates, lead, pitch, asphalt or low absorption bricks. They emerged during the Victorian era and are commonly found in buildings from around 1900.

Damp-proof courses are now required in the construction of new buildings to prevent rising damp and in some situations to prevent penetrating damp. Approved document C of the Building Regulations, Site preparation and resistance to contaminants and moisture, suggests that a damp-proof course may be a, ‘…bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture.’

Approved document C requires that, to prevent rising damp, a damp-proof course should be:

Damp proof course.jpg[image source Approved document C, Site preparation and resistance to contaminants and moisture]

A damp-proof course may also be required:

  • In masonry walls below a coping, where the coping is constructed from a material that is not impervious to water.
  • In the joints between walls and door and window frames.
  • In suspended timber ground floors between the timber and materials that can carry moisture from the ground.

Standards for damp-proof courses are provided in BS 8215:1991 Code of practice for design and installation of damp-proof courses in masonry construction.

The absence of a damp-proof course in older buildings can be rectified by creating a moisture-impermeable layer, either by the insertion of a damp-proof course, or by the injection of water-repellent chemicals. Treatment generally also involves remedial work to any corroded or decayed elements of the building fabric, as well as hacking off and replacing existing plaster to a height of 1 m.

However, damp in older buildings is actually often caused by a leak or a defect in the wall construction, such as a cracking, rather than by rising damp, and this may not be rectified by the insertion of a damp-proof course. It is important therefore that any defects are identified and corrected first before accepting the cost and disruption of inserting a damp-proof course.

Where it is not possible to insert a damp-proof course 150 mm above the external ground level, for example if the building has a solid external wall and the internal floor level is less than 150 mm above the external ground level, external drainage solutions may be necessary, such as the installation of a french drain.

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Please note that retrofitting damp proof courses does not work in solid walled constructed buildings and can cause more harm than good. Simply direct water away from the building by having lower external ground levels than internal floor levels where possible and install free flowing land drains around the building.

Can you explain why you think it doesn't work?