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:

• Condensation (surface or interstitial).
• Penetrating damp.
• Rising damp.

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 150mm 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:

• Continuous with any damp-proof membrane in the floor.
• At least 150mm above the level of the adjoining ground if it is in an external wall.
• If it is in an external cavity wall, the cavity should extend at least 225mm below the damp-proof course, or a cavity tray should be provided with weep holes every 900mm so that water running down the cavity cannot pass to the inner leaf.

[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 1m.

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 150mm above the external ground level, for example if the building has a solid external wall and the internal floor level is less than 150mm above the external ground level, external drainage solutions may be necessary, such as the installation of a french drain.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

• Blockwork.
• Breather membrane.
• Cavity tray.
• Cavity wall.
• Cold bridge
• Condensation.
• Damp.
• Damp proof membrane.
• Defects in brickwork
• Defects in stonework.
• Dew point.
• Flashing.
• French drain.
• Humidity.
• Insulation.
• Interstitial condensation.
• Penetrating damp.
• Rising damp.
• Rising damp in walls - diagnosis and treatment (DG 245).
• Vapour barrier.
• Wall ties.
• Wall tie failure.
• Weep hole.

Comments

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.

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