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Last edited 16 Dec 2021
Types of damp-proof courses
These terraced houses near Bridport were made from brick with the lower courses of the wall made of Bothenhampton stone (also known as Baunton stone). This local stone is naturally impervious to water and acts as a damp-proof course.
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).
Approved document C of the Building Regulations, Site preparation and resistance to contaminants and moisture, suggests that a DPC may be a, ‘…bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture.’
DPCs can be created by impervious materials or techniques, including:
- Bitumen roll.
- Slates and cement mortar.
- Mastic asphalt.
- Lead or copper sheeting.
- Tar and pitch.
- Engineering bricks.
- Chemical injections.
Bitumen is a petroleum-based hydrocarbon that occurs naturally as a concentrated substance in oil sands and pitch lakes. For standard DPC purposes, bitumen roll DPC may consist of a thin layer of non-viscous bituminous material made with a waterproof hessian cloth base. Extra protection may be provided by using a thin layer of copper or lead foil interlining instead of hessian cloth.
 Slates and cement mortar DPC
It is possible to create an effective form of DPC by constructing a double layer of slates in cement mortar. Any type of slates can be used, and the mortar can be made up of two parts clean sand and one part Portland cement.
Ancient Romans used slate without mortar as a DPC to block rising damp. In this example from the Hardknott Roman fort in Cumbria, the slate also serves to separate the original Roman construction (below) from modern reconstruction (above).
To use this method of DPC requires temperatures high enough to melt natural asphalt. Pure bitumen is then added to the heated asphalt until a workable consistency is achieved. This substance is then spread on the wall.
This can be an expensive method and may require an advanced level of expertise.
When used for DPC, copper sheets may become corroded over time.
In some instances, non-ferrous metallic materials may be susceptible to decay when attacked by salts that might occur in damp mortar. This can reduce the effectiveness as the thickness of the material decreases.
Early forms of impervious bricks were sometimes referred to as blue bricks. A higher firing temperature typically gave these bricks their blue colour (although in some instances they may appear black).
 Chemical injection DPC
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