Damp Proof Course (DPC) Treatment in London

What is a Damp Proof Course?

Damp Proof Course is a form of barrier installed at the lower level of the wall of a building to prevent water from rising up that way. They can be made of non-absorbent, water-resistant materials such as slate, bitumen and plastic. 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 150 mm 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 225 mm below the damp-proof course, or a cavity tray should be provided with weep holes every 900 mm so that water running down the cavity cannot pass to the inner leaf. Damp-proof courses are now required in the construction of new buildings to prevent rising damp and in some situations to prevent penetrating damp.

Over the years, construction practices have changed as has the DPCs used in our homes. Initially, many homes used slate tiles, yet with the modern plastic damp course the cost is far lower, it has become the new standard. Stainless steel is also used in many modern construction projects.

Where do you find your damp proof course?

Since raindrops typically bounce approximately 150mm after hitting the ground, this ensures the wall above the damp proof course is well protected from water saturation. Damp Proof Course is often a black material and should be visible (not mortared over). If it is mortared over (often done for aesthetic reasons) the mortar can act as a ‘bridge’ between the brick layers above and below the damp proof course reducing its benefit.

What Materials are used for damp proofing?

These are generally classified into two: Flexible Materials for Damp Proofing and Rigid Material for Damp Proofing.

1) Flexible Materials for Damp Proof Course

These are materials which do not crack, break or deform under pressure or load. Most commonly used examples include;

a) Bitumen Mastic (Mastic Asphalt)
These materials demand special care when laying. It consists of asphalt or bitumen mixed with fine sand in a hot state to form an impervious mass. Due to this consistency, it can be spread (when hot) to a depth of 2.5cm to 5cm, which sets on cooling. It provides good impervious layer.

b) Bitumen Felts (Sheets)
It consists of 6mm thick sheet of bitumen prepared in rolls having a width equal to that of a brick wall.

c) Hot laid Bitumen
This material is used on a bedding of cement concrete or mortar. This should be applied in two layers at the rate of 1.75kg/m2 of the area.

d) Metal Sheets
Metal sheets of Copper, Aluminum, or Lead are used to prevent dampness, but they are costly. Sheets of these materials are used throughout the thickness of the wall. The sheets of Lead are laid over lime mortar and not with cement mortar. Due to the chemical reaction of Cement over the Lead, the sheets of metal should be coated with asphalt. The thickness of the sheets should not be less than 3mm.

2) Rigid Materials for Damp Proof Course

These materials do not resist transverse stresses and cracks when subjected to severe loading.

a) Rich Concrete
1.2cm to a 4cm thick layer of Rich Concrete (1:2:4) painted with two coats of hot bitumen is used as horizontal D.P.C. It also prevents moisture penetration by capillary action. These layers are laid where the damp is not excessive.

b) Mortar:
A 2cm thick layer of Rich Cement and Sand Mortar (1:3) is applied on the inner face of the external wall. This is a vertical D.P.C. The surface is then painted with two coats of hot bitumen.

c) Bricks:
Over burnt or dense bricks in one or two layers can be used as cheap and effective DPC. They are laid in Rich Cement and Sand Mortar. Bricks are rarely used as DPC except in cheap houses.

d) Stones or Slates:
Two layers of stone slabs or slates laid in lime, cement and sand mortar (1:1:6) make the best DPCs. They can also be laid in cement and sand mortar. It is used where a good quality stone is easily and cheaply available.

What are the Benefits of Damp Proof Course?

1. No Foul Smell
2. Improved Appearance
3. Increase the Value of your Home
4. Damp proofing can get rid of excess mildew and bacteria. These could be triggers for people suffering from asthma or bronchitis.
5. Damp Proofing prevents moisture from seeping into the walls and can get rid of residual moisture.

FAQ’s