Damp proofing

Damp proofing in construction is a protective measure applied to building foundation walls to prevent moisture from passing through the walls into interior spaces. A damp-proof course (often abbreviated to DPC) is a horizontal barrier in a wall designed to prevent moisture rising through the structure by capillary action - a phenomenon known as rising damp. A damp-proof membrane (DPM) performs a similar function for a solid floor.

Background

Rising damp can arise for various reasons - the failure of an existing damp proof course, bridging due to the raising of external ground or internal floor levels, or in older buildings, the complete absence of a damp proof course.

Brick, stone and mortar are porous allowing damp from the ground to rise by capillary action, carrying with it ground salts including chlorides and nitrates. These salts from the ground can absorb moisture from the atmosphere leading to wall dampness in conditions of high relative humidity. Also they can ruin decorations and break down internal plaster.

A Roman damp-proof course at Hardknott Roman Fort

Damp-proof courses have been used since ancient times. The Romans used a horizontal course of slate inserted in a wall to act as an impervious barrier.

Building standards in many countries require most new buildings to incorporate a DPC/DPM at the time of construction. This may consist of a thin strip of plastic, a course of engineering brick or slate, or a layer of bitumen.

Construction

A DPC is usually a thick plastic strip bedded into the mortar between two courses of bricks or blocks. It can often be seen as a thin plastic line in the mortar near ground level.

A DPM is usually a thick polythene sheet laid under the floor slab, to allow the slab to dry out and keep out groundwater. It is often laid on a bed of sand, to prevent the sharp edges of the hardcore damaging it.

To create a continuous barrier, pieces of DPC or DPM are welded together. In addition, the DPC is welded to the DPM around the outside edges of the ground floor, completely sealing the inside of the building from the damp ground under it.

In a cavity wall, there is usually a DPC in both the outer and inner wall. In the outer wall it is normally 150-200mm above ground level (the height of 2-3 brick courses). This allows rain to form puddles and splash up off the ground, without saturating the wall above DPC level. The wall below the DPC may become saturated in rainy weather. The DPC in the inner wall is usually below floor level, (under a suspended timber floor structure), or, with a solid concrete floor, it is usually found immediately above the floor slab so that it can be linked to the DPM under the floor slab. This enables installation of skirting boards above floor level without fear of puncturing it. Alternatively, instead of fitting separate inner and outer DPCs, it is common in commercial housebuilding to use a one-piece length of rigid plastic, (albeit an angled section), which fits neatly across the cavity and slots into both walls (a cavity tray). This method requires the need for weep vents to enable rainwater ingress to drain from the cavities otherwise rising dampness could occur from above the DPC.

Remedial DPC

In old buildings there may be a DPC made from lead. The DPM may be non-existent, leading to damp problems, or it may rely on an impermeable floor finish such as ceramic tiles to keep most of the damp out.

Where a DPC is absent or inadequate, there are various means of retrospectively fitting one. A common method in masonry walls is to drill holes into the wall at regular intervals and inject a penetrating liquid (e.g. silicone) into the holes. The chemical is absorbed into the masonry where it cures to form a waterproof barrier. More recently, damp-proofing creams have been introduced which are faster to install and do not require specialist pumping equipment. Whether in liquid or cream form, the effectiveness of chemical damp-proofing products depends on a number of factors including product strength, the types of active ingredients in the formulation, the delivery system (e.g. solvents and surfactants), and the suitability of the system for the substrate that it is being injected into. Some forms of the chemical are odour free, some have a strong odour, this should be taken into account when purchasing.

Damp proofing remedies include:

• Silane Diffusion: Utilising a concentrated thixotropic silane / silicone ‘cream’
• Siliconate Transfusion: A gravity fed system with no wastage via hidden voids
• Siliconate Injection: Recommended occasionally for single brick walls

Health and Safety

Some DPC materials may contain asbestos fibres. This was more commonly found in the older, grey sealents as well as flexible tar boards.

Other possibly hazardous materials include the use of lead sheets as a DPC material.

References

• Dampness in Buildings and Damp Course