Damp proofing

A damp proofing barrier

Damp proofing in construction is a type of waterproofing 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 resist 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. Moisture resistance is not necessarily absolute: it is usually defined by a specific test method, limits, and engineering tolerances. In simpler words, DPC is used to stop dampness in buildings. In theory, due to capillary movement of water, water rises from earth to the building. Passing through foundation it rises higher to reach walls. Reaching water to walls may damage them by creating cracks, breaking cement-paint bonds and creating dark-spots on wall etc. So, to avoid water from reaching to walls, we lay DPC layer at plinth level (the joint level of walls and foundations). DPC layer is usually laid below all the walls unaffected from the issue that the respective wall is a load bearing wall or a partition wall. Usually, DPC membrane is 4" to 9" wide.

In practise, Rising Damp almost never occurs. Nearly every occurrence of so called 'Rising Damp' can be properly diagnosed as related to condensation, penetrating damp, or the improper use of impermeable materials. For example, the widespread use of impervious gypsum plasters in solid wall, lime mortar constructed houses traps condensation and creates damp problems which are incorrectly interpreted by chemical salesmen and 'damp surveyors' as Rising Damp. Similarly, cement render applied to the outside of walls built in this way will create the same symptoms. It is vital to correctly diagnose causes of damp problems - many older, solid wall construction houses now have several useless injection damp proofing courses as a result of incorrect diagnosis.

Background

Rising damp in theory can occur 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. in older days, stone-slab was mostly used as DPC material.

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.

Materials

Materials widely used for damp proofing include:^[1]

• Flexible materials like hot bitumen, plastic sheets, bituminous felts, sheets of lead, copper, etc.
• Semi-rigid materials like mastic asphalt
• Rigid materials like impervious bricks, stones, slates, cement mortar or cement concrete painted with bitumen, etc.
• Stones
• Mortar with waterproofing compounds
• Coarse sand layers under floors
• Continuous plastic sheets under floors

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, mold health issues or generally a poor indoor air quality, 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 retrofitting 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; others have a strong odour.

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 sealants as well as flexible tar boards.

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

References

1. ^ P.C. Varghese (2005). Building Materials. PHI Learning Pvt. Ltd.. p. 230. ISBN 8120328485. http://books.google.com/books?id=MBamtq45OykC&pg=PT248&lpg=PT248#v=onepage&q&f=false.