Damp Proofing Information

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Water proofing

It is common where walls and floors are located below ground level for ingress of either dampness or even running water. This lateral and rising damp penetration is caused either by capillary action or by hydrostatic pressure. The term for overcoming such waterproofing situations in cellars and basements is 'tanking'. A good tanking system can perform well enough to enable cellars and basements to be transformed into habitable living rooms and bedroom areas.

A trend over the last decade has evolved whereby property owners are actively having basement areas added to houses which were originally built without them. This has become very much the vogue in areas of high property value such as London, where in some areas there is hardly a street without several houses having 'basement creations' constructed. Whilst the techniques for adequate waterproofing of new basements is excellent, the disruption and diversion such operations can have on the local water table, are in some cases inadvertently causing neighbouring properties significant damp and flooding problems.

Tanking traditionally took the form of an asphalt lining applied to both the basement walls and floors. Today the method more often constitutes a cemetitious render coating or a cavity air gap membrane.

Cavity air gap membranes /cavity drainage membranes

This membrane is a high density polyethylene extruded membrane which is fixed mechanically to the walls, ceilings and floors but with an air gap for the movement of water vapour. The membrane has studs varying from 3mm to 20mm which retain an air gap between the wall and the membrane. Any water egress behind the membrane will discharge down the wall and into a secret perimeter drainage system. This drainage system is laid at a slight fall and water is then collected in a sump unit which incorporates a pump to then discharge the water to the nearest drain. The wall membrane is mechanically fixed through the studs using a waterproof fixing, complete with rubber waterproof grommet. An independent stud partition is then erected on the inside of the membrane which then accommodates wiring, pipe work, plasterboard and finishes in the usual manner. Metal studding is particularly suitable for basement applications.

Floor membranes similarly have a small air gap and the polyethylene membrane is extremely crush resistant thereby allowing a floor screed to be applied as a useable finish. Alternatively a floating floor comprising polystyrene insulation and tongue and grooved chipboard or Weyroc can be used.

Cavity air gap waterproofing systems are relatively fast to install and commonly have a 10 to 30 year guarantee. It is normal to have the sump and pump unit maintained annually to ensure no build up of grit or debris. With the current and future increase in flood risks some consideration should be given to an independent power pack that provides a battery back up supply should a power cut occur. Similarly high level water alarms can be fitted to advise if the unit is failing or water is rising unexpectedly fast.

It should be expected that any membrane waterproofing system would carry a BBA (British Board of Agreement) Certificate for the membrane material.

Cemetitious Multi-Coat Render (tanking)

Another popular method of basement waterproofing is a cemetitious render whereby multiple coats of render, with a waterproofing agent in a dense sand/cement mix, is applied to the walls and floor. The technique endeavours to create a fully lapped joint of each wall and floor layer of render, thereby creating a fully integrated waterproof lining.

It is vital that the basement surface is fully prepared to form an excellent key for bonding purposes with no loose material or contamination. Whilst this method of waterproofing has proved successful in the past, there is no mechanism for the release of any hydrostatic build up of water pressure and thus a sealed membrane system is often a preferred method. Additionally the cemetitious render must not be punctured in any way thus fixing of sockets, pipes and light fittings can be an issue and an inner stud partition leaf is often used.

Damp Proof Course treatments.

Rising damp is the blight of thousands of buildings throughout the UK. Modern houses tend to have a full damp proof membrane located just above outside ground level. Previous to this a blue brick course was used for a degree of damp proofing but this did not stop some penetration through the mortar joints. Many older existing houses have no damp proof course whatsoever.

Chemical Injection Damp proof Courses

Typically a chemical injection damp proof course is created by drilling holes at about 100mm centres in the brickwork or mortar joints at ground level. A damp proof course fluid is then pumped into these holes under pressure. The fluid reacts with carbon dioxide to form a water repellent silicone based resin within the capillaries of the bricks or mortar. This resin stops the further passage of water but is permeable to water vapour thereby allowing the materials to breathe and dry out.

Many companies also offer an 'organic' based injection method whereby the damp proof course fluid reacts with the water present in the brickwork to form the impermeable barrier.

With most chemical damp proof course systems it is vital to remove the lower 1m of plaster from the internal wall since this is usually contaminated with hygroscopic salts. The walls are then re-plastered with a suitable 'renovating plaster'. The renovating plaster is typically a cemetitious, lightweight plaster containing lime which acts to inhibit mould formation. If renovating plaster is not used a 1:3 sand cement render can be used but the sand must be sharp sand to BS1199 and a salt retardant additive is required. A layer of thistle board finishing plaster or a multi finish is then applied. Carlite finish is not suitable.

Gel and Cream Injection Damp Proof Courses

In recent years a new generation of damp proofing products has emerged. Water soluble thrixotropic gels can be injected into any damp substrate using a simple cartridge gun or compression pump. This gel then reacts to form a silicone repellent which is completely water resistant.

Silicone emulsion creams are injected into any form of masonry at low pressure through holes pre-drilled. This formulation then creates polysiloxanes once the liquid cream has migrated into all of the pores of the substrate.

Where very thick walls require damp proofing or those which perhaps have an element of rubble filling or partial voids, it is possible to inject a waterproof mortar. This mortar comprises cement, waterproofing chemical and a quartz aggregate. This method is ideal for old and historic buildings where the wall structure is often not fully consistent.

Finally, an electric charge system known as electro osmosis can be used. In this system platinum coated, titanium wire anodes are drilled into the wall of the building. These are linked to each other by titanium wire and form a complete circuit around the building. The circuit is then connected to a low voltage power supply. By adding an earth rod to the circuit a short distance away (a cathode), a potential difference is created effectively stopping the water rising and effectively pulling any moisture back towards the ground. The system can be installed internally or externally and thus is ideal for listed buildings. Externally, the titanium wire can be placed within a bed joint of the masonry and over pointed. Electro osmotic damp proof courses have been tried and tested over many years with very successful results.