How to damp proof walls

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How to repair or replace a damp proof course.

What is a damp proof course?

A damp proof course is a physical or chemical barrier that works to prevent building materials from being saturated by water from the ground.

Most modern houses are built with a physical damp proof course in the walls. A plastic or rubber strip at 150mm above the outside ground level which is positioned in horizontally in the mortar course.

How do damp proof courses fail?

Every now and then in new build properties the damp proof course omitted from the construction or not installed in the wrong place.

Older buildings may not have an effective damp proof course.

Bridging of the damp proof course. This is when the outside ground level has been raised to above the damp proof course. As damp proof courses only work by stopping water from rising up from underneath bridging will stop it from working.

Internal floor levels may have been raised to above the position of the damp proof course. This is common when a timber floor is replaced with a solid concrete slab as part of refurbishment project.

Occasionally some types of damp proof course may break down over time and fail or be damaged due to structural movement, though this is rare.

Why is rising damp a problem?

A lack of an effective damp proof course can allow the walls to be saturated by moisture from the ground. Water is able to rise up through the masonry by capillary action, a process called rising damp.

Rising dampness can cause the following problems:

Damage to wall finishes, paints and wallpapers.

Groundwater will often contain salts which will contaminate wall plasters causing them to expand and fail.

Timbers embedded in the wall can decay when exposed to moisture.

Wet walls are difficult to heat. Rising Damp can cause a big increase in energy bills.

How to repair a damp proof course.

While it is possible to chase out the mortar course and insert a physical damp proof course, this would be very labour intensive and expensive.

Koster Crisin can be used to install a new damp proof course into a wall. Koster Crisin contains water repellent silicones and resins that prevent water from being able to rise up through the masonry. The quality of the damp proofing cream is important, some cheaper products rely on silicone only and struggle to work on really wet walls.

If the external ground level has been raised the new damp proof course will be installed at 150mm above the outside ground level. We recommend to seal the area below the new damp proof course to prevent dampness from tracking across at low level. Koster NB1 Grey can be used to seal against moisture and salt.
It is very likely that any internal wall plaster will be contaminated with salts and will need to be replaced. If old unsuitable renders are not removed and renewed, contaminant salts can act to suck moisture from the air and remain damp.
Koster Restoration Plaster can be used to replace the existing wall plaster. Typically wall plas-ter is replaced up to a height of 1200mm. Koster Restoration Plaster is both salt resistant and breathable. This means that it wont be damaged by any residual salt left in the wall and also allows the wall to breath, speeding up the drying out process.

An alternative to a wet render system is to use Delta PT Slimline. This 4mm thick membrane is fixed to the walls using Delta PT Plugs. The mesh on the surface allows for the direct application of plasterboard by dot an dab adhesion.

For our full range of damp proofing systems click here.