Damp Away Course

What is a damp away course and why do I need one?

A damp away course or dpc is an essential part of the construction process in that its function is to prevent rising dampness.

Rising dampness is a result of dampness in the ground being drawn up through the capillary channels in any given building fabric.

Generally a dpc will be installed when the building is constructed. In new buildings the damp away course is physical membrane. This will be laid into a mortar course at the base of the walls. In older buildings it is common for slate or bitumen to be used as the damp away course and some older buildings will not have a damp away course at all.

As said, all of the above provide a physical barrier against rising dampness but why is this necessary? In short it prevents spoiling of the internal wall finishes. When a wall suffers from rising dampness the associated affects to the internal finishes is staining, salting and peeling paint or paper, blown plaster, rotten skirting boards etc.

A damp away course is designed to stop the dampness at low level before it has a chance to do any damage higher up the wall.

Sometimes an older dpc will fail. If it does or if there isn't one can be installed as a remedial or retrofit measure. Typically one of the following methods would be used:

• Chemical damp away course

This involves drilling a series of holes 150mm above the external ground level along the base of the walls and injecting a silicone fluid or cream. The idea is that the silicone content in these products is designed to repel moisture rising up the wall.

My opinion is that this method is a little hit and miss in that the density and quality of the wall will have a big baring on whether it will be affective as will the application method in terms of drilling hole to the correct diameter and depth and volume of chemical installed which is often very hard to gauge. This is why you will often fins a chemical damp away course is recommended along with the removal of internal plaster and re-plastering with a waterproofing additive. The idea is if one doesn't work hopefully the other will.

• Physical Damp away course installation

Believe it or not is possible to retro fit a physical damp away course. Mortar joints along the walls are cut out with an electric joint saw and a new physical damp away course is inserted in sections. The wall is then made good around it.

My opinion is that long term effectiveness should be very good however it is likely to be expensive and externally disruptive because someone is literally cutting through the walls of your house. Imagine the dust!!

• Electro osmosis damp away course

A titanium wire is passed around the walls to be treated and at regular intervals this wire is pushed into holes made in the brickwork, then it is earthed and a safe voltage is applied, this stops the dampness from rising up the masonry.

A pretty unusual method which has opinions divided as to its effectiveness. In my opinion has never really been proved to be an effective damp away course and as such is very rarely seen or used these days.

• Cavity membranes

Not a damp away course as such but more an alternative to one. These systems are becoming ever more popular as an effective solution to rising dampness in buildings.

Cavity membranes are high density polypropylene and polyethylene plastic membranes. The rolls of plastic are molded into a stud formation to give and "egg box" type appearance and for damp proofing purposes they usually have a mesh welded to the surface to provide a base to plaster or dab fix plasterboard onto.

They are described as cavity membranes because the stud profile creates an air gap between the membrane and the wall to which it is applied. Once applied to the wall (usually up to at least 1m) The membrane allows any moisture in the wall to evaporate into the air gap but because it is free to move unhindered it is not pushed to adjacent areas or higher up the wall and the membrane unlike any damp away course will provide an impermeable separation barrier between the damp wall and the new wall finished which is permanently affective against damp, salts, staining etc. The membranes are attached using special sealed mushroom fixings and despite some concerns the membranes can be easily fixed through retrospectively if required. Plastered finish and dab fixed plasterboard and skim provide the rooms usual aesthetic appearance with the peace of mind that the membrane will prolong their longevity. In my opinion this is a good and cost effective method of treatment which can be used in conjunction with or instead of any of the above.

Read about course here