What is a damp proof course (DPC) and why it matters

There is a thin layer built into the base of every proper brick wall that most people never see. It is called the damp proof course, and when it is missing or bridged, you get damp walls.

What a DPC does

Ground holds moisture, and brick and mortar will wick that moisture upward like a paper towel. The damp proof course is a waterproof barrier laid in the wall, usually a couple of courses above the ground, that stops the moisture rising any further.

In newer homes it is a strip of tough plastic membrane laid in the mortar bed. In older Melbourne homes it might be slate, blue engineering brick, or in some cases nothing at all, because the house predates the rule.

Signs the DPC is failing or bridged

The DPC itself rarely breaks. The usual problem is that something lets moisture jump over it:

• A garden bed, path or concrete poured up against the wall above the DPC line.
• Render carried down past the DPC onto the footing.
• Mortar or debris fallen into the cavity, forming a bridge.

What you see inside is a tide mark on the wall, bubbling paint, salt deposits or a musty smell low down. The damage stays near the floor and dries higher up.

Why it matters in Melbourne

Our reactive clay soils hold water through winter, and plenty of older suburbs have homes built close to the ground. Over the years, garden levels creep up and new paths get laid, and the DPC quietly gets buried. Once moisture is rising, it carries salts that eat at mortar and plaster from the inside.

The fix depends on the cause. Sometimes it is as simple as lowering the soil or cutting back a path. Sometimes a new chemical DPC has to be injected into the wall. Either way you have to find why the moisture is rising before you patch the damage, or it comes straight back.

If you have a damp patch near the floor, send a photo and your suburb and we will help you work out what is going on before it spreads.