A damp-proof course is the lowest section of a facade, from the foundation to approximately 30 cm above ground level, that is specially protected against rising damp. The damp-proof course is made with water-resistant materials (special trass mortar or a waterproof membrane) to prevent ground moisture from travelling up through the wall.
Why is a damp-proof course needed?
Bricks and mortar are porous — they absorb water. Without protection, ground moisture rises through the foundation into the wall (capillary action). This leads to:
- Damp patches and salt efflorescence on interior walls
- Peeling plaster and wallpaper
- Mould growth
- Long-term damage to the brickwork
The damp-proof course interrupts this capillary action.
How is a damp-proof course made?
Method 1: Trass mortar
The lower part of the wall is laid with trass mortar — mortar to which trass (a volcanic material) has been added. Trass mortar is water-resistant and is standard practice below ground level.
Method 2: Waterproof membrane (DPC)
A strip of waterproof membrane (Damp Proof Course) is laid in a mortar joint at approximately 15 cm above ground level. The membrane blocks the moisture.
Method 3: Injection (existing buildings)
In existing walls without a damp-proof course, a chemical DPC can be retrofitted: holes are drilled in the mortar joint and a water-repellent liquid is injected to seal the pores of the brick.
Damp-proof course in DIY projects
When building a masonry garage, shed or garden
wall:
1. Pour foundation — Strip foundation at the correct depth
2. Lay first courses — With trass mortar, from foundation to approximately 15 cm above ground level
3. Insert DPC membrane — In the joint at 15 cm above ground level
4. Continue laying — Above the DPC with normal mortar
Related terms
- Strip foundation
- Mortar
- Joint
- Half-brick wall
- Drip sill