Look at older houses and you will often see a band of peeling paint and white salt patches along the bottom metre of the walls. That is rising damp: ground moisture creeping up through the masonry. Two simple elements at plinth level stop it, and both cost very little when the house is built.
How rising damp works
Bricks, blocks and mortar are full of tiny pores. Like a wick in an oil lamp, those pores draw water upward from wet soil by capillary action. As the water evaporates from the wall surface, it leaves behind dissolved salts, which crystallise and push off paint and plaster.
What the plinth beam does
A plinth beam is a reinforced concrete beam running along the walls at plinth level, connecting all the columns. It spreads wall loads evenly onto the foundation, reduces uneven settlement and cracking, and improves the building's earthquake resistance by holding the columns together near the ground. It also provides a level, solid base for the DPC.
What a DPC is made of
| Type | How it is laid | Notes |
|---|---|---|
| Cement concrete DPC | A thin layer of rich concrete, often around 40 to 50 mm, with a waterproofing compound mixed in | Most common in Indian houses |
| Bitumen coating | Hot bitumen or a bituminous coating over the concrete layer | Adds an extra barrier |
| Membrane DPC | A sheet membrane laid across the wall | Common in Europe and North America |
Five rules for a DPC that works
- Lay it continuously along every wall at the same level. Gaps let moisture bypass it.
- Keep it above the finished ground level outside, so rainwater splashing up does not get above it.
- Do not plaster across the DPC on the outside with a continuous coat that connects the wet soil to the wall above.
- Never raise the outside ground or paving above the DPC later. This is the most common way old DPCs are defeated.
- Cure the DPC concrete properly before building on it.
Fixing rising damp in an existing house
- Lower the outside soil or paving if it has crept above floor level.
- Fix leaking pipes, blocked drains and garden beds against the wall first.
- Chemical DPC injection, where a water-repellent liquid is injected into drilled holes along the wall, can create a new barrier.
- Remove salt-damaged plaster and replaster with a breathable or salt-resistant mix after the wall dries.
Frequently asked questions
Is a plinth beam compulsory?
For framed houses it is standard good practice and often required by the structural design. Your structural engineer will specify its size and reinforcement.
Can waterproof paint stop rising damp?
No. Sealing the surface traps moisture inside the wall and pushes the damp higher. Stop the source instead.