Damp Floors / Concrete Foundations 

Rising damp in a house with a concrete foundation isn’t supposed to happen—but in the real world, it absolutely can. When it does, it’s usually a sign that something has failed, been bridged, or was never installed correctly in the first place.

What actually causes rising damp?

Rising damp is driven by capillary action—moisture from the ground gets drawn upward through tiny pores in materials like concrete, mortar, and brick.

In homes with concrete slabs or footings, the main causes are:

1. Failed or missing damp-proof course (DPC)

A DPC is a moisture barrier (usually a membrane) designed to stop water rising from the ground. If it’s:

  • Damaged
  • Poorly installed
  • Missing (common in older homes)

…moisture can wick straight up into walls.

2. Bridging of the damp-proof course

Even if a DPC exists, it can be bypassed if:

  • Soil, garden beds, or paving are built up too high against the wall
  • Render or plaster extends below the DPC line
  • Debris accumulates in cavities

This effectively gives moisture a “bridge” to climb over the barrier.

3. Porous masonry + constant moisture supply

Concrete and brick are not waterproof—they’re porous. If the ground around the home stays wet (poor drainage, leaking pipes, high water table), moisture is continuously available to be drawn upward.

4. Cracks in slab or footings

Cracks allow water to bypass barriers and enter the structure directly, feeding moisture into walls.

5. Poor site drainage

If water pools around the base of the house due to:

  • Inadequate fall away from the slab
  • Blocked or missing drainage
  • Downpipes discharging near walls

…you’re essentially “feeding” the rising damp problem.

Consequences of rising damp

This isn’t just cosmetic—it can turn into a serious building and health issue:

Structural & material damage

  • Salt deposits (efflorescence) on walls
  • Crumbling plaster and mortar
  • Peeling paint and bubbling coatings
  • Timber rot in skirting boards and framing

Indoor environment & health

  • Persistent musty odours
  • Increased humidity indoors
  • Mould growth (because moisture + dust = food source)
  • Potential respiratory irritation over time

How to stop or fix rising damp

There’s no single fix—it depends on the cause. But these are the proven approaches:

1. Improve drainage first (critical)

If you don’t remove the moisture source, nothing else will last.

  • Ensure ground slopes away from the house
  • Extend downpipes well away from foundations
  • Install or repair ag drains if needed

2. Remove bridging issues

  • Lower soil or garden beds below the DPC level
  • Cut back render or plaster that extends too low
  • Clear wall cavities if blocked

3. Repair or install a damp-proof barrier

Options include:

  • Chemical injection DPC systems
  • Physical membrane installation (more invasive but very effective)
  • Waterproof coatings (only as part of a broader system—not a standalone fix)

4. Seal cracks and entry points

  • Repair slab and footing cracks
  • Seal penetrations (pipes, conduits)

5. Manage internal moisture

Even after fixing the source:

  • Use ventilation and extraction fans
  • Keep humidity low
  • Clean dust regularly (reduces mould risk)

Prevention (especially important in humid climates like SE QLD)

  • Keep external ground levels at least 75–150 mm below the DPC
  • Maintain good drainage year-round
  • Avoid garden beds against walls
  • Regularly check for leaks and cracks
  • Use breathable wall finishes (avoid trapping moisture inside)

The key takeaway

Rising damp isn’t just “water coming up through concrete”—it’s almost always a system failure involving moisture, materials, and poor detailing.

If you tackle only the symptom (like repainting or sealing walls), it will come back. If you remove the moisture source and fix the pathway, you stop it for good.