Damp-Proof Course (DPC)

Definition and construction

• Let’s define the damp-proof course (DPC).
• In the construction of a house, to keep it simple, we start with the foundation. The foundation goes in and is laid with concrete.
• Once this is cured, the developer then builds plinth walls.
• Plinth walls rise from the foundations until ground floor level, at which stage the sand from the foundations is backfilled against the plinth walls and levelled and compacted. Thus, plinth walls are sub-soil, and this is the area we damp proof.
• There is a lot more that goes into it, but for the simplicity of this explanation, a DPC plastic sheet, typically a minimum of 350 micron, is laid on top of the compacted backfill.
• A concrete slab is then thrown, and the damp-proof course extends through the exterior house wall, on which the wall continues to be built on top of the concrete and on top of the damp-proof course, using the outer skin of the house as the continuous course.

Leak effects inside

• When a leak occurs inside the house (like from a bathroom, plumbing, etc.), water enters on top of this plastic which spans through the entire house.
• It can then gravitate and wick through the porosity of the concrete or screed or flooring material, which is sometimes porous like wooden decking material, and be delivered deep within the house.
• This wooden flooring material rots and causes mould, and this is where damp through mould occurs.

Mould hazard

• Mould is a health hazard.
• It should be cleaned with white vinegar, not bleach, as bleach will propagate the mould.
• Skirtings are another area where mould is visible. Once the skirting has swollen, we notice dimples from the nails which have remained at the original height of inception, and the skirting has swollen around the nail, creating dimples.
• Once the skirting is removed, black mould can be seen on the back.

Black Mould

• Black mould is created in the presence of moisture, food, and oxygen. Once one of them is removed, the mould will cease to exist.
• The reason mould is so bad for us is because it propagates in the bronchial system, much like the seeds of a mushroom spore entering the atmosphere which, once ingested, germinate in the ENT system of our bodies (ear, nose, and throat).
• Once this germinates, flu-like symptoms are experienced.
• After my 25 years of experience, the worst cases I find are noses that stream with blood, especially in younger children.
• Multiple doctor visits don’t treat the cause; they alleviate the symptoms.
• Wooden flooring and wooden skirtings are very sensitive to mould growth.
• We also notice mould growth on carpets and parquet flooring.
• We always advise tiling the inside of the house for a mould-free environment.
• We have processes to waterproof the floor slab and walls so that you can enjoy wood finishes.

Water Ingress from Plinth Walls (Design Construction Fault)

• Water ingress from damp through the plinth walls, which is a design construction fault.

Old industry standard

• Our industry standard many years ago was to have these plinth walls built as face brick.
• This allows the water trapped under the damp-proof course plastic to evaporate through osmosis due to the pull of the sun / atmospheric relative humidity.
• The water trapped under the damp-proof course needs a way to exit.
• This is where we use damp proofing and air bricks punched through the plinth walls.

Air bricks have different prices:

• If you go surface-mounted
• If you enter through one brick

• Or if you core drill through two bricks into the sub-soil backfill

• This then allows the wall to breathe through the backfill.

• Multiple air bricks placed in close proximity allow air to move inside the building and create a path of least resistance for the water to be scavenged.
• Air bricks placed one inside and one outside will remove water as the air passes through the wall thickness

Industry shift

• The industry standard was face brick, and you notice this on the old builds of 30–50-year-old houses (as dated in 2024).
• The industry trend now is to plaster these plinth walls.
• In the old style of building, we would have face brick at the bottom and plaster above.
• By extending the plaster to the floor without damp-proof precautions, damp pushes its way through the new plaster

How Water Decomposes Plaster

Historical context

• In the old days, we did not have cement.
• We went into caves, chopped out stalagmites and stalactites, powdered them, and added them to sand.

Erosion process

• As water moves, it erodes.
• The heavier the water flow, the greater the erosion.
• As water moves through the building, it leaches lime and deposits it on the surface after evaporation.

Effect on plaster

• Sand particles are released once lime is removed.
• As the sand is released, anything adhered to it comes loose.
• That is why when damp bubbles are popped, sand runs out.

Correct solution

• The only long-term solution is to stick new plaster to a solid surface — the brick.

Bricks and Hardness

Brick properties

• Bricks change form from their original composition.
• Clay bricks are endothermic.
• Cement stock bricks change form chemically.
• These bricks cannot decompose as quickly as mortar or plaster.

Hardnesses (MPa)

• Mortar: ~4 MPa
• Plaster: ~4 MPa
• Cement stock bricks: 7–12 MPa
• Clay bricks: variable

Implication

• Bricks don’t decompose as plaster or mortar do.
• Sticking damp-proof systems to brick is the only long-term solution.

Expansion and Movement

• Different materials expand and contract at different rates.
• This movement causes cracks and gaps where water enters.
• Damp-proofing prevents water ingress and preserves the structure.

Subsoil damp proofing

• This is the area we specialize in
• Good examples can be seen on our YouTube videos
• Prime examples are underground cellars
• Repairs require treating the floor, walls, and ceiling

Conclusion

• Our damp-proof systems are designed using material limits to produce strong, affordable solutions.
• Kindly contact us for a free assessment to rejuvenate your home for long-lasting effects.