16/11/2025
Never Allow Your Contractor To Put Dpm (Nylon) Beneath Your Foundation Blinding, Here's Why:
Few days ago, I made a post addressing a dangerous construction trend gradually creeping into the industry: placing a Damp Proof Membrane (DPM) beneath blinding concrete before blockwork begins.
This method directly contradicts the requirements of both the Nigerian Building Code (NBC) and the British Standards, which guide proper and safe construction practices.
For clients and young builders who may not understand the terms:
Blinding is a thin layer of concrete placed on the compacted soil to provide a clean, level surface for foundation walls or reinforcement.
DPM (Damp Proof Membrane) is a tough plastic sheet used to stop moisture from rising into the building.
According to the Nigerian Building Code NBC 2018, Part 2, Section 3.34:
"A damp proof membrane shall be provided under ground floor slabs and above the hardcore to prevent the passage of moisture from the ground into the building."
This aligns with British Standards BS 8102 & BS 8215, which also place the DPM above the hardcore and directly under the floor slab, not under blinding.
Placing the DPM under blinding is simply wrong.
Here is why it should be avoided completely:
1.
It traps water
When DPM is placed under blinding, rainwater and groundwater become trapped between the soil and the membrane. Instead of allowing water to escape, the membrane locks it in, creating a moist environment that encourages soil instability and deterioration of the blinding concrete.
2.
It reduces bonding
Concrete needs proper grip with the soil or hardcore beneath it for stability. A plastic sheet under the blinding destroys this bond. The blinding becomes a floating surface rather than a stable base, which affects the entire foundation system.
3.
It weakens the concrete’s strength
Concrete gains part of its strength through friction and proper seating on its base. When you place nylon under it, that friction disappears. The result is weakened blinding concrete that can shear, shift, or crack under load.
4.
It shows poor site practice
Any experienced builder, architect, or engineer knows that placing DPM under blinding is never allowed. Seeing it on a site immediately reveals poor supervision, quackery, or a disregard for building codes. It signals that the foundation may have other hidden structural errors.
5.
It can lead to cracks in the foundation system
Since the membrane prevents proper bonding and support, the blinding becomes vulnerable to movement. As the soil expands, contracts, or settles, the blinding can crack. These early cracks eventually transfer into the blockwork, the floor slab, and even the walls, making the entire building unsafe.
Construction is not guesswork. Every stage is guided by codes, standards, and decades of engineering research. When you ignore these principles, defects are not a matter of “if” but “when.” Putting DPM under blinding is one of those shortcuts that look harmless but create long term structural problems.
If you want a durable, safe, and moisture-free building, follow the codes:
Hardcore → DPM → Floor slab.
Never place the membrane under blinding.
Quality construction is not expensive, repairs are.
