A damp-proof membrane (DPM) is a polyethylene sheet laid over concrete slabs to prevent moisture rising through the floor. Standard specification is 1200 gauge (300µm) polyethylene, meeting BS 8102.

How much overlap do DPM sheets need?

DPM sheets must overlap by a minimum of 150mm at all joints. The overlaps should be taped with DPM jointing tape to create a continuous moisture barrier.

Do I need DPM on all concrete floors?

All ground-floor concrete slabs should have a DPM unless the slab already has an integral DPM cast in. For existing slabs without DPM, a surface-applied liquid DPM or sheet DPM is required before laying floor finishes.

How do I prepare the slab for DPM?

The slab surface must be clean, dry and free from sharp protrusions that could puncture the membrane. Fill any holes or cracks with a rapid-setting compound. On rough slabs, lay a sand blinding layer first.

Should DPM turn up at the edges?

Yes. The DPM should turn up at walls and around any penetrations by at least 150mm, lapping with the wall DPC (damp-proof course) where possible. This creates a continuous moisture barrier.

TradeCalculator

Floor DPM Calculator — Rolls, Lap Allowance & Tape

Calculate 1200g DPM roll coverage with 150mm lap allowance, joint tape and slab preparation.

Room Length

Room Width

DPM Type

300\u00b5m (1200g) is standard

Price Per Roll (optional)

How We Calculate This

This calculator determines the DPM sheet area needed including lap allowances at joints, plus tape and upstand requirements.

The formula

Strips = Room width ÷ (Roll width − Lap)

DPM area = Strips × Roll width × Room length

Tape = (Strips − 1) × Room length × 1.1

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.