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Floor DPM Calculator — Rolls, Lap Allowance & Tape
Calculate 1200g DPM roll coverage with 150mm lap allowance, joint tape and slab preparation.
Minimum 150mm overlap at sheet joints, sealed with DPM tape (BS 8204-1 / Approved Document C).
Standard 1200-gauge DPM roll is 4m wide (BBA-approved UK product).
Standard roll is 25m long (4m × 25m = 100 m²); half-rolls are 12.5m.
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How We Calculate This
This calculator determines the DPM sheet area needed including lap allowances at joints, plus tape and upstand requirements.
The formula
The first strip overlaps nothing and covers the full roll width; each further strip loses the lap from its effective width. So if the room is no wider than one roll, a single strip does the job.
Strips = 1 + ceil((Room width − Roll width) ÷ (Roll width − Lap)), or 1 if the room is no wider than the roll
DPM area = Strips × Roll width × Room length
Joint tape = (Strips − 1) × Room length × 1.1 (the ×1.1 adds a ~10% trade allowance for cutting waste — it is a rule of thumb, not a standard figure)
Example: an 8m × 4m room laid with a 4m roll needs just 1 strip (32 m² of DPM, no joints), because the room is exactly one roll wide.
Frequently Asked Questions
A damp-proof membrane (DPM) is a polyethylene sheet laid above or below a concrete slab to stop ground moisture rising through the floor. The standard ground-floor specification is 1200 gauge (300µm) polyethylene sheet conforming to BS EN 13967, as required by Approved Document C of the Building Regulations.
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.
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.
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.
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.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.