Damp Proof Membrane Calculator
Floor DPM, cavity drainage, tanking slurry and radon barrier - with upstands and tape
The membrane is turned up the wall - typically 150-300 mm. Set to 0 to ignore upstands.
50 m rolls - for sealing joints and upstand perimeter
10% for regular rooms; 15-20% for awkward shapes or many internal corners.
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Total inc. Upstands
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Frequently Asked Questions
What thickness of damp proof membrane do I need?
Standard ground-bearing floor: 1200 gauge (300 micron) polyethylene sheet - the minimum under Approved Document C. Gas-resistant membranes (radon/methane protection): 300 or 450 micron with fully gas-tight sealed laps. Basement/high water table: a standard polythene DPM is not sufficient - use a cavity drain membrane or tanked waterproofing system.
Where does the damp proof membrane go in a floor construction?
Below the slab (sandwich DPM): laid on compacted hardcore/blinding sand before concrete is poured - most common position. Above the slab (surface DPM): laid on hardened concrete before screed or floor finish - must link to the wall DPC. All laps must be at least 150 mm and taped or welded to form a continuous barrier.
How much overlap do I need at DPM sheet joints?
Approved Document C: minimum 150 mm overlap at all sheet joints, taped with polythene or butyl tape to form a continuous barrier. Wall upstand: turn up 150-300 mm above finished floor level and connect to the wall DPC. All pipe penetrations: seal with proprietary sealing collars or tape to maintain continuity.
What is a radon barrier and when do I need one?
Radon is a radioactive gas that seeps from certain rocks and soils. In Basic Protection areas: a 300 micron membrane with sealed laps as the DPM is sufficient. In Full Protection areas: a radon sump and fan system must also be installed. Check your site on the UKHSA postcode radon checker or BRE Report BR 211 Affected Areas mapping.
What is a gas membrane and when is it required?
Required where a site is at risk from ground gases (methane, CO², radon) - typically brownfield sites, land near landfill, coal mining areas, or made ground. Gas membranes are 300-450 micron polyethylene or specialist multi-layer membranes with all laps welded or taped gas-tight. Risk identified via Phase 1 desk study and Phase 2 site investigation; protection measures must be designed by a suitably qualified engineer.
How do I calculate how many rolls of DPM I need?
DPM rolls are typically 4 m wide x 25 m long (100 m² per roll). Add floor area + wall upstand area (perimeter x upstand height, typically 0.15-0.3 m) + 10-15% for laps and waste. Example: 10 x 8 m floor: 80 m² floor + 7.2 m² upstands x 1.12 = 97.6 m² - one 100 m² roll covers it. For multiple sheets, count sheet widths and add 150 mm per lap.