HesapHanem

Damp Proof Course Calculator

Enter the wall perimeter, roll length and roll price to see rolls needed and total cost.

How it is calculated

The tool takes the wall perimeter P in metres and adds a flat 10 % for laps at joints and corners: required = P × 1.10. It then divides by the roll length R in metres and rounds up to whole rolls: rolls = ceil(required ÷ R). Cost is rolls × price per roll, and the offcut is rolls × R − required. The 10 % is a single allowance for the whole run; it does not count your actual joints or corners, so a plan with many corners may need more. Perimeter and roll length must be above zero; a price of 0 is accepted. Lap sizes and DPC rules differ between countries and building codes.

Worked example

A building with a 42 m wall perimeter, rolls 20 m long at 18.5 per roll. Required length is 42 × 1.10 = 46.20 m. Dividing by the roll length gives 46.2 ÷ 20 = 2.31, rounded up to 3 rolls. Cost is 3 × 18.5 = $55.50. Three rolls give 60 m, so the offcut left over is 60 − 46.2 = 13.80 m, enough to keep for a corner repair or a small extension.

Try this example →

Common mistakes

  • Entering the room's floor area, or the outside perimeter when you need to line the inner leaf as well. Measure the length of wall the DPC will actually run along, and add each leaf separately if you lay two.
  • Typing the roll width instead of its length. A roll might be 0.1 m wide and 30 m long; only the 30 goes into the roll-length field.
  • Assuming the 10 % allowance covers every case. It is flat, so a building with many small returns and corners can use more lap than the tool assumes.
  • Ignoring rounding. You must buy whole rolls, so with 2.31 rolls of need the tool bills 3, and the leftover can be 40 % of a roll or more.
  • Leaving the price at the default. The cost line then reflects the default per-roll price, not your merchant's.

About this calculator

A damp proof course (DPC) is a continuous horizontal barrier built into a wall to stop rising damp from moving up from the ground into the brickwork above, and “continuous” is the operative word — every joint where one roll ends and the next begins must overlap, commonly by around 100mm at horizontal joins and more at corners, or the barrier isn’t actually continuous. Total DPC length needed is therefore wall perimeter plus the extra length consumed by these laps, not simply the bare perimeter measurement. This calculator allows for the laps with a flat 10% added to the perimeter, whatever the roll length or number of joints and corners, so count your own joints if the wall has many corners.

Builders and DIY renovators calculate DPC length, roll count and total cost before ordering because DPC membrane is sold in fixed roll lengths, and underestimating the lap allowance is a common reason a job runs short mid-installation — a gap in the DPC anywhere along the wall defeats its purpose, since damp will simply rise through the unprotected section.

Wall perimeter plus 10% gives the DPC length; divide by the roll length and multiply by the roll price to see how many whole rolls to buy, what they cost and the offcut left over.

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