Moisture Barrier Calculator

How the moisture barrier calculator works

This calculator sizes the strip of waterproof membrane laid in a mortar joint near the bottom of a masonry wall to stop ground moisture climbing into the wall above. In the UK, Ireland, Australia and New Zealand it is called a damp-proof course, or DPC. In North America the same job is done by base-of-wall through-wall flashing, also sold as a dampproofing course. Either way it comes in long, narrow rolls, and the question is how many to buy.

Enter the length of wall, the wall (leaf) thickness, the roll length, a price per roll and an allowance for laps and offcuts. The calculator adds the allowance to the length, divides by the roll length and rounds up to whole rolls: N = ⌈P × (1 + w/100) ÷ L⌉, where P is the wall length, w the allowance in percent and L the roll length. It also picks the first stock roll width that covers the wall and tells you how much will be left on the last roll. It does not size floor membranes. A damp-proof membrane or vapor retarder under a slab is sold by area and needs an area calculation.

Measuring the wall

Measure the total length of every wall that needs a DPC, at the level where it will be laid. External walls are the obvious ones, but loadbearing internal walls built off their own foundations usually need one too. Cavity walls catch people out. Each leaf gets its own DPC, so a house with a brick outer leaf and a block inner leaf needs two runs, often in two different widths. Run the calculator once for each leaf. If the DPC stops at a wide opening such as a garage door, leave that width out.

Choosing the roll width

A flexible DPC should run the full width of the leaf. Visqueen's DPC design guide, quoting the British Standards guidance PD 6697, says it should be bedded on mortar and laid in continuous lengths for the full width of the leaf. So the calculator picks the first stock width at least as wide as the wall. On the metric side it uses the range of Visqueen polyethylene DPC, sold in 30 m rolls in widths of 100, 112.5, 150, 225, 300, 337.5, 450, 600, 900 and 1200 mm. A 100 mm block leaf takes 100 mm DPC, a 102.5 mm brick leaf takes 112.5 mm, and a 215 mm solid wall takes 225 mm.

With US units the calculator uses the widths of W. R. Meadows' Air-Shield thru-wall flashing, a product sold for thru-wall flashing and as a dampproofing course: 4, 6, 9, 12, 16, 18, 20 and 24 in, in 75 ft rolls. There is a catch with brick veneer. The Brick Industry Association's Technical Note 7B says base flashing should start outside the face of the brick, cross the air space and turn up the backing at least 8 in. That makes it far wider than the brick. To size it, enter the whole width as the wall thickness: a 3⅝ in brick, a 1 in air space and an 8 in upturn add up to 12⅝ in, which calls for a 16 in roll.

Laps, corners and the allowance

Where one length of DPC meets the next, the two are lapped. The Visqueen guide calls for laps of at least 100 mm in straight runs and full laps at corners. The BIA's Technical Note 7 asks for flashing pieces lapped at least 6 in and sealed with a compatible sealant or adhesive. A manufacturer may allow less for its own product (the Air-Shield datasheet gives 2.5 in), but where a lap rule and a product sheet differ, the larger lap is the safe one.

Laps use less material than people expect. On a plain rectangle of 25 m, one roll joint at 100 mm and four corner laps on a 100 mm wall use about 0.5 m, roughly 2% of the length. The default 10% allowance also covers the cuts at openings, piers and junctions, and the short ends that are not worth joining in. Drop it towards 5% for a simple rectangle; keep 10% or more for a plan full of bays and returns.

Worked examples

With the metric defaults, 25 m of 100 mm wall with 10% for laps and offcuts needs 27.5 m of DPC. One 30 m roll covers it, with 2.5 m to spare.

A cavity-walled extension has a 40 m brick outer leaf and a 38 m block inner leaf. The brick leaf needs 44 m of 112.5 mm DPC, which is two rolls with 16 m left over. The block leaf needs 41.8 m of 100 mm DPC, two more rolls with 18.2 m left. Four rolls in all, because the two widths cannot share a roll.

In US units, 120 ft of 4 in wall with a 10% allowance needs 132 ft, so two 75 ft rolls of 4 in material with 18 ft spare. The veneer flashing described above, 150 ft of wall at 12⅝ in, needs 165 ft of 16 in flashing: three rolls, with 60 ft left over.

Where the DPC goes

Height matters as much as quantity. In England, Approved Document C says the DPC in an external wall should be at least 150 mm above the adjoining ground and continuous with any damp-proof membrane in the floor. That keeps rain splashing off paving from bridging it. In the US, the BIA says continuous flashing must be placed above grade at the base of walls, with weeps immediately above it; most codes allow weeps no more than 33 in apart, and the BIA prefers open head joints at no more than 24 in. Other countries set their own heights and details, so check the local code before laying the first course.

What the result leaves out

The count covers the horizontal DPC at the base of the wall. Cavity trays over openings, vertical DPCs at door and window reveals, and floor membranes are separate items, as is the mortar to bed the DPC. Retrofitting a DPC to an existing wall is a different job again, usually done by chemical injection, and one for a specialist damp surveyor to assess.

Using this calculator with other BuildMetricLab tools

The brick and mortar calculators count the walling above the DPC, the brick tie calculator covers the ties across a cavity, and the foundation waterproofing calculator deals with walls below ground. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.