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The standard allowance most suppliers and estimating guides assume for ordinary work.
The total veneer wall area needing cavity drainage.
The area of the DRAINED cavity, which is shorter than the wall at both ends: the mat starts at the through-wall flashing and finishes below the top of the veneer, because it has to deliver water to the weeps rather than carry on past them. Measuring the elevation instead over-orders on a low wall and, on a tall one, hides the fact that the flashing level was never checked.
Extra material for roll overlap, cuts around ties, and openings.
On this product the allowance is dominated by the TIES, not by the openings. A mat behind a tied veneer is pierced wherever a tie passes through it, so a wall with a tie grid is cut hundreds of times, each cut producing a small offcut and a place where water can pass behind the mat if it is not lapped or taped. Walls with dense tie patterns need a larger allowance than their area suggests.
Drainage mat needed
352 sq ft
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Cavity Wall Drainage Mat Area Calculator: 352 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Drainage mat coverage matches the wall's cavity area, with a waste allowance for roll overlap and cutting around ties and openings
Inputs used
- Wall Area
- 320 sq ft
- Waste Factor (%)
- 10
What this calculation does not cover
- The waste factor is applied to whatever area you type in, so openings are either counted twice or not at all depending on how you measured. Enter the gross elevation of a wall carrying glazing and the 10% allowance is added to the window openings as well, returning more than the netted-out wall needs; enter the net area instead and that same 10% has to absorb every offcut around the reveals.
- Wall area is the right basis only for a full-height drainage mat or mesh. Products that are mortar-collection devices sitting in the bottom few courses of the cavity are bought by the linear metre of wall, so a 10 m long wall needs 10 m of that product, not the 33 m² (355 sq ft) this page returns for a 30 m² (323 sq ft) elevation.
- The waste factor does not know your roll width or your lap width. Side laps consume a fixed width at every vertical seam rather than a percentage of the area, so a 1 m wide roll lapped 100 mm loses roughly 11% to overlap alone, which is more than the 10% default and more than twice the 5% floor this field accepts, before a single cut around a tie, a cavity tray or a reveal.
- Sizing the mat says nothing about whether it can drain. It only works if it runs down onto a cavity tray or DPC and terminates above open weep holes, and the mat itself consumes cavity width, so the residual clear cavity, the wall tie length and the tie embedment into each leaf have to be checked against the specification separately. None of those follow from an area.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Drainage mat coverage matches the wall's cavity area, with a waste allowance for roll overlap and cutting around ties and openings
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