Waterproofing
Cavity Drain / Drainboard Calculator
Calculates dimpled drainboard for basement walls or under-slab drainage
Updated May 13, 2026 · Live
What this tool does
Calculates dimpled drainboard for basement walls or under-slab drainage.
Formula Used
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How the cavity drain / drainboard calculator works
Calculates dimpled drainboard for basement walls or under-slab drainage. The calculator takes your dimensions and supplier rates, applies a standard US construction formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical US waterproofing wastage
Membranes and elastomerics waste 5–10% on overlaps and cutouts. Add more at penetrations, inside corners, and around floor drains. Our defaults reflect common US trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm building code compliance. Those remain the responsibility of a suitably qualified designer, engineer, or your local building official.
On-site considerations for cavity drain / drainboard
Below-grade waterproofing must be designed to the ICC-ES report or ASTM C898/C981 — sheet membrane, fluid-applied, or crystalline. Mixing systems voids manufacturer warranty and creates failure points.
Codes and compliance
Habitable basements need a complete waterproofing system per IRC R406. Foundation drainage (footing drains to daylight or sump) is mandatory in expansive or wet soils. When in doubt, file a pre-application question with your local building department — early clarity is cheaper than a corrective inspection.
Before you order
Buy the complete system from one manufacturer — primer, membrane, sealant, protection board. Mixing brands voids warranty and shifts blame if it leaks. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this cavity drain / drainboard calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labor-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the math.
Sources & methodology
This tool estimates the number of dimpled drainboard rolls required to cover a basement wall or under-slab drainage area, then calculates total material cost. Roll quantity is computed as the ceiling of the total area divided by the effective coverage per roll after accounting for seam overlap, using the formula: Rolls = ⌈Area ÷ (Roll Coverage × (1 − Overlap% ÷ 100))⌉. The default overlap allowance is 10%, a common mid-range assumption for drainboard seam laps. Total cost is the roll count multiplied by the price per roll entered by the user.
Frequently asked questions
Are cavity drain / drainboard calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical US trade allowance for waterproofing. Increase it for complex cuts, awkward shapes, or first-time DIY. The default wastage allowance reflects common trade practice; values lower than the default may underestimate offcuts.
Does this replace professional advice?
No. This tool is a planning estimator. For work that affects structure, building code compliance, gas, electrical, plumbing, or drainage to a public sewer, consult a licensed contractor or design professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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