How is below-grade waterproofing coverage different from a roofing membrane take-off?

Three ways. Verticals dominate: a foundation job is mostly wall area plus a footing turn-down and often a top termination detail, so height accuracy matters more than footprint. Laps are bigger — many below-grade sheet systems require 100-150 mm side and end laps plus double coverage at corners and penetrations. And primer or conditioner for the substrate is a separate consumable priced per square metre of the SAME area, so the wall area you compute here gets bought twice in different products.

Why round the roll count up?

Membrane rolls are sold as whole units, and a partial roll still requires a full roll to be purchased, so the result is always rounded up to the next whole roll.

Does this account for overlap between rolls?

No — this calculates the base roll count from the wall area and each roll's rated coverage area, and you should assume that figure is GROSS. Data sheets normally publish the roll's plain width × length, which deducts nothing for the 100-150 mm side and end laps this system needs; the claim that most manufacturer figures are already net of overlap was wrong and contradicted this page's own coverage-rate help. Either reduce the coverage figure by the lap allowance before entering it, or factor overlap into the wall area before entering it.

Where does the roll coverage area come from?

Use the coverage area printed on the product's technical data sheet — it depends on the roll's specific width and length and varies by manufacturer and product line.

How much does a lap take off one roll, in numbers?

A tenth of the gross area of a roll 1 m (39 in) wide lapped 100 mm (4 in) over its neighbour, before any end lap, corner patch or offcut, because the side lap runs the full length of every roll. That roll covers only 900 mm (35 in) of new wall per drop. Carried through this page's own default, 80 m² (about 860 sq ft) of wall on a 9.3 m² (100 sq ft) roll returns 9 rolls; the same roll entered at 90 per cent of its gross area, 8.37 m² (90 sq ft), returns 10. A narrower roll loses a larger share of its width to the same lap, so the deduction has to be worked from the width on the data sheet rather than carried over from another product.

Is a pre-applied sheet, laid before the concrete, measured the same way?

The arithmetic is the same and the surface is different. A pre-applied sheet, called blind-side waterproofing in the US, is fixed against the earth retention — sheet piles, lagging, a piled wall or sprayed concrete — or against the outer formwork, with its bonding face towards where the concrete will go, and it bonds to the wall as the concrete is poured and cures. So the area to enter is the retention face the sheet is fixed to, measured once any levelling or facing it needs is in place. Where the same sheet continues under the base slab, that floor is not wall and is not in the default; it has to be added to the area entered, or counted on its own with that product's roll size. A post-applied sheet goes on the cured wall from the excavation side, which is the case the substrate note on this page describes.

Should the wall be measured inside or outside the basement?

Outside, because that is the face the sheet goes on, while room sizes and many drawing dimensions are taken inside. On a rectangular basement the outside perimeter is longer than the inside one by eight wall thicknesses, two at each corner: a 200 mm (8 in) wall adds 1.6 m (5 ft 3 in) of run, and at a height of 2.4 m (about 8 ft) that is 3.84 m² (about 41 sq ft) of membrane the inside measurement never saw. On any plan built from right angles the difference stays at eight wall thicknesses, because each extra outside corner comes with an inside corner that takes the same length back. Height is measured on the outside too: from wherever the specification takes the sheet down to at the footing, up to its termination, rather than the depth of the dig or the ceiling height indoors.

Does a deeper wall or a higher water table change the roll count?

Not on this page. The count uses two numbers, wall area and roll coverage, so 80 m² (about 860 sq ft) returns the same rolls at 1 m (3.3 ft) deep or 4 m (13 ft) deep, in dry ground or wet. What depth and water change is which product is fit to be there: standing water presses on the wall with about 9.8 kPa for every metre of head (0.43 psi per foot), and a sheet chosen for damp ground may not be rated to resist that. A heavier sheet, a second ply or a different system brings its own roll width, length and lap, each of which has to be re-entered here, so the product choice comes before the count. A second ply is also a second count: the page covers the area once, so a two-ply specification is run once per ply, each with that ply's own roll size and laps, and the counts added.

Materials & Quantities

Below-Grade Waterproofing Membrane Roll Calculator

Calculate how many rolls of below-grade waterproofing membrane are needed for a foundation wall.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Foundation Wall Area

The total below-grade foundation wall area to be waterproofed.

Measure the height times length of each below-grade wall face that will receive the membrane, and sum them.

Roll coverage area

The coverage area of a single membrane roll, per the manufacturer.

Check the roll's rated coverage area on the product label or technical data sheet — this varies by roll width and length.

Membrane rolls needed

9 rolls

High confidence

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Roll count = ceil(wall area ÷ roll coverage area), the standard method for estimating rolled sheet membrane material

Inputs used

Foundation Wall Area: 860 sq ft
Roll coverage area: 100 sq ft

Final result9 rolls

What this calculation does not cover

Nothing here asks whether the wall will actually accept the membrane. Self-adhered and torch-applied sheets bond only to a sound, dry, clean substrate, and most carry a minimum application temperature. Honeycombing, unfilled tie holes, form release still on the face, or a wall that is too green or too cold all have to be dealt with first — an unbonded sheet is a channel that carries water sideways behind it, which is worse than no sheet at that spot.
The membrane still has to survive backfill, and what protects it is not in this count. Sheet membrane punctures easily on angular fill and under the machine placing it, so a protection board or drainage composite normally goes on over the same area. Unlike a miscounted roll, a sheet torn during backfill is buried: it cannot be found or repaired without excavating the wall a second time.

Estimated cost — your price

Price per roll ($)

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 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Roll count = ceil(wall area ÷ roll coverage area), the standard method for estimating rolled sheet membrane material

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To tank the walls and a basement's floor (waterproofing). Generic types, no brands, no prices.

Protection this work requires

Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
A dust mask does nothing about solvent vapour: ventilate through, and use an organic-vapour (A1) cartridge respirator in an enclosed room.
Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
Breakers and grinders cause permanent nerve damage: limit continuous trigger time, keep hands warm, and stop if fingers tingle or blanch.
Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.

Tank the walls and a basement's floor (waterproofing): Applying a slurry or a coating to its maker's instructions is within reach of a competent builder. Which grade of dry space the basement needs (in the UK, BS 8102's grades 1 to 3), whether a barrier on its own will do or a drained cavity is needed with it, and how every joint, penetration and junction is detailed are the waterproofing designer's, and the standard asks for a waterproofing specialist in that design; this site flags the grade and never chooses it. A barrier on the inside face holds back water under pressure only where the wall behind it can carry that pressure, a structural question this site never sizes. On a wall that holds back no ground, where a damp specification names this protection, that specification sets which walls and how high, and the basement grade above does not apply.

Show the 6 tools this job needs

Essential

Brushes
Mixing bucket or tub
Paddle mixer
SDS rotary hammer

Recommended

Plastering trowel and hawk

Optional

Utility knife

Also needed as materials: replacement blades, SDS bits and chisels.

what each waterproofing tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Falls, Tanking and the Gully in a Wet Room Flooruses this calculator
A walk-in shower cut into a floor that already exists: the depth budget under it, the gradient band the codes barely leave open, and the tanking above it.
Backfilling Against Waterproofing
Building a Green Roof the Structure Can Carry

Called something else where you work? Tanking and below-ground waterproofing · Wet room and curbless shower · Flat roof covering — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Sheet vs Liquid-Applied Waterproofing — the factors that actually differ, with no invented prices.

Already have some? How much foundation wall will my rolls of waterproofing membrane cover?

Worked example: Bathroom gutted and retiled — step 5 of 9

How to calculate below-grade waterproofing membrane roll in 3 steps

Foundation Wall AreaThe total below-grade foundation wall area to be waterproofed.
Roll coverage areaThe coverage area of a single membrane roll, per the manufacturer.
Membrane rolls neededThe tool computes the membrane rolls needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Membrane rolls needed by foundation wall area

Page defaults, not your figures above.

Foundation Wall Area	Membrane rolls needed (rolls)
400 sq ft	4
600 sq ft	6
800 sq ft	8
1,000 sq ft	10
1,200 sq ft	12
1,400 sq ft	14
1,600 sq ft	16

Frequently asked questions

How is below-grade waterproofing coverage different from a roofing membrane take-off?: Three ways. Verticals dominate: a foundation job is mostly wall area plus a footing turn-down and often a top termination detail, so height accuracy matters more than footprint. Laps are bigger — many below-grade sheet systems require 100-150 mm side and end laps plus double coverage at corners and penetrations. And primer or conditioner for the substrate is a separate consumable priced per square metre of the SAME area, so the wall area you compute here gets bought twice in different products.
Why round the roll count up?: Membrane rolls are sold as whole units, and a partial roll still requires a full roll to be purchased, so the result is always rounded up to the next whole roll.
Does this account for overlap between rolls?: No — this calculates the base roll count from the wall area and each roll's rated coverage area, and you should assume that figure is GROSS. Data sheets normally publish the roll's plain width × length, which deducts nothing for the 100-150 mm side and end laps this system needs; the claim that most manufacturer figures are already net of overlap was wrong and contradicted this page's own coverage-rate help. Either reduce the coverage figure by the lap allowance before entering it, or factor overlap into the wall area before entering it.
Where does the roll coverage area come from?: Use the coverage area printed on the product's technical data sheet — it depends on the roll's specific width and length and varies by manufacturer and product line.
How much does a lap take off one roll, in numbers?: A tenth of the gross area of a roll 1 m (39 in) wide lapped 100 mm (4 in) over its neighbour, before any end lap, corner patch or offcut, because the side lap runs the full length of every roll. That roll covers only 900 mm (35 in) of new wall per drop. Carried through this page's own default, 80 m² (about 860 sq ft) of wall on a 9.3 m² (100 sq ft) roll returns 9 rolls; the same roll entered at 90 per cent of its gross area, 8.37 m² (90 sq ft), returns 10. A narrower roll loses a larger share of its width to the same lap, so the deduction has to be worked from the width on the data sheet rather than carried over from another product.
Is a pre-applied sheet, laid before the concrete, measured the same way?: The arithmetic is the same and the surface is different. A pre-applied sheet, called blind-side waterproofing in the US, is fixed against the earth retention — sheet piles, lagging, a piled wall or sprayed concrete — or against the outer formwork, with its bonding face towards where the concrete will go, and it bonds to the wall as the concrete is poured and cures. So the area to enter is the retention face the sheet is fixed to, measured once any levelling or facing it needs is in place. Where the same sheet continues under the base slab, that floor is not wall and is not in the default; it has to be added to the area entered, or counted on its own with that product's roll size. A post-applied sheet goes on the cured wall from the excavation side, which is the case the substrate note on this page describes.
Should the wall be measured inside or outside the basement?: Outside, because that is the face the sheet goes on, while room sizes and many drawing dimensions are taken inside. On a rectangular basement the outside perimeter is longer than the inside one by eight wall thicknesses, two at each corner: a 200 mm (8 in) wall adds 1.6 m (5 ft 3 in) of run, and at a height of 2.4 m (about 8 ft) that is 3.84 m² (about 41 sq ft) of membrane the inside measurement never saw. On any plan built from right angles the difference stays at eight wall thicknesses, because each extra outside corner comes with an inside corner that takes the same length back. Height is measured on the outside too: from wherever the specification takes the sheet down to at the footing, up to its termination, rather than the depth of the dig or the ceiling height indoors.
Does a deeper wall or a higher water table change the roll count?: Not on this page. The count uses two numbers, wall area and roll coverage, so 80 m² (about 860 sq ft) returns the same rolls at 1 m (3.3 ft) deep or 4 m (13 ft) deep, in dry ground or wet. What depth and water change is which product is fit to be there: standing water presses on the wall with about 9.8 kPa for every metre of head (0.43 psi per foot), and a sheet chosen for damp ground may not be rated to resist that. A heavier sheet, a second ply or a different system brings its own roll width, length and lap, each of which has to be re-entered here, so the product choice comes before the count. A second ply is also a second count: the page covers the area once, so a two-ply specification is run once per ply, each with that ply's own roll size and laps, and the counts added.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.