Two take-offs, not one
A below-grade membrane order is really two orders that get written on the same line. There is roll goods for the flat wall, which is easy to measure and easy to install, and there is detail material for the places where two planes meet or something comes through, which is neither. They run out at different rates and the second one runs out first.
On the defaults — 140 ft of wall, 8 ft high, a 3 ft roll lapped 3 in — the field maths is straightforward. Net coverage is 2.75 ft per strip, so the run takes 51 strips, each 8 ft long, which is 408 linear feet of roll. Ten percent waste makes it 448.8, and at 100 ft a roll that is 5 rolls. The wall is 1,120 square feet and you have bought 1,500 square feet of membrane to cover it.
| Line | Default wall | Where it comes from |
|---|---|---|
| Wall area | 1,120 sq ft | 140 × 8 |
| Net coverage per strip | 2.750 ft | 3 ft roll − 3 in lap |
| Strips | 51 | 140 ÷ 2.75, rounded up |
| Field membrane | 408.0 lin ft | 51 × 8 |
| Plus 10% | 448.8 lin ft | |
| Rolls at 100 ft | 5 | 500 ft bought |
| Side laps to seal | 51 | closed run, 408 ft of seam |
| Footing joint | 140.0 ft | the full run |
| Four inside corners | 32.0 ft | 4 × 8 ft |
| Four outside corners | 32.0 ft | 4 × 8 ft |
| Three penetrations | 7.5 ft | 3 × 30 in |
| Detail run | 211.5 ft | 1.51 ft per foot of wall |
One and a half feet of hand-formed detail for every foot of wall. That ratio is what the estimate usually misses, and it is worse than it looks, because detail work is slow: a corner is cut, worked into the angle without a void behind it, and rolled, and a pipe is worse again. The field goes on at a walking pace.
The side lap decides the roll count, and it is not yours to choose
Every inch of side lap comes straight off the coverage. A 3 ft roll lapped 2 in covers 2.833 ft and needs 50 strips; lapped 4 in it covers 2.667 ft and needs 53. That is a 6 percent swing in the whole order from a two inch change, which is why guessing the lap is a bad idea and reading it off the data sheet is a good one. It is also why a wider roll helps disproportionately on a long run — the lap is paid once per strip and a wider roll has fewer strips.
The end lap only bites when the wall is taller than the roll is long, which on a house it never is and on a deep commercial wall it can be. When it does bite, each strip gets spliced and the calculator adds a lap to the strip length and a seam to the count.
What the page does not decide
It has no view on which system suits your wall, what primer goes under it, how far up the wall the membrane should come, whether the footing joint takes a fillet or a strip, or whether you need a drainage board, a drain or both. Those come from the designer and the manufacturer. What it also cannot do is tell you the water is gone: a membrane on the wall and a drain at the footing address different halves of the problem, and if the water arrives during rain from one place, grading and downspouts may be the actual fix. Sort out which you have with the water entry organizer before anything gets excavated.
If the wall is standing in groundwater rather than in wet soil, the load on it is a different question again — the hydrostatic pressure calculator puts a number on that, and the number goes to an engineer. Inside the house, the interior drain trench and crawl space liner pages cover the work that happens when digging outside is not an option.
Questions people ask
How much waterproofing membrane do I need for a foundation wall?
More than the wall area, because of the side lap. On the defaults here — 140 ft of wall 8 ft high, a 3 ft roll with a 3 in side lap and 10 percent waste — the wall is 1,120 square feet and the order is 5 rolls, or 1,500 square feet of material. That is 1.34 square feet bought for every square foot of concrete. A narrower roll or a wider lap pushes that ratio up.
Why does the calculator count corners and pipes separately?
Because they are a different material, a different rate of work and a different failure mode. The default wall takes 211.5 feet of detail strip against 408 feet of field membrane — a third of the linear footage, on maybe 4 percent of the area. Almost every below-grade leak that gets investigated turns out to be at a corner, a lap end, a tie hole or a penetration rather than in the middle of a sheet, so counting the details is the part of the take-off that reflects the actual risk.
How much primer does a foundation wall take?
Whatever the primer data sheet says for your substrate, divided into the wall area. The form opens at 250 square feet per gallon, which over 1,120 square feet is 4.48 gallons or one 5 gallon pail with half a gallon spare. Read that rate off the can rather than trusting the default: a rough form finish, a block wall or green concrete can pull far more than a smooth poured face, and this page has no way to know which you have.
Do I need protection board over the membrane?
That is the assembly designer and the membrane manufacturer talking, not a calculator. What the page does is count it if you say you are using it: 140 feet of wall covered 8 feet high is 1,120 square feet, which at 32 square feet a sheet and 10 percent waste is 39 sheets. Set the board height to zero to leave the line out. Note that protection board and drainage board are not the same product doing the same job, even though they often occupy the same place in the wall.
Is the material cost the cost of waterproofing a foundation?
Not close, on a retrofit. The cost section here prices membrane, detail strip, primer, board and termination bar at figures you type in, and that is all it prices. Excavating an existing foundation to full depth, keeping the hole open safely, dealing with the water while the work happens, and backfilling and compacting afterwards are usually several times the material, and the excavation is also the dangerous part. On new construction with the hole already open the balance is very different.