The area is small and the perimeter is not
Take a plain 60 by 36 alcove. The floor is 15 square feet. Three walls, 156 inches of run, waterproofed 72 inches up, is 78 square feet. A 36 inch curb with a 16 inch girth over the profile is 4 square feet, and one niche adds 4 more. That is 101 square feet, and at any sensible price it is not a large material order.
Now count the lines. The floor meets a wall for the whole 156 inches. There are two inside vertical corners at 72 inches each. Two curb ends at 6 inches. Three band runs along the 36 inch curb. Three penetrations at 24 inches around. One niche at 84 inches. That is 48 feet of band on 101 square feet of shower, and four preformed corners.
| Line | Default alcove | Arithmetic |
|---|---|---|
| Shower floor | 15.00 sq ft | 60 × 36 in |
| Walls | 78.00 sq ft | 156 × 72 in |
| Curb | 4.00 sq ft | 36 × 16 in |
| Niche | 4.00 sq ft | 1 × 4 sq ft |
| Total area | 101.00 sq ft | |
| Floor to wall | 13.00 ft | 156 in |
| Two inside corners | 12.00 ft | 2 × 72 in |
| Two outside corners | 1.00 ft | 2 × 6 in |
| Curb, three runs | 9.00 ft | 3 × 36 in |
| Three penetrations | 6.00 ft | 3 × 24 in |
| One niche | 7.00 ft | 84 in |
| Banding | 48.00 ft | 0.48 ft per sq ft |
| Band rolls at 33 ft | 2 | 52.8 ft with waste |
Half a foot of banding for every square foot of shower. A 5 inch band over 48 feet covers 20 square feet, which is 19.8 percent of the area — and that fifth of the shower is where essentially all of the leaks happen. Nobody has ever traced a shower leak to the middle of a wall.
Sheet or liquid changes the order, not the geometry
Both branches read the same measurements. On the sheet branch, a 3.3 ft roll lapped 2 in has 3.133 ft of net width, so a 16 ft roll gives 50.13 square feet of coverage and 101 square feet plus 10 percent waste takes 3 rolls. On the liquid branch, the field is counted as the total area minus the banded strip so nothing is paid for twice: 81 square feet of field at 40 square feet per gallon and 20 square feet of band at 20, both with waste, comes to 3.33 gallons.
Note the band rate is half the field rate on that default. That is deliberate and it is the usual shape of the thing — the band gets a coat under it and a coat over it, and starving it leaves fabric threads that wick water straight through. If you are working from a data sheet with only one coverage figure on it, ask the manufacturer what the banded detail takes rather than assuming it is the same. The wet mil audit works the same problem from the other end, checking what film the pails you have already opened actually left behind.
What is not on this page
The drain, deliberately. A bonded flange, a clamping ring with a liner, and a linear channel are three completely different sequences at the one place where the water is meant to leave, and none of them is a square footage. The slope of the pan is not here either — that is a mortar bed question. And if the entry is level rather than curbed, the number that governs is the recess in the framing, which the curbless recess calculator works out from the whole build-up.
Downstream, the tile calculator handles what goes on top, the caulk page covers the movement joints at the changes of plane that stay flexible, and glass is measured after the tile is on and not before.
Questions people ask
How much waterproofing does a standard shower need?
On a 60 by 36 alcove waterproofed 72 inches up, with a curb and one niche, it comes to about 101 square feet plus 48 feet of banding. As sheet membrane on a 3.3 ft roll with a 2 in lap that is 3 rolls of 16 ft plus 2 rolls of band. As a liquid at 40 square feet per gallon in the field and 20 over the band it is 3.33 gallons. The waterproofing height, the coverage rates and the band width all come from your system, not from this page.
Why is the banding counted separately from the area?
Because it is a different material, a different cost per foot and a different failure rate. The 48 feet of band on a default shower covers about 20 square feet at a 5 inch width — a fifth of the area — and essentially every shower leak that gets opened up turns out to have started at a corner, the floor-to-wall joint, the curb or a penetration. Counting the changes of plane makes the risky part of the take-off visible rather than burying it in a square footage.
Do I need preformed corners or can I fold the band?
That is the manufacturer of the system talking, and different systems answer it differently — some supply moulded inside and outside corners and expect them to be used, some show a folded detail, some allow either. The page counts the corners either way: the default alcove has two inside and two outside, so four. What it cannot tell you is which method your product allows, and that is exactly the sort of thing a warranty turns on.
How high does waterproofing have to go up the shower wall?
This page does not answer that and will not, because it is set by the code your jurisdiction has adopted and by the system manufacturer, and both vary. The height field opens at 72 inches because it is a common number, not because it is a requirement. Note also that the waterproofing height and the tile height are separate decisions, and that a shower with a hand shower or body sprays wets parts of the room that a fixed head never reaches.
Does the drain get counted in the area?
No, and that is deliberate. A bonded flange, a clamping ring over a sheet liner, and a linear channel are three different details, installed in three different sequences, and each one has its own instructions about how the membrane meets it. It is the single most important junction in the shower and it is not a square footage, so the page leaves it out rather than giving you a number that would suggest it had been handled.