Uncoupling Membrane: A Worked Example

One uncoupling-membrane order starts with a two-area measurement record and ends with a single roll choice that survives the strip-width math. This page runs a 120 by 144 in bathroom floor plus a 60 by 84 in closet through the full plan: strips, a reused offcut, a 10% overage, and the roll that clears the ordered length. If the areas are not measured yet, measure the membrane first: every step below assumes the record table exists.

How many feet of membrane does this job need?

61 ft on the order: 48 ft for the bathroom, 7 ft of new membrane for the closet, and a 10% overage. One DITRA 30M roll covers it. The larger-looking DITRA-XL/175 roll falls 7.75 ft short on length, even though its coverage number is bigger.

What does the measurement record carry?

Two rectangles define this job:

Each rectangle keeps its own strip plan. One combined area would hide the fixed 39 in roll width that controls the count.

How do the bathroom strips work out?

The roll width is 3 ft 3 in, which is 39 in. Divide the 120 in room width by 39 in and round up:

1. 120 ÷ 39 = 3.0769, so 4 strips.
2. Run the strips along the 144 in length: 4 × 144 in = 576 in = 48 ft.
3. The fourth strip covers the last 3 in of room width. The leftover rip from that strip is 36 in wide and 144 in long. Keep it; the closet needs it.

Strips = room width ÷ 39 in, rounded up

Linear feet = strips × strip length in feet

How does the closet fit the leftover rip?

Plan the closet on its own, then check whether its narrow strip fits the bathroom's leftover rip.

1. 60 ÷ 39 = 1.5385, so 2 strips, each 84 in long: 2 × 84 in = 168 in = 14 ft of membrane.
2. The second strip is only 21 in wide. The bathroom's rip is 36 in wide and 144 in long, and 21 ≤ 36 with 84 ≤ 144, so cut that strip from the rip.
3. New membrane for the closet: one full 84 in strip = 7 ft. The rip leaves 36 × 144 − 21 × 84 = 3,420 in² (23.75 ft²) as spare.

Treating the closet as its own 14 ft purchase would double its order. The rip is free material only when the needed piece actually fits inside it.

Which roll clears the order?

Add the new linear footage, apply the overage, then check the roll's length, not just its coverage.

1. New linear: 48 ft + 7 ft = 55 ft.
2. 10% overage for two simple rectangles: 55 × 1.10 = 60.5, rounded up to 61 ft.
3. DITRA 30M is 98 ft 5 in long, which is 98.42 ft. That clears 61 ft in one roll.
4. DITRA-XL/175 is 53 ft 3 in long, which is 53.25 ft. It falls 7.75 ft short, even though its 175 ft² coverage beats the 155 × 1.10 = 170.5 ft² area check.
5. Fallback: DITRA 150 (45 ft 9 in) plus DITRA 5M (16 ft 5 in) gives 62.17 ft of length, which also clears the order.

How do you check the arithmetic?

Run two independent cross-checks before ordering.

1. Area path: net area 120 + 35 = 155 ft²; with the 10% overage, 170.5 ft². The ordered 61 ft of 3 ft 3 in membrane covers 61 × 3.25 = 198.25 ft², which is ≥ 170.5 ft².
2. Inch path: 576 in + 84 in = 660 in, and 660 ÷ 12 = 55 ft, which matches the pre-overage linear total.
3. Rounding check: 120 ÷ 39 = 3.0769 rounds up to 4 strips, and 60 ÷ 39 = 1.5385 rounds up to 2. Both counts moved upward, so no strip was dropped.

The three paths agree, so the strip counts, the rip fit, and the ordered length are consistent. Keep the unrounded 3.0769 and 1.5385 on the record so a different product's roll width can be substituted without remeasuring.

Which mistakes change the count?

• Ordering from area math alone. The 170.5 ft² check passes while the roll comes up 7.75 ft short on length.
• Forgetting the strip-width roundup. 3.08 strips is 4 strips, not 3.
• Adding an overlap allowance at the seams. Sheets abut; overlaps waste material and raise the floor.
• Using one roll's length for another product. Published lengths run from 16 ft 5 in to 98 ft 5 in.
• Rounding the ordered length down. 60.5 ft needs 61 ft.
• Buying the closet's full 14 ft. Its 21 in strip fits the bathroom's leftover rip.
• Merging the two areas into one 155 ft² before planning strips. Each rectangle needs its own strip count, because the 39 in roll width wastes the partial last strip.

Limits

The 10% overage is a planning rule of thumb for simple rectangles, not a manufacturer specification. Raise it to 15% for cut-up rooms with niches, penetrations, or many small areas. This fixture uses Schluter's published 3 ft 3 in roll width; other brands publish different widths and lengths, so read the product being bought.

When the floor must be waterproof, seal the seams and floor-to-wall transitions with KERDI-BAND. That is a separate ordering line, not extra membrane area. This worksheet plans quantities. It does not choose the membrane product, approve the substrate, or set movement joints. Those decisions follow the manufacturer's handbook, the project specification, and the responsible installer.