Wetrooms
Decoupling or waterproof sheet membrane rolls and seam tape
- where
- useful cover from one roll (m²)
- roll width (mm)
- side overlap (mm)
- roll length (m)
- end overlap (mm)
- rolls to buy
- area to cover (m²)
- waste allowance (%)
- length of side seams to tape (m)
Explain this equation
Use this to order rolls of decoupling matting or sheet waterproofing and to estimate how much seam tape the joints between strips need.
Wr and Lr are the roll width and length. so and eo are overlaps that are lost from each roll: zero for butt-jointed mats like DITRA, typically tens of mm for overlapped sheets. w covers cutting waste.
The useful width is the roll width less the side overlap, and the useful length is the roll length less one end overlap; the 1000s turn mm into m. Rolls are the area with waste divided by the useful cover, rounded up. Laying strips side by side, the seam length is roughly the area divided by the useful strip width.
A 1 m × 30 m roll covers 30 m² butt-jointed, so 25 m² with 10% waste is one roll and about 25 m of seams. With 50 mm overlaps each roll loses about 5%.
Watch out: perimeter, upstand and pipe details need extra tape and corners on top of the seam length.
Works out rolls of decoupling mat or sheet waterproofing for a floor, and the length of seams to tape.
Enter your values to see the result.
Good to know
Schlüter DITRA comes 1 m wide (30 m rolls) and is laid with tight butt joints; where it acts as waterproofing the joints are covered with KERDI-KEBA at least 125 mm wide. Overlapped sheet membranes lose the overlap from the useful width. The seam length here is the side seams across the area (area ÷ effective width); add the perimeter and any end joints for upstand and junction tape.
Common mistake
Forgetting that a 50 mm overlap on a 1 m roll loses 5% of the cover before any cutting waste.
Limits
Rectangular areas laid in parallel strips. Small or awkward rooms waste more. Check roll sizes and overlap rules for the product you buy.
Sources
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.