The classic mistake: you measure the terrace, it comes to 20 m², you order a 20 m² roll and you run short halfway through the job. Surface metres and membrane metres never match, and the difference is predictable once you know where it comes from.
The four things you're missing
1. The real surface includes the vertical parts
The membrane doesn't stop where the floor stops. On terraces and roofs it runs up the parapet a minimum of 20 cm. In a shower it runs up the walls of the wet area to the height of the tiling.
It's the part people forget most and it usually adds between 15 and 40 % to the total.
2. The overlap
The 0.5 and 0.8 mm membranes are laid with a 10 cm overlap. With a 1.5 m wide roll, the effective width drops to 1.4 m: you lose 7 % at every joint.
The 1.25 mm ones are the exception: they go butt jointed, with no overlap. You don't lose any width, but you need joint tape for every linear metre of joint.
3. Offcuts
Corners, junctions, drain openings. Allow 5 % on clean, rectangular surfaces, and 10 % if there are a lot of changes of direction.
4. Preformed pieces are counted separately
They don't come out of the membrane, they're ordered by the unit:
- One corner for each junction, internal or external.
- One reinforcement for each pipe that passes through.
- One scupper or one drain for each drainage point.
- Joint tape: measure the linear metres of perimeter and of joints.
Example 1: a tiled shower
A 1.2 × 1.2 m tray, with two walls tiled up to 2 m high.
- Floor: 1.2 × 1.2 = 1.44 m²
- Walls: 2 × (1.2 × 2) = 4.80 m²
- Subtotal: 6.24 m²
- +10 % for offcuts and overlap: 6.9 m²
A 7.5 m² roll of ECODRY50 or DRY50. Add 2 internal corners and one reinforcement for each exposed pipe.
Example 2: a terrace
A 4 × 5 m terrace, with a parapet all the way round.
- Floor: 20 m²
- With overlap (effective width 1.4 m instead of 1.5): 20 ÷ 1.4 × 1.5 = 21.4 m²
- 20 cm parapet over 18 m of perimeter: 3.6 m²
- +5 % for offcuts: ≈ 26 m²
A 30 m² roll of ECODRY80 or DRY80, plus 4 external corners and the scupper or the drain for the outlet.
Available formats
The membranes come in rolls of 5, 10, 20 and 30 m long by 1.5 m wide (7.5 · 15 · 30 · 45 m²). The ECODRY50 and the DRY50 also come in a 1.2 m width, useful in narrow bathrooms because you waste less.
And for a one-off shower there's the 2.4 m² membrane pack, which saves buying a whole roll.
Round up, always
Having half a metre of membrane left over costs a few euros. Being half a metre short costs a new order, a few days' wait and an extra joint right where you didn't want one.
On every product page you have the m² calculator: you enter the surface and it tells you the units. And if the project has awkward shapes, send us the measurements and we'll work it out with you before you pay.
