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Decoupling membrane calculator

A decoupling membrane takes tile adhesive twice: a thin bond coat to stick the mat down, then more to fill its cavities and fix the tiles. Enter the room and the tile to get membrane rolls and both adhesive layers in whole bags.

The formula · Worked example · Frequently asked questions ↓

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1

Project settings

Units
2

Room geometry

Area 1 · Shape
4 m3 m

Net area 12.00 m² · perimeter 14 m · skirting 13.2 m

Exclusions (0)

Kitchen island, bathtub, shower tray, built-in wardrobe…

Doorways (1)
3

Tiles

Shape
Sold as

Substrate prep

Adds membrane rolls, a bond coat under the mat and the tile adhesive that fills it and fixes the tile.

Adhesive

Grout

Grout

Accessories

More options
%
kg/dm³
%
kg

Leave 0 to estimate it from the coverage.

Defaults to the total door width.

4

Floor height and doors

Keep more if the gap under the door is the air path for heating or ventilation.

Door 1

From the bottom of the door to the current floor. Leave 0 if not measured.

0 if the next room keeps its floor.

Layer, bottom to topThickness
Tile adhesive bond coat under the membrane1 mm · 1/16 in
Decoupling membrane (thin), incl. the adhesive that fills it3 mm · 1/8 in
Tile adhesive bed (10 mm notch)2.8 mm · 1/8 in
Porcelain tile10 mm · 3/8 in
Grout—
New layers16.8 mm · 11/16 in
Old covering taken up—
The floor goes up by16.8 mm · 11/16 in

Door 1

Enter the gap under the door to check it.

Step to the next room 16.8 mm · 11/16 in: too high for a reducer. Use a threshold or ramp profile, or rethink the build-up.

Adhesive beds are estimated from their coverage (1 kg of powder ≈ 0.7 mm per m²). Thicknesses are typical — check your products.

37 tiles · 10 boxes

37 tiles · 10 boxesShow shopping list

How much tile adhesive with a decoupling membrane?

Budget adhesive for two separate jobs. The bond coat goes on the subfloor with a 6 × 5 mm V-notch trowel and holds the membrane down: about 1.4 kg of powder per m². On top, adhesive first fills the square or dovetail cavities of the mat — about 2.2 kg/m² for a thin membrane, 3.9 kg/m² for a thick or underfloor-heating one — and then fixes the tiles with the notch the tile size calls for.

That second layer replaces the usual adhesive line, so the shopping list shows three lines instead of one: membrane rolls, bond coat, and fill and fix. Back-buttering, the uneven-substrate allowance and 10% waste apply as usual, and the tiles, grout and trims come from the tile calculator.

Membrane rolls and seams

Rolls are butted edge to edge, not overlapped, so the floor area plus about 5% for trimming is enough. Typical rolls cover 5, 15 or 30 m², and the calculator picks the combination with the least left over. In a shower or wet room the seams and the perimeter need sealing tape: switch on waterproofing under Extras and it is added to the list.

The membrane formula

membrane = A × 1.05; bond coat = A × r_bond × 1.10; fill and fix = A × (r_fill + r_notch) × 1.10

A is the floor area, r_bond the bond-coat rate (1.4 kg/m² with a 6 × 5 mm V-notch, 2.5 kg/m² with a 6 mm square notch), r_fill the adhesive that fills the mat and r_notch the fixing rate for your tile, including back-buttering. Each line is rounded up to whole rolls or bags only at the end.

Worked example

  • Room 4 × 3 m = 12 m², 600 × 600 mm tiles (10 mm notch, 4.0 kg/m²), thin membrane.
  • Membrane: 12 × 1.05 = 12.6 m² → 1 roll of 15 m².
  • Bond coat: 12 × 1.4 × 1.1 = 18.5 kg → 1 bag of 25 kg.
  • Fill and fix: 12 × (2.2 + 4.0) × 1.1 = 81.8 kg → 4 bags of 25 kg, one more than the 3 bags without a membrane.

Which adhesive goes where

Many membrane makers specify one adhesive under the mat and another — often a particular class or their own product — on top, because the layer sealed between the mat and the tile cures differently. That is why the two layers are separate lines: use the adhesive your membrane maker specifies for each, and the trowel they recommend for the bond coat.

When a membrane is worth it

A decoupling membrane lets the tiles move a little independently of what is underneath. It earns its cost where the subfloor moves:

  • Timber floors and plywood overlays, where timber movement can crack tiles and grout.
  • New, partly dried or cracked screeds and slabs.
  • Underfloor heating, where heating cycles move the floor.
  • Large-format tiles, which are less forgiving of movement.

How much height it adds

A thin membrane adds about 3 mm plus its bond coat; an underfloor-heating membrane about 7 mm. The floor height calculator adds it to the rest of the build-up and checks the gap under each door.

Frequently asked questions

How many bags of adhesive for a DITRA-type membrane?

For a thin membrane, plan on about 1.4 kg/m² for the bond coat and 2.2 kg/m² plus your fixing rate on top. 12 m² of 600 × 600 mm tiles takes 1 bag for the bond coat and 4 bags for fill and fix. DITRA is a trademark of its owner; CalcMyFloor is not affiliated with it.

Which tile adhesive goes over the membrane?

Follow the membrane maker. Many specify a different adhesive on top from the one underneath; the two layers are separate lines so you can buy each.

Do I need a membrane on a concrete floor?

Not always. A sound, dry, flat slab or screed without cracks often takes tiles directly. Use one over cracks, new screeds, underfloor heating or timber.

Is the membrane waterproof?

Many decoupling membranes are waterproof once the joints and corners are sealed with tape. Without the tape, the mat alone is not a waterproof system.

How much height does it add?

About 3 mm for a thin membrane with its bond coat, about 7 mm for a thick or underfloor-heating membrane. See the floor height calculator for the whole floor.

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