Do I need a decoupling membrane under a tiled floor?

Tiles and grout are brittle and almost everything they get laid on moves a little. Bond the two together rigidly and the movement goes straight into the tile. A decoupling membrane is a thin layer whose two faces can shift slightly relative to one another, so the movement is absorbed inside the layer rather than transmitted upwards.

Step by step

1. Establish whether your substrate will move.A timber deck always moves. A new screed shrinks for months. A slab with underfloor heating expands and contracts every day it runs. A mature, sound concrete slab in a stable room barely moves at all.
2. Check for existing cracks and judge whether they are active.A stable shrinkage crack in an old slab is a good candidate for a membrane. A crack that is opening, stepped or associated with movement in the walls is structural, and a membrane will not save the tiles.
3. Use one over a timber floor.This is the strongest domestic case after underfloor heating. Timber decks flex and move with humidity, and a decoupling layer over a properly prepared, adequately stiff deck is the standard way of making a tiled timber floor last.
4. Use one over underfloor heating.A heated screed cycles daily and puts continuous stress into a bonded tile bed. Membranes designed for this also help distribute the heat and reduce the thermal mass to be warmed.
5. Use one over a new screed you cannot wait for.Screed shrinks as it dries and the drying period is long. Where the programme will not allow the full wait, a decoupling membrane is the accepted way to tile earlier — but the moisture requirement for the tile adhesive still applies.
6. Do not use one to hide a bad base.The membrane goes onto a substrate that is already flat, clean, sound and correctly prepared. It is a movement layer, not a levelling layer and not a strengthening layer.
7. Fill the dimples solid.The most common installation failure. The adhesive must fill the membrane's dimples or waffles completely so the tile is fully supported, otherwise you have deliberately created the voids that cause hollow tiles and point-load cracking.
8. Use the adhesives the membrane manufacturer specifies.There is usually one specification for bonding the membrane down and another for bedding the tile on top, and they are not always the same product. Follow the system rather than mixing brands.
9. Keep the movement joints.A membrane does not remove the need for perimeter and intermediate movement joints in a tiled floor. Large areas, doorways and any junction with a different substrate still need them.

Alternatives

• Decoupling or uncoupling matting: A dimpled polymer sheet bonded down and tiled over. The most common domestic form, and the one that also carries heating cables in some systems.
• Fleece-backed membrane: A thinner sheet product with a fleece face for bonding. Lower build height, useful where floor level is tight.
• Tile backer board: Over timber, a rigid backer board screwed down does a different job — it adds stiffness and a stable, water-tolerant surface. It is often used with a membrane rather than instead of one.
• A better substrate: Stiffening a bouncy timber floor, or letting a screed dry properly, addresses the cause rather than accommodating it. Where it is possible, it is always the better answer.

Questions people ask