How to Install an Uncoupling Membrane for Floor Tile

Why this matters

An uncoupling membrane is a thin plastic sheet with a pattern of pockets on top and a fleece on the bottom. Thinset bonds the fleece to the subfloor; more thinset fills the pockets and holds the tile. The plastic between them lets the subfloor and the tile move a little independently, so the small in-plane movement of wood does not crack the tile above it. It works only if the fleece is bonded to the subfloor over its whole area. A membrane that looks flat but is stuck to the floor in stripes is a sheet of plastic floating on ridges, and the tile over it cracks and sounds hollow exactly where the bond is missing. Every step below is about getting that one bond right and proving it before tile hides it.

Start state

• The floor is stiff enough. A membrane is not a stiffener. The deflection and subfloor requirements for ceramic and stone are owned by Tile Assemblies Over Wood and Concrete, and the membrane maker's handbook states its own minimum subfloor build for each tile type. The stricter of the two governs.
• The subfloor is flat, clean and sound. Fastened, squeak-free and within the tile flatness tolerance (see related: Floor Flatness Tolerances by Floor Type). Dust, paint and old adhesive stop the fleece bonding.
• The membrane maker's handbook in hand. It names the thinset class for bonding the fleece to the subfloor, the class for filling the pockets and setting tile, the trowel notch, how seams are treated, and where movement joints go.
• Tools: the notched trowel the handbook names, a wood or rubber float or a flat trowel for embedding, a utility knife and straightedge, a mixing paddle and drill, and kneeling boards so nobody walks on bare membrane.

Step 1: Choose the thinset the membrane maker names

This is a product choice, not a preference. Many polyethylene membrane makers call for one mortar class to bond the fleece to plywood, often a modified mortar rated for plywood under ANSI A118.11, and for an unmodified dry-set mortar under ANSI A118.1 in the pockets and under the tile, because a modified mortar enclosed between an impervious membrane and dense tile cures very slowly. Some makers now approve specific modified mortars above the membrane. Read the current handbook for your product and match it; see related: Mortar + Grout Selection Reference.

What breaks if skipped: the wrong mortar above the membrane can stay soft for a long time under porcelain, and a tile floor grouted on soft mortar moves.

Step 2: Mix to the consistency that wets the fleece

Mix with the water ratio on the bag, slake for the time the bag states, and remix. The handbook usually asks for a mortar loose enough to wet the fleece fully but stiff enough to hold the trowel's ridges. Too stiff and the fleece sits on the ridge tops; too wet and the ridges slump.

Hazard: dry thinset powder contains crystalline silica and is caustic. Open and pour the bag low and away from your face, mix with a paddle at low speed so the powder does not cloud, and keep wet mortar off skin, since it causes alkaline burns with prolonged contact; gloves and eye protection for mixing and spreading. Exposure controls fall under 29 CFR 1926.1153; see related: Respirable Crystalline Silica and the Construction Standard.

Mix batches you can use within the mortar's pot life at the room's temperature. A batch that has started to stiffen in the bucket goes in the trash, not back under the paddle with more water.

Step 3: Spread one membrane's width at a time

Cut the membrane strip to length before spreading. Then trowel the bond mortar on the subfloor with the notch the handbook names, combing the ridges in one direction, over only the area the next strip will cover. Spread too far ahead and the mortar skins before the fleece reaches it.

Step 4: Embed the fleece with a float

Lay the strip fleece-down into the fresh mortar, aligned to your layout line. Press it in with the float or the flat side of a trowel, working from the center of the strip out to the edges and ends, in the direction of the ridges, so air is pushed out rather than trapped. The goal is to collapse the ridges into the fleece across the whole strip.

Step 5: Lift a corner and look

Within a minute or two of embedding the first strip, peel back a corner or an edge and look at the fleece. Full contact means mortar across the whole fleece surface you exposed and on the subfloor beneath it, ridges collapsed, no dry fleece showing in lines. Press the corner back down and re-embed it.

Do this on the first strip of the job, on the first strip after every new batch, and any time the room has warmed or the crew has been held up.

Step 6: Treat the seams the maker's way

Butt adjacent strips edge to edge; do not overlap them, because an overlap is a double thickness that telegraphs through tile as a ridge. Where the membrane is also serving as the waterproofing, the handbook calls for its banding strip over every seam and up the wall at the perimeter, set in the mortar it names. Where it is not, a butted seam is enough.

Leave the perimeter and the movement joint locations the handbook and Movement Joints in Tile Floors call for. Do not fill the gap at the walls with mortar.

Step 7: Fill the pockets

Fill the pockets with the tile mortar, using the flat side of the trowel to force it in and strike it off level with the membrane's top. Many makers allow or prefer this the same day as tile setting, immediately ahead of the tile; others allow filling first and setting later. Tile then goes down in a fresh layer of mortar over the filled pockets, at the coverage owned by How to Lay Out and Set a Tile Floor.

Protect it. Bare membrane is easy to crush or tear underfoot. Lay boards to walk on until tile covers it.

 SECTION, not to scale
 tile                [========][========][========]
 thinset, tile       ..............................
 membrane top      --+  +----+  +----+  +----+  +--
 pockets, filled     |..|    |..|    |..|    |..|
 with thinset        +--+    +--+    +--+    +--+
 fleece, embedded  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 bond thinset      ................................
 plywood subfloor  ////////////////////////////////
 joist                  [  ]                [  ]

Worked example: an 11 by 13 kitchen over plywood

The floor: 11 by 13 ft, 143 sq ft, a single layer of 23/32 inch tongue-and-groove plywood glued and screwed to joists at 16 inches on center, checked against the porcelain tile requirements in Tile Assemblies Over Wood and Concrete and the membrane handbook, both passed. Porcelain tile, 12 by 24 inches. The handbook named a plywood-rated modified mortar for the fleece and an unmodified mortar above.

The run: the membrane strips ran across the 11 ft width, so each strip was cut to about 11 ft less the perimeter gap, and the room took strips side by side down its 13 ft length.

The first lift: on strip 1, the corner showed full fleece contact. On strip 4, about 40 minutes after its batch was mixed in a kitchen that had warmed with the afternoon sun, the corner showed the fleece wet in lines with dry stripes between them where ridges had skinned before the strip went down. That is a partial bond across the whole strip, not just the corner.

The fix: strip 4 came up while the mortar was still soft. The crew scraped both the subfloor and the fleece clean, mixed a fresh, smaller batch, spread again for one strip only, embedded it, and lifted the corner: full contact. For the rest of the room they mixed half-batches, enough for two strips, and lifted a corner on every strip, not just the first after each batch.

The pockets were filled the next morning ahead of the tile, with the unmodified mortar the handbook named.

How to verify you got this right

The bond is verified before the pockets are filled, because once mortar and tile are on top, the fleece can no longer be seen.

• Every lift showed full contact. Corner lifts on every strip in the kitchen after strip 4, each one showing fleece covered with mortar end to end of the exposed area.
• No hollow areas. Once the bond mortar has set, press and slide a flat hand or a soft-soled foot across every strip, and tap lightly with a knuckle. A bonded membrane feels and sounds solid; a hollow area flexes or sounds drummy under the plastic. Mark any one found, cut that section out and rebond it with a patch of membrane before the pockets are filled.
• Seams butted, flat. No overlaps, no lifted edges along seams.
• Perimeter open. The gap at every wall and fixed object free of mortar, ready for the movement joint.

What changes the call

• Over concrete, the membrane is often used over a cracked or green slab rather than plywood, and the bond mortar is the one the handbook names for concrete, usually a different class from the plywood one.
• Heated floors. Some membranes are made to hold heating cable in their pockets; the cable is laid before the pockets are filled, and the heating maker's spacing and continuity tests come before any tile goes down.
• Large-format tile above about 15 inches on a side often needs a different trowel and back-buttering for the tile layer; the membrane steps here do not change.

References

• The membrane maker's installation handbook, current edition (mortar classes, trowel, seams, movement joints)
• ANSI A118.1 and ANSI A118.11 mortar specifications, as named by the membrane maker
• 29 CFR 1926.1153, respirable crystalline silica in construction
• See related: Tile Assemblies Over Wood and Concrete
• See related: Mortar + Grout Selection Reference