28 Metres of Perimeter Movement Joint Placed with Schluter Dilex on an Open Plan Ground Floor

Twenty-eight metres of perimeter around an open plan ground floor leaves plenty of edge detail exposed to failure. Wherever the tile field meets a load-bearing wall, doorway upstand, or column base, the edge needs a soft joint so the field can move independently of the structure. Schluter Dilex EK and Dilex BWS are common choices for this work: EK at the wall-to-floor angle, BWS for field joints once the run becomes long. Both use a flexible PVC or CPE insert held between anchoring legs that bed into the tile adhesive.

A cementitious screed keeps shrinking for weeks after it appears dry, and a warmed slab expands by a measurable amount each time the underfloor heating manifold opens. If the tiled field is bonded rigidly into four walls, stress tends to show first at the grout line nearest the perimeter. After that, a row of tiles may debond and give the familiar hollow sound under a tap.

Reading the slab before cutting profiles

Start with a moisture reading, because the adhesive, membrane, and finished tile all depend on it. A 50mm sand-cement screed should be below roughly 75 percent relative humidity on an in-situ probe, or under 2 percent CM for cement screeds, before large format porcelain is installed over it. A calcium sulfate screed sits lower, near 0.5 percent CM. Skipping that check can turn a subfloor moisture barrier screed into a vapour trap, pushing moisture into the adhesive and weakening the bond months after the room looks finished.

Snap a chalk line 10mm off each wall and treat that strip as the movement zone. Across a 28 metre perimeter, there will usually be bowed walls, proud door thresholds, and internal returns where two profiles must meet at a mitre. Mark those junctions before ordering. Dilex EK comes in defined lengths, while BWS comes in 2.5 metre bars, and mismatched insert colours across one open space make the perimeter detail look patched together.

Run a laser level around the room and find the worst high and low points. If the field varies by more than 3mm under a 2 metre straightedge, correct the slab first. Large format tile adhesive can take out shallow hollows, but a 12mm dip under a 1200mm slab is beyond what the bed can safely disguise without cracking the tile.

Uncoupling membrane and perimeter movement

Over a young or doubtful slab, a Schluter uncoupling membrane such as Ditra 25 sits between screed and tile so the two layers can shift slightly in relation to each other. Ditra 25 handles small in-plane stress within the tiled field, while the perimeter Dilex profile manages movement between the tiled field and the surrounding structure. Confusing those two functions is a common source of failure on heated ground floors.

Bedding Dilex EK along the wall line

Comb large format tile adhesive at the perimeter with a 10mm or 12mm notched trowel, covering only the length that can be tiled before the adhesive skins. Press the anchoring leg of the Dilex EK into the fresh bed so the flexible insert sits flush with the intended tile surface. Back-butter the first row of tiles and set them against the profile’s spacer edge.

For a 20mm porcelain slab, choose an EK height that matches the tile thickness plus the adhesive bed. The profile range runs from around 8mm to beyond 20mm, so the selected size should put the insert crown level with the finished floor. A proud insert turns a 28 metre wall line into a long raised edge.

At an internal corner, cut both EK lengths at 45 degrees and bring the inserts together cleanly. A gap at that mitre gives mop water a route under the profile. Some fitters add a bead of Schluter Kerdi-Fix behind the mitre, especially in a wet room zone where water can sit against the skirting line.

Use a tile levelling system on the first perimeter row even when the slab reads flat. Clip-and-wedge sets such as Raimondi or the Rubi Delta keep adjacent large format edges coplanar during cure. The row against a Dilex profile shows lippage quickly under raking light.

Check that adhesive fully supports the anchoring leg. Voids beneath it let the profile move, and the insert can eventually pop. On the first metre, lift a test tile after ten minutes and confirm full adhesive transfer before carrying the method around the room.

Quantities for the 28 metre run

An open plan ground floor measuring 9m by 5m has a wall perimeter of 28m. With Dilex EK in 2.5m bar lengths, the perimeter requires 12 bars once cutting waste at mitres and the two door openings is allowed for. Ordering 13 gives enough margin, because a short splice in the middle of a wall tends to show through the finished grout line.

Field movement joints matter once any dimension exceeds 8m. The 9m length needs one intermediate Dilex BWS across the field, dividing the floor into two bays under 5m each. That adds 5m of profile, or 2 more bars. Underfloor heating lowers the practical threshold, and many manifold specifications call for a field joint every 40 square metres or at any doorway. This floor is 45 square metres, so one intermediate run can be justified along the natural threshold between the two zones.

Adhesive coverage for 45 square metres of large format porcelain, using a 10mm to 12mm notch with back-buttering, is close to 5kg per square metre. That makes 225kg, or about 15 bags at 15kg. Epoxy grout for a wet room covers far less per unit than cementitious grout, and the perimeter Dilex inserts receive no grout. Their function depends on the soft insert remaining free to move.

Epoxy grout and rigid tile joints

Epoxy grout in a wet room, using a two-part product such as Kerapoxy or Fugalite, is chosen for zero water absorption and chemical resistance. Once cured, it is rigid. Movement capacity on this floor sits in the Dilex inserts at the perimeter and in the single BWS field joint, while the grout lines between the tiles remain hard. If the BWS insert is filled with grout, the movement joint is locked.

Work in disciplined batches across a 45 square metre floor. Most two-part epoxies have a pot life of 45 to 60 minutes at 20 degrees, with a shorter working time when the slab is warm from commissioning heat. Wash off with the correct pads and clean water in two passes while the haze is still active. Cured epoxy haze on matt porcelain is difficult to remove later.

Luxury vinyl changes the perimeter detail

Change the finish from porcelain to luxury vinyl over underfloor heating and the perimeter design changes with it. Click LVT is a floating floor, so it needs an 8mm to 10mm expansion gap around the perimeter, concealed under skirting or scotia. Dilex has no role in that build-up because the planks move as a floating raft.

Glued LVT over UFH is a different build-up again. It is fully bonded and expands minimally, so it tolerates a warmed slab better than a click system that can open at the seams if the heat ramps too quickly. The slab moisture reading still matters for vinyl, often sharply, because most vinyl warranties are void above a defined RH and glue-down adhesive is water-sensitive. Movement is accommodated in a concealed perimeter gap for floating LVT and in exposed Dilex inserts for a tiled porcelain floor.

If the client has not chosen between a 20mm porcelain slab and glued vinyl over the same heated screed, the unresolved item is the point at which the profile order and grout specification must be fixed against the drying programme. How much float remains in the schedule once the perimeter detail depends on a finish that has yet to be chosen?