Perimeter Expansion Strip for Levelling

Every material in a floor moves. It expands a little when it warms up and contracts when it cools, and a poured compound also shrinks slightly as it dries. If the levelling layer is poured tight against the wall, these movements have nowhere to go. The stress then releases where the layer is weakest, and a crack appears, often in corners or near doorways.

The strip creates a compressible layer around the edge. When the floor expands, the strip squeezes together. When the floor contracts, the strip springs back. It also:

• stops footstep noise travelling from the floor straight into the walls,
• protects the bottom of the walls from compound running onto them,
• gives a clean, straight edge for the final floor covering to meet.

The usual choice is a polyethylene foam strip. It is light and flexible, does not absorb water and bends easily into corners. Three things decide which strip suits your job.

Height

The strip must always be taller than the planned levelling or screed layer. The part that sticks up is trimmed off once the compound has hardened. If the strip is too low, the compound flows over it onto the wall and the expansion gap no longer works.

Thickness

For a thin levelling layer in a flat, a thinner strip is usually enough. A thicker screed or a heated floor moves more, so a thicker strip is chosen. If you have underfloor heating, follow the heating system manufacturer's instructions, as they usually specify the thickness of the perimeter gap.

Self-adhesive strip and strip with a film skirt

A self-adhesive backing holds the strip against the wall during the pour, so it does not slide or peel away. A strip with a film skirt has a band of plastic film along the bottom edge, which you lay flat on the subfloor facing into the room. The poured compound weighs the film down and seals the joint between the strip and the floor. With very runny self-levelling compounds, or when pouring over insulation, this is the more practical option because the compound cannot seep under the strip.

The tradesperson first measures the perimeter of the room, including alcoves, piers and the area around doors. They also check how thick the levelling layer will be, so they can pick a strip of the right height.

Next they clean the bottom of the walls, removing dust, grease and any loose plaster or paint. Self-adhesive strip will not hold on a dusty surface. By this stage the subfloor is often already primed, or it is primed just after the strip goes on, depending on the compound manufacturer's method. What matters is that the primer is fully dry before the strip is fixed and before the pour.

The strip goes on in one continuous run along every wall. In corners it is folded, not cut. Joints are butted tightly or overlapped slightly so there is no gap anywhere. The strip also goes around everything that passes through the floor: columns, heating pipes, waste pipes, chimney breasts and door frames. These spots are a frequent source of cracks.

During the pour, someone keeps an eye on whether the strip is holding and the film is lying flat. Once the compound has hardened, the excess is cut off with a sharp knife level with the floor. The rest of the strip stays in the joint permanently and is later hidden by the skirting board. This part keeps doing its job for the whole life of the floor, so it is never pulled out.

A simple rectangular room with straight walls is done quickly. More time goes into:

• an irregular layout with lots of corners, alcoves and piers,
• many pipes and penetrations that the strip has to wrap around neatly,
• uneven, crumbly or dusty walls that need cleaning or consolidating first,
• a thick screed or underfloor heating, where a taller, thicker strip with a film skirt is needed,
• awkward spots behind radiators or under built-in furniture.

• Gaps in the strip. One short stretch of bare wall, for example behind a door, is enough for the compound to bond to the wall. A crack often forms right there.
• Forgotten pipes. A heating pipe cast solid into the floor transmits movement and sound, and the floor around it cracks.
• A strip that is too low. The compound flows over the top and joins the wall.
• A strip that has come loose. On a dusty or damp wall, self-adhesive strip peels off during the pour and the compound runs in behind it.
• Cardboard or offcuts of polystyrene instead of strip. Cardboard soaks up water and goes soft, polystyrene is too rigid. Neither works as a proper expansion joint.
• Pulling out the rest of the strip after curing. This leaves an empty joint that fills with dirt over time and loses its flexibility.

If you are levelling the floor yourself, you can easily fit the strip on your own. All you need is a tape measure, a sharp knife and a little patience in the corners.

1. Measure the perimeter of the room, including alcoves and pipes, and add a margin for joints.
2. Check the planned thickness of the levelling layer and choose a strip that is taller.
3. Vacuum and clean the bottom of the walls, removing loose plaster and grease.
4. Prime the subfloor according to the manufacturer's instructions and let it dry.
5. Start in a corner, peel off the backing and press the strip against the wall, right down to the floor.
6. Fold the strip in the corners rather than cutting it.
7. Butt the joints tightly or overlap them slightly.
8. Wrap the strip around every pipe, column and door frame.
9. Lay the film skirt, if the strip has one, flat on the subfloor facing into the room.
10. Go round the whole perimeter before pouring and press back any sections that stand away from the wall.
11. Once the compound has cured as the manufacturer specifies, trim the excess strip level with the floor.

• The strip runs round the whole perimeter without a break, including behind doors and in corners
• All pipes, columns and penetrations are wrapped
• Before the pour, the strip stands above the planned level of the compound
• After the pour, the compound is not bonded directly to the wall anywhere
• The excess strip is trimmed cleanly level with the floor
• The rest of the strip is still in the joint and has not been pulled out
• There are no visible cracks or lifted patches along the walls