Installation of Dry Screed (Keramzit + Rigidur Boards) to Level

The floor is made up of several layers. At the bottom is a separating layer. On a concrete slab this is usually a polythene sheet that protects against residual moisture. On a timber floor it is paper or fleece, which stops the fine granules from trickling through gaps between the boards. An edge insulation strip runs around the walls. Next comes the levelling fill of expanded clay. This is fired clay: light, non-combustible and it does not soak up water. Rigidur floor elements made of gypsum and cellulose fibre go on top. They are usually around 20 to 25 mm thick and often consist of two layers bonded with an offset, which forms a stepped edge or rebate at the joints.

Expanded clay is sold under several names, such as LECA or Liapor. Technically it is the same material, and the system works the same way. Contractors often include slightly different things in this job, so before you accept a quote, check:

• What fill depth the quote assumes and what happens if more is needed
• Whether the separating layer, edge strip and screws are included
• Which type of floor element will be used and whether the joints are both glued and screwed
• Whether sanding the joints and preparing for your chosen floor covering are included

A dry screed makes most sense where a wet screed would cause problems. A timber joist floor copes badly with moisture and heavy loads. In an older block of flats you want to keep extra weight off the floor structure, and waiting several weeks for a screed to dry may not be an option. In a loft, carrying materials up and mixing on site is awkward. The expanded clay also improves heat and sound insulation slightly, which helps above an unheated cellar or in a flat with neighbours below.

The system can take laminate, vinyl, engineered or solid wood, carpet and ceramic tiles. For tiles, follow the board manufacturer's instructions, use a flexible tile adhesive and, in a bathroom, the manufacturer's waterproofing system. Do not use a dry screed in rooms with constant moisture or heavy point loads unless the manufacturer confirms it is suitable.

If you live in a flat, tell the building management or owners' association before work starts, mainly because of noise and materials being carried through shared areas. If the floor structure is old or damaged, ask a structural engineer to check its load capacity. Replacing a floor build-up does not usually need approval. If the work affects the load-bearing structure, ask your local building authority what your local building regulations require.

A competent DIYer can lay this floor if they work slowly and carefully. A level fill matters far more than speed. Before you start, read the board manufacturer's installation guide, because screw spacing, adhesive type and minimum fill depth differ between systems.

1. Remove the old floor covering and check that the subfloor is sound, dry and free of loose material.
2. Vacuum the subfloor, chip off loose bits and fill larger holes.
3. Lay the separating layer with overlaps and turn it up the walls.
4. Fix the edge strip around the perimeter so it stands above the finished floor height.
5. Mark the finished floor height with a laser or spirit level and subtract the board thickness.
6. Set guide rails or battens exactly at the height of the fill.
7. Pour the expanded clay, screed it off along the rails with a straightedge and lightly compact it as the manufacturer recommends. Build up deep fills in layers.
8. Remove the rails, then fill and level the grooves they leave.
9. Lay the first row of boards from the wall opposite the door. From now on, walk only on laid boards, never on the levelled fill.
10. Apply the system adhesive to the rebates, lay the boards with staggered joints and screw the joints together.
11. Once the adhesive has cured, scrape off any squeezed-out glue and sand down raised edges.
12. Check the floor with a straightedge, trim the edge strip and leave a gap at the walls for the skirting board to cover.

The simplest case is a rectangular room on a solid concrete slab with only small height differences. The deeper the fill, the more work there is: more material to carry, more layers to compact. Awkward layouts, alcoves, and pipes or cables in the floor that need careful filling around them all add time. So do doorways, where heights must match the next room exactly. The job also takes longer if an old floor has to be broken out first, if the floor structure needs repairs, or if materials must be carried up several storeys without a lift. Tiled floors and bathrooms add waterproofing and closer screw spacing.

• Uneven or poorly levelled fill. The board above it flexes, the joint creaks and laminate locking joints break.
• Walking on the finished fill. Footprints leave hollows that nobody can see once the boards are down.
• Too little adhesive or the wrong adhesive. The boards do not act as one slab and the joints move.
• No movement gap at the walls. The floor has no room to expand and noise travels into the walls.
• Dust and mortar debris under the separating layer. The fill settles unevenly.
• Overlooking the floor's load capacity. Even a light build-up has weight, and old timber floors need checking.

Before the floor covering goes down, walk over the floor with the installer. Use a 2 m straightedge to check flatness. For ordinary floors, a deviation of roughly 2 mm over this length is a common target. Large-format tiles or thin vinyl need a flatter surface, so check the covering manufacturer's requirements.

• The straightedge never shows a bigger gap than the covering manufacturer allows
• The boards do not flex, rock or creak when you walk on them
• Joints are staggered and glued, with excess adhesive removed
• Screw heads are countersunk and nothing sticks up
• A movement gap remains around the whole perimeter
• The height at the door matches the adjoining rooms
• In a bathroom, waterproofing runs up the walls and corners are reinforced with tape