Levelling Underfill Expanded Clay (Liapor) Lightweight Granules

Expanded clay is made by firing clay at a very high temperature. The clay puffs up and forms porous granules that are light but strong enough to carry a floor. According to manufacturers, it weighs several times less than the same volume of concrete. On an old floor structure, that makes a big difference.

An underfill is worth considering mainly when:

• The base is uneven and the differences are too large for a self-levelling compound
• The floor structure is timber or otherwise weak and cannot take a heavy layer
• You want a dry floor build-up with no wet trades
• Cables or pipes need to run within the floor
• You want to improve the thermal and sound insulation of the floor

Why not simply pour concrete? A concrete screed is heavy and brings a lot of water into the building. At usual thicknesses it takes weeks to dry, and you cannot lay the final floor covering until it has. A dry underfill, once levelled and covered with boards, lets work carry on straight away. Concrete is still the better fit for heavy point loads, industrial spaces, or where you want the floor to store heat.

With timber floors in older houses, ask a structural engineer to check the load capacity, especially if you are adding a thicker layer or a heavy finish such as tiles. If the work would involve changes to the load-bearing floor structure, check with your local building authority beforehand what approval is needed. In a flat, let the building management or owners' association know about the work.

Expanded clay is sold in several grain sizes. For floors you will most often come across these:

• Fine grade, roughly 1 to 4 mm: for fine levelling and thinner layers, easy to screed flat
• Medium grade, roughly 4 to 8 mm: the usual choice under dry floor systems in flats and houses
• Coarse grade, roughly 8 to 16 mm: for large height differences, often as a bottom layer topped off with a finer grade

The main rule is to follow the instructions of the dry floor system you are laying on top. System manufacturers usually state which grain size to use, the minimum thickness, and how thick a loose fill may be. For thicker layers they often require a bonded fill or an extra layer of boards so the granules do not shift under load.

The fill is covered with gypsum fibreboards, cement-bonded particle boards or ready-made dry floor elements. OSB is also used, but in a floating floor you must stick to the thickness and jointing the manufacturer specifies, otherwise the board will flex on the loose base.

If your floor structure is sound and you are working in a single room, you can lay the underfill yourself. Above all, you need patience to get it truly flat. For larger height differences, prepare screed rails, a long aluminium straightedge and a spirit level or laser level.

1. Remove old layers, dust and loose material, then vacuum the base.
2. Check that the base is dry and that timber joists or floorboards are not damaged.
3. On a concrete base, lay polythene sheeting with overlaps turned up the walls. On a timber floor, lay a vapour-permeable protective layer that stops the granules trickling through, as the system manufacturer specifies.
4. Fix edge insulation strip around all walls.
5. Find the highest and lowest points and set the finished floor height, including boards and floor covering.
6. Set the screed rails level and check them with a spirit level.
7. Pour expanded clay between the rails and draw it flat with the straightedge.
8. Compact the layer lightly, for example with a board or as the manufacturer describes, then level it again.
9. Lift out the rails, carefully fill the gaps they leave and level them.
10. Walk only on load-spreading boards and lay the covering boards one by one, starting from a corner.

Cover any services in the fill so there is enough material above them and the board does not rest directly on them. Leave electrical connections to a qualified electrician and water pressure testing to a plumber.

How much work the job takes depends mainly on:

• The size of the height differences and the layer thickness needed
• The condition of the base, for example rotten floorboards or old parquet that has to come up
• How many pipes and cables have to be worked around
• The shape of the room, alcoves and door thresholds
• Access: carrying bags up several floors without a lift takes longer
• Whether the fill is loose or bonded

On the other hand, a simple rectangular room, a firm dry base and a carefully planned finished floor height all make the work easier.

In practice, the same mistakes come up again and again, and they usually only show once the floor covering is down:

• Poor levelling and compaction: the fill settles later and the floor flexes or creaks
• Missing edge strip: the boards touch the wall, pass footstep noise on and have no room to expand
• Wrong separating layer: no moisture protection on concrete, or an impermeable sheet on a timber floor where it does not belong
• Wrong grain size for the thickness: coarse granules in a thin layer do not stay stable
• Miscalculated height: the thickness of the boards and covering is forgotten, so doors will not close or a step appears into the hallway
• Walking directly on the fill: this leaves dips that cannot be fixed once the boards are on

Before the fill is covered with boards, go through this list:

• The fill is flat and gaps under a 2 m straightedge are no larger than the board manufacturer allows
• A continuous edge strip runs around all walls
• The separating layer is intact and the overlaps are turned up the walls
• Services are covered and no board will rest directly on them
• The height matches the plan, including boards and floor covering
• Once the boards are laid, the floor does not flex, even when you walk along the walls
• Transitions to doors and neighbouring rooms work without unexpected steps