Dovetail floor sheets are thin steel sheets with a dovetail-shaped profile that serve both as permanent formwork and as reinforcement for a thin concrete floor. They stay in the structure after the pour: the dovetail form keys into the concrete and provides both bond and stiffness. Lewis® is the best known brand name for this type of sheet.
The combination of steel sheet and concrete, together with the joists beneath, forms a load-bearing floor. That makes the system particularly suited to renovation: a concrete floor can be built on an existing timber joist structure without lifting in heavy precast units.
Why are they used?
- Sound insulation. The mass of the concrete substantially improves the airborne sound insulation of a timber floor — the main reason for using it in flats and converted buildings.
- Fire resistance. A concrete layer over the joists improves the fire resistance of the floor separation.
- Water resistance and stiffness. In a bathroom or kitchen over a timber floor, the concrete layer gives a flat, stable, moisture-tolerant base for tiling.
- Underfloor heating. The concrete layer can house heating pipes and spreads the heat.
- Low build-up height. The floor can be executed from around 36 mm thick, which is often decisive in a conversion.
Installation in outline
- Check the joists first. This is the critical step. A 50 mm concrete layer weighs roughly 110 to 120 kg per square metre — on top of the existing loading. Have a structural engineer assess whether the joists and bearings can take it; strengthening or adding joists is sometimes necessary.
- Lay the sheets at right angles across the joists, so that each sheet bears on several joists.
- Overlap the sheets at the head ends and click them together lengthwise. Keep to the overlap specified by the manufacturer (of the order of 50 to 100 mm at the head end).
- Fix the sheets to the joists as specified, and set edge formwork and a perimeter isolation strip against the walls.
- Place and compact the concrete, level it off and keep the floor sufficiently damp while it cures.
The clear span between joists is limited; on the known systems it is at most of the order of 2.5 metres. Exact maximum span, sheet thickness and concrete specification are in the manufacturer’s instructions — those govern, not a rule of thumb.
Points to watch
- Always recalculate the weight. This is the most common mistake: laying sheets on a joist structure never designed for the load.
- Drying time. Concrete is not immediately ready for tiling. Allow several weeks before the residual moisture is low enough for a vapour-tight floor finish; measure the moisture rather than guess.
- Isolate the edges. Without a perimeter strip against the walls, impact sound carries through and the slab can crack from shrinkage.
- Work under a watertight roof. Concrete poured onto joists in a building that is not yet weathertight causes moisture damage in the timber below.
- Services first. Electrics, underfloor heating and drainage must be in place before the pour; chasing afterwards into a thin slab with steel sheets is asking for trouble.
Related terms
- Permanent formwork
- Formwork
- Formwork plate floor
- Reinforcement
- Screed
- Liquid screed
- Sound insulation
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.