Can screed be laid on a timber floor?

It is possible, but never by laying the mortar directly onto the boards. A screed on a timber floor must always be designed as afloating, decoupled screed, and the real question is not technical but structural: can the floor support the load?

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Load-bearing capacity first, everything else second

A traditional 5 cm screed weighs about 100 kg per square metre. On old joists sized for a floor and a carpet, this extra load is rarely acceptable. Before making any decisions, you must know the section of the joists, their span and their spacing, and have the load validated by a structural engineer if in doubt.

This check is not just a formality: a floor that flexes under the screed will cause it to crack, and the problem then becomes irreparable without removing everything. Theweight of a screed per square metreis the first figure to put on the table.

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How to lay a screed on timber

If the load-bearing capacity is sufficient, the principle is that of afloating screed: the wood and the mortar must never be in contact.

• Consolidating the floor: screwing down boards, replacing damaged elements, and eliminating squeaks.
• Laying a decoupling layer and rigid insulation, which distributes the load and breaks acoustic bridges.
• Edge insulation strips around the entire perimeter, extending above the finished floor level.
• Reinforcement with mesh, recommended on deformable substrates.

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When it is better to give up on a wet screed

When load-bearing capacity is limited, or when the available height is less than 6 or 7 cm, a traditional screed is no longer the right solution. Thedry screed, made up of panels laid dry, weighs three to four times less and introduces no moisture into an old floor, which is a decisive factor in renovation projects. A lightweight screed or a thin-set anhydrite screed are other options to consider.