Cement Screed on CLT floors

In public buildings like schools, a cement screed on an impact board on top of a CLT slab is a very common solution, see figure below. It should be a very good solution creating a stable and nice floor to walk on and sufficient sound insulation in all directions if applied correctly. However, nothing comes without an acoustic risk.

A thin screed of approximately 50 mm transmits a lot of structural sound horizontally. To prevent excessive sound transmission the screed must be cut in places with high or normal sound insulation requirements between rooms or different spaces. Due to production efficiency this cut is often done with a saw after the screed is applied. This cutting strategy is risky since the cut is not always done all the way through the screed and the edges might still be connected since the cutting machine cannot reach all the way. The screed is also very often continuous in door openings creating a structural bridge. This screed connection, albeit small, is enough to destroy the impact sound insulation.

This type of important details is not always clearly described in the detail design drawings, but only in text in the acoustic report. It is important that key acoustic measures like this is displayed more clearly to avoid risk for failure. The workers on site must know exactly how the cutting is carried out, otherwise costly repair solutions in the finished building can be expected. The production team and the acoustician should always discuss these crucial steps in early separate meetings and keep close contact during the screed installation.

Workmanship errors are also easy to detect with early indicative impact sound measurements, but they must be performed ASAP, preferably directly on the screed. The cost of an extra measurement is negligible in comparison to a system problem that is discovered too late.