CROSS Safety Report
Performance of acoustic separation in a floor system
Overview
The reporter started to investigate the performance of standard details for acoustic separation in the floors between flats and is concerned about the structural, and possibly the fire safety implications.
Key Learning Outcomes
For designers:
- If standard details are to be specified, their suitability for all design criteria should be checked
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This report is from a qualified structural engineer who is concerned about their newly built, first floor flat and the performance of the floor. After moving in, the reporter considered the floor to be excessively bouncy and so investigated further.
During investigations of the floor structure, the reporter found that it was made of oriented strand board (OSB) web engineering joists at 600 centres with OSB structural decking over the top. On top of this there was a floating floor comprising of acoustic composite timber battens 45x67 millimetres deep at 600 centres, 19mm thick plasterboard and 22mm chipboard on the plasterboard.
The plasterboard is fixed to the top of acoustic battens. Furthermore, the reporter has determined that some if not all internal walls in the flat are erected on the floating floor. The reporter is concerned that the acoustic details used may result in structural safety implications.
The reporter started to investigate why the construction details had been used and found that a number of parties produce standard details that show this construction - although all specify that walls should be built off the structural floor.
The reporter considers that the acoustic design should be coordinated with the structural design and is concerned that this has not been the case
To avoid post completion sound testing, developers notify building control in their submissions about the details they want to use. Proof of intent to use these details is all that is normally required and means that no further checks have to be undertaken.
The reporter considers that the acoustic design should be coordinated with the structural design and is concerned that this has not been the case. In particular, the reporter questions the use of plasterboard as part of the floating floor, pointing out that plasterboard is just gypsum sandwiched between two layers of paper.
The reporter has approached plasterboard manufacturers for their opinion and has been advised that they recommend that plasterboard should not be fixed over the battens but rather should be secured between the joists.
The reporter questions the fact that nobody confirms or designs the structural suitability of the acoustic floor structure
Whilst initially concerned with the structural implications of incorporating plasterboard in a floating floor build up, the reporter is also concerned that any leakage of water in (say) a bathroom or kitchen will result in softening of the plasterboard and lead to extensive and expensive repairs.
Finally, the reporter would like to draw attention to engineers stating on their drawings either: “the acoustic floor is to be confirmed by others” or “the acoustic floor is to be designed/approved by others”. Local Authorities are presumably accepting such submissions because they are satisfied that the standard acoustic floor details have been fully approved. The reporter questions the fact that nobody confirms or designs the structural suitability of the acoustic floor structure.
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Expert Panel Comments
An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.
This is a good report highlighting how complex even 'simple’ designs can be. Laying plasterboard over battens makes the board vulnerable to local crushing.
The natural development of minor cracking due to structural movement or changes in temperature or moisture content, accidental water ingress or accidental load and service condition are likely to undermine key functionality such as structural integrity, smoke containment and potentially acoustic performance.
Loadbearing partitions including racking walls should be built off structural decking, but light weight partitions that are not structural, or fire separation walls, can be satisfactorily constructed off a suitable floating floor.
The adoption of standard details for one criterion (in this case, acoustics) does not guarantee that they are suitable for another (in this case, structural). Designers need to ensure that when they specify such standard details, they have fully considered their suitability.