Spandrels: specifications for separation

Although in the true architectural sense it's incorrect, the term 'spandrel' is commonly applied to the usually triangular infill panel that separates one roof space from the next. As well as being structurally sound and fire-resistant, spandrels also need to limit the transfer of sound between adjoining dwellings, so there are a few points to be aware of when they are used in conjunction with Robust Details separating walls.

The Part E Robust Details Scheme is based on the construction pattern book approach, where particular forms of construction have been built, tested and reproduced faithfully and then tested again at least 30 times. If the form of construction has provided acoustic insulation that is on average 5dB better than that required by the Building Regulations, with no test giving less than a 2dB improvement, the construction can be considered for approval as a Robust Detail and be published as a pattern in the book.

With masonry walls, the introduction of the ceiling treatment to the flanking sound transmission path can mean that nothing other than the blockwork is required in the roof space. The full specifications for these scenarios are given in the relevant sections in the Robust Details handbook.

Alternatively, and where the Robust Detail mentions it, the generic spandrel specified in Appendix A1 of the handbook can be used – but the requirements vary with the type of wall construction.

For masonry, the junction of a smooth timber panel with a rough blockwork surface is particularly weak, as it can result in gaps that especially high-frequency sound could penetrate. To counter this, flexible sealant can be applied between the spandrel linings and the blockwork, or the spandrel can be sited on mineral wool; the leaves of the wall must also extend at least 300mm above the ceiling line. Therefore, on stepped terraces with, say, a 600mm step, this may result in both leaves being 900mm above the lower ceiling in order to be a minimum of 300mm above the higher ceiling. Timber – and light-steel-frame walls – not have the same issue, so these can stop where the ceiling lining does, and no additional sealing is required.

The single-leaf spandrel shown in Appendix A1 of the Robust Details handbook is adequate where there is a simple separating wall arrangement, for instance in a terraced row. But where for example there are a set of four cluster homes that result in a cruciform cavity arrangement, a single-leaf spandrel could not be applied without bridging one of those cavities, so twin-leaf spandrels should be used such that they follow the line of, and do not connect, the wall leaves.

It is very rare for a spandrel to be delivered to site and found equal to the full length of the wall: normally, pre-boarded sections will be joined together on site. Appendix A1 states that lapped joints in the boarding, or those occurring over timber members, do not need additional sealing; but straight-through joints, like those used where the spandrel sections come together, do need to be properly treated with sealant or have cover strips applied. In this area especially, reference should be made to other regulations and standards, such as Approved Document B.

Any voids at the ridge or eaves, such as those shown in the diagram below, are acceptable because they allow the sound to dissipate. The sloping ceiling sections, however, will have no void, so need to be double-boarded to limit the amount of sound getting above the ceiling and hitting the wall–roof junction.

Although it is, as mentioned, unacceptable to use generic Appendix A1 spandrel panels to separate neighbouring rooms in roofs, there is provision in Appendix A2 for proprietary systems incorporating timber-frame panels to be used in this setting. However, it must be remembered that these are systems, which means they comprise the spandrels and structural roof cassettes that span either horizontally or from the eaves to the ridge; consequently, they are not for use with attic trusses or any other trussed rafter.

By their nature, each of these proprietary systems is different from the others in make-up, installation or both, so each has its own page in the appendix. The other implication of these differences is that not all walls have been approved to be used in conjunction with these systems. Table 6a in the introduction to the handbook lists each of the approved wall types, and which systems can be used in combination. The Robust Details scheme gives a number of options for acoustically dividing neighbouring roof spaces, but as with any other element, the construction on site has to reflect the specification that was tested, assessed and approved.

To summarise, you should ensure all relevant guidance in the Robust Detail or Appendix A1 is followed, refer to Table 6a in the introduction to the handbook to check the compatibility of proprietary Appendix A2 room-in-roof systems where these are used, and check compliance with other regulations and standards such as Approved Document B.

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