FIG 12.0 (chapter opener) A typical low-rise CLT site (Ermine Street Church Academy by AHMM, as described in subsequent case study). CLT use requires a different, smarter approach to construction. Digital design and manufacturing enables better and more efficient coordination, reducing risks and improving quality. To best realise this potential, issues should be considered and communicated early by the team and with potential contractors or suppliers, not just at tender or construction stages. Beyond safety, key issues during manufacturing and construction include moisture control and durability, surface protection and the work of follow-on trades. Whitewoods are versatile timbers but not typically durable. Moisture content is controlled in factories and panels arriving on site are reasonably consistent at around 12% moisture content (MC). They can accommodate normal site conditions and exposure but timber will begin to degrade if moisture levels exceed 20% for long periods of time. Water trapped during construction, from weather or leaks from service installations (the source of which may not be obvious or immediately recognised) and unforeseen site delays or stand-downs, which are not uncommon, all pose risks to long-term performance. Each must be addressed in a moisture control strategy or method statement with associated responsibilities clearly defined within appointments. Regular checks using a hand-held moisture meter should be made at the panel surface or within the panel depth for those exposed to excess moisture for extended periods. Some sites have used temporary roof decks to partially cover the footprint of a new build, like a hat that is lifted into place at the end of each day. This is clearly not an option for a very large building but most schemes are generally built with upper floors exposed to the elements as they extend upwards. Others have used tarpaulins or waterproof ply boarding to provide a degree of protection or a sacrificial temporary membrane installed in optimum conditions once uppermost panels are placed (or even before they are lifted). The most robust approach will be to manage water on site, away from completed areas and detail subsequent construction whether roofs or other forms of enclosure, to allow the substrate to ‘breathe’ and dry out if required (this may impact ventilation paths, membrane positions, insulation specification etc). FIG 12.3 Moisture meter used on site to determine moisture content of slabs before being sealed under other construction. Panel edge sealant can be applied on site and this can be particularly useful in temporarily reducing risks for wall panels that may otherwise be exposed to rain along their top edge or be standing in water at their foot. Some installers may apply a flashing tape to exposed edges. Some manufacturers offer a factory applied water repellent coating to full panels and although it should be noted that this is not intended to provide long-term protection, it may offer some reassurance for projects considered to be at risk (including of site delays) and has the added benefit of limiting marking of the panels from regular site actions, footprints etc during assembly.
CHAPTER 12
BUILDABILITY AND ASSEMBLY
DURABILITY AND MOISTURE CONTENT
Keeping moisture out
Surface applied sealants