Having completed 39 light steel frame (LSF) flats over 4 storeys in Peckham, Excel are about to commence another scheme of 44 flats over 4 storeys in Camberwell for the same contractor. Both schemes had been partially constructed in timber frame before suffering well publicised fires last year. There is no doubt that a timber frame solution is well suited for this type of building and, provided adequate site protection is provided, can be as safe as any other method of construction. There is significantly less 'fuel' on a LSF site during construction - a significant factor when assessing fire risk. There are few, if any, tangible and usable sustainable characteristics that benefit one system over another. Both external wall constructions are typically A+ rated by the BRE Green Guide. Chain of custody certification for timber frame, if properly applied by the supplier, can gain additional points under Mat 2 in the Code for Sustainable Homes. There can also be some benefits in reduced waste on site with the LSF solution.
Timber frame shrinks in service. As well as careful design detailing and control of following trades, the sequence and timing of various applications during the construction process is critical to managing the risk of potential defects occurring from this shrinkage (plus non-elastic deformation and compression of joints between timber). Contractors often plump for joint fillers which do not demonstrate suitable compression characteristics to account for differential movement. This is rarely well managed by architects and contractors (sorry chaps!) and subsequent defects are experienced.
There is no shrinkage in LSF structures - a major benefit to build programme can be delivered because of this and risk of potential remedial defects can be significantly reduced.
Whilst initial prime costs for light steel frame may be higher, the sequence of build allows contractors to value more works earlier in the process delivering a notable cash flow improvement. Arguably, light steel frame can be built over a shorter contract period as brickwork can be installed before cladding dead loads (roof tiles, plasterboard etc) are applied or windows fitted. This will obviously be subject to the contractor arranging following trades appropriately.
What about New Building Regs Part L? Well we await accredited construction details with eager anticipation but my view is live with the default value for thermal bridging or let us calculate and provide the psi/Y values for the SAP assessor.
What is really interesting is how we have been able to discuss a number of potential solutions with our clients without being driven by a manufacturing business that offers one basic solution. There can be a varying number of influences on each project. As previously noted 'one size does not fit all'. 'Horses for courses' and all that..... (anyone got any tips for the Grand National?)
