Current affairs

frame method of construction, using Celcon aircrete elements manufactured in Germany. The panels are 600mm wide and 100mm thick, and typically have a life expectancy of 50 years. Slightly reinforced with steel bars for transportation, the panels lend themselves to two storey houses, some with a room in the roof, but are not currently used in commercial properties. Subcontractor SIG installs them on site, working with Barratt Homes, housing associations and others. With a typical roll out of five days, I-House is attractive to the social housing market. Describing the build method and flow, he said the

elements are placed on top of the damp proof course using a mortar bed and adhesive, then propped and fixed together with ties and nails. Bracing allows the mortar to set. Insulation is ‘full filled’ to avoid any flue effect in the cavity. Aircrete jumbo blocks fill in along the joists around the edge of the floor cassette system, then the next floor is built similarly, with vertical elements. The roof is installed, but windows and the outer leaf of brick, rendered blockwork or timber cladding are usually installed later. Celcon is classed as non combustible against

Approved Document B (ADB), Class A1 against EN 13501-1: 2007, and Class 0 for surface spread of flame (two hours for a load bearing wall; four for a non load bearing wall). It has third party approval from Lucideon, Buildoffsite Property Assurance Scheme (BOPASS) and the National House Building Council (NHBC).

Timber frame builds Next, James Walker of the Structural Timber Association (STA) identified the largest timber frame (TF) market as low rise housing, new builds occupying around 30% in England and 80% in Scotland. Multi storey, mainly residential buildings are also built, mostly limited to six storeys for fire reasons, though some newer builds are higher. Ten storey Dalston Lane in Hackney, hailed as the largest cross laminated timber (CLT) building in the world, is really a hybrid, using steel frame and concrete cores combined with CLT panels. And many bespoke timber buildings (supermarkets, office blocks etc) exist. ‘Off site’ construction was driven partly by the

2016 Farmer review, Modernise or die, and shortage of labour to meet increased housing demand, but as Mr Walker’s graph showed, actual construction is not increasing. Other off site drivers were reduced waste, improved quality and cost, and better thermal performance and construction. However, he pointed out that a building must often try to satisfy conflicting requirements: structural, thermal, acoustic and fire. Traditionally, timber systems were ‘stick built’ by

carpenters from raw materials delivered to site, but are now mostly factory built. Methods include ‘post and beam’ forming the building envelope, used more for bespoke commercial buildings and traditional oak framed buildings. More common is panelised, flat pack housing, built in factories ‘to varying degrees’. ‘Open panels’, using timber studs to provide the

structure with an outer sheathing board for rigidity, are assembled on site, then insulation and internal lining added. ‘Closed panel’ solutions, eg structural insulated panels (SIPs) with timber boards on each side of an insulating core, and CLT, are fitted out in the factory. They consist of studs, sheathing board, internal plasterboard, linings and insulation. Windows, doors and services can be applied on site or in the factory. Government grants have helped ‘volumetric construction’ take off. Here, the whole housing block is factory built, then externally clad and assembled on site. Timber suits this modular building concept, well, he explained, as it’s strong, lightweight and easily workable. A number of companies are rolling these out. Around half of the calls and emails to the

STA helpline concern in-service and construction fires. ADB contains fire requirements for in-service buildings, but not for those under construction, or for preventing fires on site or limiting adjacent damage and harm from fire. STA’s activities include research, training and guidance, including on fire safety, published with the Health and Safety Executive (HSE). The HSE is now inspecting sites and prosecuting where this guidance is not followed. Collaborative guidance is also created with stakeholders such as BM TRADA, which has written structural guidance and design detailing for durable timber buildings. Recently, the STA Assure scheme for independent building assessments was introduced. Responding to a question, Mr Walker said there’s

no restriction on the type of insulation used ‘as long as the wall assembly meets the thermal and acoustic requirements – in common with other buildings, what can be put on the external face of closed panel FEBRUARY 2019 47


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