Building nine two-bed apartments is all in a day’s work for Hunter Off-Site – well, two days’ work actually. The timber frame house manufacturer is just about to deliver 24 of its factory-finished modules to a site near the busy high street in Tonbridge, Kent, and the whole lot will go up in 48 hours.
It’s not something that fazes the team because with its volumetric off-site manufacturing system, putting up houses in a matter of days is like a typical day at the office for the rest of us. And because the modules arrive on site fully insulated, with electrics and plumbing, and fitted out with everything from exterior joinery to the kitchens, bathrooms and even the light bulbs, once they’re in place, all that’s needed is to fit the staircase, connect the services, and apply the exterior finish.
“Apart from a couple of days’ work per dwelling on site connecting services and finishing the staircase, you lock the door and there’s nothing else to do,” said managing director Russell Edmondson. “Typically our clients can save 50% off their master construction programme.”
Situated just off the M20 in Aylesford, Kent, the company was set up five years ago by Russell Edmondson to supply small developers with an “off the shelf, easy solution” and using timber was an obvious choice.
“We had a choice of steel or timber. We didn’t mind selling clients the off-site concept but we didn’t want also to have to sell them the concept of a steel frame system so we decided on what everyone understood, and that was timber frame,” said Edmondson.
It’s a decision that’s given Hunter a unique position and a head start in the sustainability stakes. “It’s allowed us to really stand out because all of our modular competitors are steel frame. Being a timber frame supplier you can really promote yourself on sustainability and the building envelope performance,” said Edmondson.
But being a volumetric off-site manufacturer doesn't mean producing a one-size-fits-all design. “Something else that makes us unique is that we don’t have standard house types. Sometimes there’s a perception that with manufacturers you have to buy one type of layout or so many before it's cost-effective,” he said.
About three years ago, the Hunter team decided to move away from the private market when they realised their manufacturing system and product were ideally suited to affordable housing, especially as the Code for Sustainable Homes began to take shape.
“It wasn’t something we set out to do but we automatically give people the predictability, the quality and the environmental requirements,” said Edmondson. “Manufacturing gives you an improved level of quality that you can’t achieve on site. That’s not just finished quality, it’s the whole building quality – how it’s going to perform.”
Off-site manufacturing means these assurances can be given at the design stage. “We can commit to performances that you could never commit to on site because we know it’s built in the same environment. The modules we’re manufacturing now will perform the same way as the modules we produced last year,” said Edmondson.
Because housing associations keep their buildings for stock, they are interested in quality, not only in terms of energy performance but also durability. To meet the latter requirement Hunter uses a plasterboard that is denser and more robust than the norm. “Once it’s fixed to the timber frame you don’t need noggins; you can hang kitchen units and radiators directly from it, and it's very hard to damage.”
Unconstrained by site logistics, Hunter buys the plasterboard in 6x2.5m sheets that weigh a hefty 0.25 tonnes each as their size has benefits for the build. “It means we don’t have any joins in our walls so when the house relaxes over its lifetime it rarely cracks, which is a benefit for our clients,” said Edmondson.
Hunter’s modules are made to a maximum 4.8m wide x 12m long which are strapped together on site to form a complete dwelling. It offers two options for the structure – 38x89 CLS and 38x140 CLS. It is the 89mm option that delivers the “real serious units” for the code.
“We insulate the stud, put OSB on the outside and then wrap the whole module in 60mm rigid insulation with a foil face. It means we minimise the cold bridging through the frame and achieve a U-value of 0.15.”
Internal walls are 38x89 CLS with plasterboard, and insulation between every stud. A row of 100mm ceiling joists supports the plasterboard. “They're made in cassette form on assembly tables, very much like normal timber frame panels. We just repeat the process for however many floors we have,” said Edmondson.
The floors are constructed using Versalam beams and TJI-joists, and because the modules are lifted into place from them, lifting equipment is built in.
When erecting flats, instead of acoustic battens to stop noise transmission, a thin acoustic mat is laid in the 9mm gap between modules. “It means we don't need much additional height for floor build-up,” said Edmondson.
As the modules are watertight before they leave the factory, topping out is not a critical stage so roofs are usually constructed on site using trussed rafters.
Unlike many projects, where construction begins before design is finalised, Hunter needs sign-off on every detail before the first piece of timber goes on the production line. Manufacture is so quick, it’s key there’s no delay. “Typically, from cutting the first piece of wood it takes 15 days to build a three-bed, usually Code level 4, house in the factory,” said Edmondson.
The production line procedures and systems allow Hunter to build houses to a 1-2mm tolerance and if there are problems, the response is that of a manufacturer: continuous improvement by identifying the issue and then updating the system.
However, the modules can take up more tolerance on site than conventional build methods and a pre-delivery digital site survey of the substructure and incoming services alerts Hunter’s installers to any problems.
Transport is a “big logistics exercise” but one that Hunter has down to a fine art. Usually around 12 modules a day can be delivered to site and they are off-loaded using the bespoke lifting equipment Hunter has developed with its haulage company. The module size is dictated by transport width restrictions but modules are easily connected on site. “They are bolted together with a strapping system,” said Edmondson. “They don’t actually need to be bolted together or to the floor; through their own dead weight they're not going anywhere but people are happier if they see the metal straps.”
Hunter’s quality control continues on site, with the assembly and commissioning carried out by its own team. External joinery is all Secured by Design and the timber throughout is FSC certified.
Hunter’s supply chain partners, including timber frame suppliers, are crucial to its ethos and process. “It’s important they buy into what we do, as it has a huge knock-on effect for us if they don’t,” said Edmondson.
This means Hunter relies on its suppliers to provide exactly what it wants and just-in-time delivery. Those relationships are about to be tested further as the company is increasing output, introducing two eight-hour shifts. That seems contrary to the current housebuilding market but, with its social housing client base and unique system, Hunter has fared better than many during the recession. While the cost may initially appear higher than brick and block, Edmondson points out the saving in time, build and performance quality, and ease. “Instead of placing a dozen subcontractor orders, the client has to place only four – with us, a ground worker, bricklayer and a landscaper.”
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The modules are craned into place |
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