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Absolutely pre-fabulous

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technical & practice

Prefabrication seems to be coming into its own at last. Here we examine some modern versions of an old idea John and Charlotte Thompson's new extension has added a modern dimension to their Art Deco home in Handforth, Cheshire. Nothing extraordinary about that - except that theirs was a prefabricated extension, delivered to site, that, using a rendered steel construction, manages to retain the period feel of the property.

On the cranked roof of the existing house a previous architectdesigned circular 'observatory' had fallen into disrepair; its aluminium frames were leaking, the double glazing had failed and, in the words of John Thompson, 'it was in a very dilapidated state'. He decided to make the most of the refurbishment by adding more usable space and repairing the 'botched' repairs that had accumulated over the years.

The new design, by Paul Hughes of Hanson-TiS, is a curved roof extension, housing a bedroom with en-suite facilities. This connects to the full-height bow-frontage, containing a spiral stair. Large glass doors open out onto a timber deck over the existing roof. It is, says Thompson, 'an ideal getaway from the kids'.

The extension itself is built of heavy stock steel box sections at 2-3m centres, clad in composite sandwich panels - pultruded GRP skins containing 100mm extruded polystyrene insulation. These panels are glued to the external face of the framework steel box sections with two-part polyurethane glue; they are also mechanically fixed, and the bottom edge is held on a lip to prevent gravitational slippage. Acting as stiffeners, the panels are overclad in calcium silicate boarding as a base for an elastomeric render.

Internally, the steel cavity is packed with 100mm mineral fibre insulation (for acoustic and insulation purposes), and overboarded with plasterboard with a skim finish, resulting in a wall with a nominal thickness overall of 200mm and a U-value of 0.18W/m 2K.The roof uses a composite 60mm insulated Kingspan board and the soffit has been battened out, interwoven with mineral fibre batts and plasterboarded to minimise rain noise and give a U-value of 0.21 W/m 2K.The original observatory was cut back to sill height as a datum point, above which to build, and where the new units have been plugged on.

Because of the size of the prefabricated structure to be added to the house, after factory assembly it was broken into two pieces for ease of transportation.

From start to finish, work has taken just six weeks in the factory, where all fixtures and fittings were built into the unit. The site operations to offload and fix the units took place between 9am-5pm on 4 April, so that the Thompsons were able to sleep in their new penthouse suite the very same night. Connections and general interface items took a further week.

lFor further details contact Hanson TiS on 0161 877 2370, or e-mail j. thompson@hanson-tis. co. uk

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