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Whirlwind walls

A house under construction in Cornwall, designed by architect de Rijke Marsh Morgan, is the first UK project to make use of Steko, an innovative wall system that will advance the use of engineered timber The past few years have seen a revival of interest in timber as a 'green' structural option, yet its use as an engineered material here in the UK has been slow to materialise.

As the Velux case study (page 10) shows, greater usage will probably be championed by non-domestic applications.

But recently, the introduction of a new wall system has proved that the domestic sector can also produce a few solutions that advance the use of engineered timber in construction.

Steko is such a solution. The innovative system comprises loadbearing timber blocks that allow the rapid construction of walls.

Developed in Switzerland seven years ago, it is distributed in the UK by London-based Construction Resources (www. ecoconstruct.

com) - an ecological building centre and supplier of sustainable building products and systems to the UK construction industry. The first-ever UK application of Steko is on a house designed by architect de Rijke Marsh Morgan (dRMM) which is currently under construction in Cornwall.

Requiring no adhesives or fixings, Steko's precision-made spruce blocks slot together like a Lego system, requiring no drying-out time and so enabling walls to be constructed easily and rapidly. At about £95/m 2, the system is not cheap, but this has to be balanced with the construction time saved. Steko claims that a house shell can be erected in two to three days, with typical laying rates of 1520 minutes per square metre being possible.

The vertical walls of each block - comprising 20mm-thick outer and inner panels glued to two horizontal battens - are separated by vertical studs which create a segmented cavity. Thanks to their top and bottom profiles, the blocks fit together by a large tongue-and-grooved profile, as well as by dowels which slot into the bottom of each vertical stud and locate into the studs of the blocks below. The timber elements of each block have their grains arranged at right angles to each other to ensure dimensional stability. Easily lifted by hand, the 6.5kg basic unit measures 640mm x 320mm x 160mm wide, although it also comes in quarter, half and three-quarter length variants.

Suitable for above-ground applications only, a Steko wall requires a concrete, masonry or timber upstand that is at least 300mm above ground level. A profiled timber base plate on a strip DPC sets off the construction, anchored at 1-1.5m centres into the concrete or screwed into a timber ring beam.

The blocks can then be stacked onto the level base, with the uppermost wall section capped at soffit level by a screw-fixed top plate onto which the first floor joists wil bear. Fixing the next base plate onto the floor construction starts the first-floor wal sequence. Once the walls are complete, a breather paper is stapled to the external face as it is critical the blocks do not get wet at any time during construction.

The blocks' integral hollows can be used for routing vertical services, while horizonta services are confined to below-floor finishes or above suspended ceilings. In order to avoid vibration of the wall, it is recommended that waste pipes and ducts be excluded from the wall structure and, instead, run through independent service zones.

As well as being convenient for wires and cables, the hollows can be filled subsequently with blown cellulose insulation to give a Uvalue of 0.42W/m 2K.As this is insufficient to meet recently strengthened Part L therma performance requirements, an external insulation system must also be attached. Steko claims that a 295mm-thick Steko wall construction - comprising 15mm gypsum plasterboard, the standard 160 block filled with cellulose insulation, breather paper 100mm external mineral wool and 20mm render - achieves a U-value of 0.20W/m 2Kwhich more than satisfies both English and Scottish house-building regulations Cladding options are those of traditional timber frame and can include insulated render, timber rainscreen and brickwork.

Walls can be up to 20m long; any longer and they will require vertical bracing.

Maximum buildable heights can reach 4-5 storeys, although without the horizontal bracing provided by floors and roofs the maximum wall height is 3m. Post-tensioning with steel rods is also possible where high wind loadings are encountered. A typical permissible wall loading is 64kN per linear metre when using the standard spruce base and top plates; this increases to 128kN per linear metre when the special beech base plate is used.

Special components in the system include lintels, reveal closing plates and liners, as well as solid blocks to be used where point loads are to be carried. The blocks do not come treated with preservative, but a natural borax treatment can be included if attack from longhorn beetles is anticipated. A minimum half-hour fire resistance is achieved when a single layer of plasterboard is applied.

The system also gets high marks for sustainability. Steko's lumber originates from well-managed European forests and is converted by a production process claimed to use little primary energy. Furthermore, when compared to stress-graded timber, the manufacturing process is able to use more timber from any given tree.

For the £80,000 3H House at Downderry, Cornwall, dRMM Architects opted for Steko after spotting the system at a building exhibition and promptly snapped up the entire display. Demounting it was not a problem, given the system's integral reconfigurability.

While the quantity available proved enough for the external walls, internal partitions had to be made of traditional timber studs. It was also fortunate that the blocks on display were of the fair-faced quality that has resulted in an internal finish of great warmth and character.

For Alex de Rijke, one of the attractions of Steko is that it can be assembled by nonskilled workers, in this case the client himself, leaving things like services installation to more-skilled tradesmen.De Rijke sees the system as economically viable for specific sectors, particularly self-build, although traditional timber frame, he notes, is cheaper when constructed by professionals.

dRMM did not make use of Steko's structural engineering package, preferring its own engineer who decreed that the exposed Cornish site demanded extra bracing for wind load; this was incorporated as vertical batte beneath the cladding. In some places, unr strained walls of the house extend through height of more than two storeys, so have bee post-tensioned by threaded steel rods.

The lower walls - filled with blow cellulose insulation - are clad with a ced weatherboard rainscreen on battens, whi on the upper floor, local planning requir ments favoured reconstituted slate. N external insulation was used for this proje as it was submitted before the recent amen ments to Part L were implemented.

As the layout includes some non-9 angles, a degree of on-site adaptation some blocks was necessary, although th proved easy enough: the blocks are easi sawn.Yet some difficulties were experience The blocks may be fine in a relatively d alpine climate but they need special preca tions in high-humidity coastal locatio which can cause movement problems moisture content rises above 10-12 per cen Nevertheless, de Rijke is sufficient enthused by the whole process to admit tha given the chance, he would definitely use th Steko system on future projects.

ARCHITECT de Rijke Marsh Morgan ENGINEER Buro Happold

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