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System-Built Steel Housing – 1940 to Now

Pre fabs

During the Second World War it was obvious that long term planning had to take into account the need for new housing. During the last year or so of the War over 100,000 temporary 'pre-fabs' were ordered ,many of them from factories more used to producing Lancaster and Halifax bombers. By 1948 almost 125,000 had been assembled. A typical pre-fab was built from lightweight steel sections with an aluminium, asbestos or profiled metal sheet covering and fibre board or plasterboard internal lining. Roofs were usually corrugated asbestos (pitched) or bitumen felt (flat). Although they were designed as temporary housing (with a design life of 10 years) many remain today. They are reasonably spacious, have large gardens and are often situated on small quiet estates. Although many have been demolished, some local authorities and housing associations have carried out extensive modernisation programmes - often involving re-roofing, re-cladding, new windows and central heating.

BISF

The most common non-traditional house is the BISF (British Iron and Steel Federation). These were built in large numbers after the Second World War; some 30,000 properties were completed, most of them semi-detached. They were not included as designated properties under the Housing Defects Act (see previous section) and, therefore, no grant assistance was available to owners. Unlike the pre-fabs above these were designed as permanent homes.

Pre-fabs have been very popular for a number of reasons. They were well designed, spacious, sometimes had partial central heating, and had fitted kitchens and bathrooms. In addition they had reasonable sized gardens (unknown before to many of the new tenants) and were usually sited on small estates in the suburbs.

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They are very popular with owner occupiers and tenants because, despite some of their defects, they offer very roomy accommodation. The Building Research Establishment have identified a number of defects, some of which have structural implications.

Defects 1) Cracking of the render, which can be caused by impact damage, and minor structural

movement has occurred in some properties. This can result in rusting of the metal lathing, followed by total failure of the rendering.

2) Rusting of the profile sheeting which clads the upper storey has occurred on a number of properties. It is caused by failure of the water stripping on the gable ends and by condensation. In severe cases this has led to rusting of the sheeting rails which will support the cladding.

3) Over the years the asbestos cement sheeting can deteriorate and the material becomes brittle and cracks. The profile is no longer made and the only option is to re-cover the roof with a modern alternative such as aluminium.

The frame is made from steel stanchions or columns with a cladding of profile steel sheeting to the upper storey. The ground floor cladding consists of render on metal lath which is tied to the steel columns. Internally, the frame is hidden by plasterboard fixed to timber battens bolted across the columns. Steel trusses support the roof covering which is asbestos cement sheeting. Details of the construction are shown in the diagram below.

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4) Some corrosion has been found in the stanchions, particularly those at the corners of the building.

There are other defects such as poor thermal insulation, discoloration of the steel sheeting and rusting of the chimney cowls. If you live in one of these properties do not be too alarmed. The BRE report states that, “The majority of BISF houses are structurally sound and those with deterioration of the structural framework can be relatively simply repaired by cutting out and replacing the affected members”. (BRE Report. The BISF steel-framed house, 1986)

Modern steel houses

Surebuild is a light steel framing system developed by Corus (formerly British Steel) capable of constructing housing up to four storeys in height. It has been extensively tested and has received BBA and Lantac certification, and NHBC approval. These photographs were taken at Portishead (near Bristol) where the Surebuild system is being used extensively by the developers, Crest Nicholson. The basis of the structure is a platform frame which is assembled from factory made panels and loose components comprising cold formed galvanized steel sections. Conventional joining techniques are used, typically welding or riveting for panel assembly and self-drill fasteners for site assembly. The vertical dead and imposed loads are supported by loadbearing external walls and internal partitions. The partitions comprise studs, head channels and bottom channels and, in some cases, intermediate noggings. Resistance to horizontal loading (racking) is provided by means of the diaphragm action of the floor and roof in conjunction with diagonal steel braces fixed in specific areas of the steel frame.

The steel frame is held down to the foundation by an anchorage system substantiated by design or testing data. The steel frame generally forms the inner leaf of a cavity wall and comprises the structural frame with insulation applied to the cavity face and a plasterboard lining to the room face. The insulation used is a rigid, foil faced foam board with a Class 1 surface spread of flame rating. To comply with warm frame construction principles and thus avoid corrosion risk associated with interstitial condensation, the board is fixed to the outside (cavity) face of the frame.

The external leaf is generally of masonry, and may incorporate feature panels of tile hanging or timber boarding. The masonry is tied to the inner leaf with purpose made stainless steel wire wall ties which pass through the insulation and clip to the flanges of the steel frame studs or alternatively with a face fixed dovetail type channel system. The wall incorporates a 50 mm clear cavity when a masonry outer leaf is used.

in-situ concrete slab, or a proprietary suspended ground floor system, may be used, and in all cases the conventions of lapping DPM's with horizontal DPC's must be followed to protect the floor and sole plate from damp. First floor consists of galvanized steel joist section with a covering of 18 mm or 22 mm V313 tongue-and-groove chipboard, 18 mm Douglas fir tongue-and-groove plywood, or l9mm tongue-and-groove oriented strand board.

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Party walls in semi detached houses are constructed using two independent steel framed leaves with a lining, to each house, comprising two layers of plasterboard incorporating a vapour check or finished with Driwall Sealer. Mineral wool batts are located between the two leaves of the wall to provide the required acoustic properties.

Services In designing and installing electrical and plumbing services the following precautions must be taken to avoid the possible risk of long-term damage to the steel frame or the services. To avoid flow of electric current to earth at holding down bolts, with a consequent risk of corrosion, local earth connections to the steel frame must be avoided. The frame must be connected to earth at one point and all earth connections in the circuit wired back to this point. All holes in the steel frame members, for electric cables, must be fitted with rubber or plastic grommets to avoid damage to the cable unless the holes have been formed by swaging (curves the sharp edges over) and are fully rounded over with no sharp edges.

Water The enclosure of cold water pipework within the external wall should be avoided wherever possible as condensation on the pipework could lead to wetting of the steel frame with a consequent risk of corrosion. If enclosure is unavoidable, the cold water pipework must be insulated with tubular plastic insulation, which must be accurately cut at junctions and changes of direction and held firmly in place with adhesive tape. Where hot water pipework is enclosed in the inner leaf of the wall, contact between copper, pipes and the galvanized frame must be avoided by the use of rubber or plastic grommets.

The outer leaf can be formed in brickwork or rendered blockwork. Like timber framed buildings flexible wall ties are required to allow for differential movement.

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Special precautions Erection of the buildings is carried out by Corus Framing, or companies under the supervision of Corus Framing. The arrangements for supervision have been assessed and found to be satisfactory. The following checklist is provided for clients who wish to carry out their own, additional supervision. These items are in addition to the normal building checks.

the substructure is set out accurately and level before walls panels are positioned

anchor bolts holding down the ground-floor steel frame panels are correctly in place at all the locations provided

all insulation boards are free from damage after fitting and that horizontal and vertical joints are correctly detailed

wall tie channels are correctly positioned and fixed or, if wire ties are used, they are correctly clipped on to stud flanges with the cover washer/retaining clip a close fit to the insulation face

bottom channel members are free from debris prior to dry lining and the drainage slots are clear

horizontal DPCs are correctly turned up against the channel upstands as dry lining proceeds

joints in brickwork are correctly filled with mortar

cavity barriers/fire stops are correctly located as indicated on the drawings

the steel frame is electrically earthed at one point and all earth returns are connected back to that point.

Roofs are formed using factory-made timber trussed rafters of up to 35' pitch spanning in either direction. Trusses are manufactured in accordance with BS 5268-3: 1985 and incorporate punched metal plate timber fasteners which are covered by an Agrement Certificate. Alternatively, roofs are constructed on site using conventional rafters and purlins. Rock fibre insulation 150 mm thick is placed between ceiling joists. Roofs are clad with concrete interlocking clay tiles or slates imposing a load not exceeding 0.65 kNm 2. The tiles are fitted over conventional sarking felt, specified to BS 747: 1994 and treated softwood tiling battens.