- Standard profiled metal cladding sheets fixed with profile "troughs" running horizontally: Plannja Ltd steel cladding in profile reference 35, high tensile steel 0.65mm thick, hot dip galvanised, and with PVF2 finish to visible face and epoxy finish to concealed face. Sheet joints were usually butted and backed by purpose-pressed butt strap; although at MKDGH Phase 1 sheet joints appear to have been lapped rather than butted.
- Accessories such as cills and copings: purpose-made pressings from pre-finished flat Plannja sheet as otherwise as (a) above .
- Corner pressings and related corner components: fabricated from galvanised profiled Plannja sheet, and primed/finished in PVF2 following fabrication.
- Subframes to doors, windows, louvres etc., purpose-made in aluminium by named specialist fabricator, and finished in "Synthapulvin" polyester powder coating (usually to a close colour match with the PVF2 finish to cladding components).
- Windows and external doors: from standard manufacturers range(s), in aluminium and with polyester powder coating. Windows: Alcan Ltd "Weatherguard Mark 2" system. Doors: Viking "Elan" range.
- Storey-height cladding rails (to support horizontal cladding) installed vertically at nominal 1200mm centres. Foot of cladding rail fixed to the floor slab via a continuous MS angle; head of rail fixed via cleats to the soffit of the roof slab or of the intermediate floor slab. Cladding rails in cold-rolled galvanised steel; Ward Bros "Multibeam" reference 45059; depth 114mm.
- Plasterboard lining fixed to inner face of vertical cladding rails, and retaining thermal insulation: 12.7mm vapour check wallboard with plastic film facing; unsupported joints formed using "H" sections and all joints taped: British Gypsum Ltd wallboard.
- Thermal insulation installed within depth of "Multibeam" cladding rails (i.e. nominal 114mm). Insulation used varied between difference "6M" projects, and included in-situ foamed plastics; blown mineral fibre; mineral fibre batts/rolls. (Blown mineral fibre used at MKDGH Phase 1).
- Services distribution void approximately 115mm deep, with inner lining in 2 layers 12.7mm plasterboard, separately supported on 48mm steel studs: British Gypsum Ltd components.
Particular adaptations at MK A & E Department
Cladding Sheets
Although Plannja Ltd still manufacturer 0.65mm sheet in PVF2 finish, they no longer manufacture profile reference 35, and were not prepared to consider setting-up cold-rolling facilities to re-create this profile, without considerable on-costs and delays.
Plannja initially suggested supplying plain pre-finished coil into the UK, to be purpose-formed to a matching profile, using brake-press or similar methods. They suggested Forall Metal Products as a suitable fabricator, and SWA had considerable detailed advice from Forall at pre-tender stage.
SWA specification accordingly named Forall as supplier of the cladding sheets, and defined Plannja PVF2 colour reference "66 beige" to match the existing cladding; this is a standard colour from Plannja's current range. (Subsequently, some doubt arose whether this colour reference does achieve a good match. However, note that Plannja Ltd can supply non-standard PVF2 colours to special order).
After appointment, the main contractor encountered potential difficulties with achieving colour match, and an alternative cladding supplier/subcontractor, Dialin Cladding Systems, was proposed, partly on the basis of this firms prior experience in an adjacent extension in matching construction. SWA agreed to the substitution of DCS as sub-contractor with responsibility for the supply as well as the installation of the cladding sheets.
DCS then proposed an alternative specification of the cladding material, again because of difficulties in achieving a colour match, in the limited contract time remaining.
The cladding sheets as obtained and finally installed were sourced from British Steel Strip Products, obtained via Capital Trading Company (Coated Steels) Ltd.
Although BSSP Colorcoat also manufacture PVF2 finished steel, there is a very limited range of 15 standard colours in PVF2, none of which is close to the cream used at MKDGH and commonly elsewhere in "6M" buildings. (Non-standard PVF2 colours are available from BSSP Colorcoat, but only in minimum orders of 10 tonnes, which equates to approximately 2000m² of cladding).
Accordingly, the BSSP Colorcoat sheets was purpose-pressed to a matching profile and was supplied and installed in "Colorcoat HPS200 Scintilia" organic coated steel, in standard colour reference "Honesty" (nearest BS reference 10C31), which initially achieves a very close match to the original PVF2 finish. The "Scintilla" finish has a small-scale embossed pattern, which may come to show dirt retention characteristics over time, when compared with the PVF2 original.
Colorcoat HPS200 has a guaranteed "minimum period to re-paint decision" period of 30 years, for this colour on walls in inland locations in northern Europe.
SWA specification required butt-jointing of the cladding sheets, against a purpose-made backing butt strap, 150mm wide; although there was some ambiguity in the record drawings in this respect. At construction stage, the contractor noted the use of lap joints in the original building, and it was agreed to replicate this detail in the A & E extension. A reasonably satisfactory appearance was achieved with lap joints.
Cills and copings
These components were purpose-formed using the HPS200 sheet, in an equivalent manner to the original "6M" components.
However, an extended parapet/coping was installed at A & E, partly to conceal items of rooftop M & E plant, and partly to suit standard cladding sheet sizes. See also 2.2.6 below.
External Corner components
In "6M", external corners were formed by a labour-intensive and failure-prone process of cutting and bending unfinished Plannja profiled sheet, brazing the cut joint formed within the profile troughs, and then re-finishing in a stoved PVF2 finish.
In the absence of original "6M" record specifications, SWA did not specify an equivalent process for the A & E extension. External corners were formed by the subcontractor by cutting and bending the pre-finished fluted sheet, and by pop-riveting the resulting lap joint within the trough. Pop rivets were needed at every trough, and a potential water penetration route is left, unless each trough joint is individually mastic-sealed.
A better approach to the design of the external corners could be considered, possibly by introducing a positive "stop" profile at this point; although this would not match the original 6M appearance.
Internal cladding corners
These arose at abutments to the existing building.
"6M" details for this condition assume a "W" profile backing pressing, with cladding profiles aligned, and a small break joint with profile fillers.
This detail was reproduced in SWA drawings at tender stage, but was not achieved on site, because of:
i) contractors unreadiness to strip-off and refix existing cladding sheets, necessary to achieve this detail
ii) existing cladding not aligning horizontally, at the two separate points of abutment
Accordingly, the contractor proposed a simple angled cover strip pressing at these abutments.
The resulting detail did not match 6M details elsewhere, but presented a reasonable appearance.
Subframes to doors, windows etc.
Purpose-made pressings were specified by SWA, generally to match the original "6M" specification. A named specialist fabricator was not specified. No apparent problems arose in sourcing these purpose-made components, although the manufacturer failed to achieve a close match in respect of the pitch of the angled face of the subframes, despite original detailed drawings being available. A reasonable colour match was achieved in polyester powder coating finish.
Miscellaneous timber packings were introduced by the contractor to support the subframes; these would not have been required if the original detail had been followed in full, and present a potential point of long-term decay.
Windows, external doors, louvers
The original Alcan Ltd "Weatherguard Mark 2" windows, and original Viking "Elan" doors, are believed no longer to be commercially available.
SWA's specification for these components was "open" in the form of a general performance specification for powder coated double glazed aluminium windows. No particular manufacturer or range was identified at the tender stage.
At construction stage, the contractor proposed Glostal Ltd "System 320" windows and "System 202" external doors, which met the performance criteria and generally appear satisfactory and somewhat more substantial than the original "6M" windows and doors. These components were supplied and installed by Messrs. Deane & Amos of Northampton, as domestic subcontractors.
Louvres were specified from the Greenwood "Airvac 50" louver range, and installed within subframes to match windows and doors. Appearance was generally satisfactory, except that the louvers were installed "overdeep" within the subframes; a simple packing detail could have resolved this aspect of installation.
At an early design stage, it became apparent that AHU's would need to be independently provided to service the A & E extension. Because of further expansion plans, these AHU's were installed within the ceiling void, rather than on the roof top over. This resulted in the need for a number of louvres being installed in the elevations; this is not typical of "6M" buildings generally. Because of this M & E design decision, it then became necessary to simplify the perimeter beams; to allow flexibility in the positioning of louvers in order to align with windows and doors.
At the perimeter, a simple UB was substituted for the double lattice beam conventionally used in "6M" structure.
Cladding rails
The 114mm "Multibeam" used in "6M" construction is no longer available from the manufacturer, Ward Bros. An alternative cladding rail has specified, reference CW12716, by Ayrshire Metal products. This is a C-Section cold-rolled galvanised rail, overall size 127 x 65mm, which gave a slightly deeper insulation zone whilst allowing the overall external wall depth to be maintained at the "6M" module of 300mm.
The structural engineer advised on the selection of this section and provided setting-out drawings. The cladding rails were supplied and fixed under the structural steelwork "package" rather than as part of the cladding package.
The fixing detail at the foot of the cladding rail remained similar to "6M" details, with a continuous fixing angle bolted to the slab, except note that:
i) the SE specified a 125 x 75 RSA, resin anchored; compared to the original "6M" 65 x 50 RSA fixed by "Tapcan" self-tapping concrete screws.
ii) The larger RSA clashed with the framing of the plasterboard inner lining at a later construction stage
iii) "Tapcan" anchors are no longer available under that trade name; but equivalent products should be sought out, as probably more economic than resin anchors
iv) the need for a larger-leg RSA arose from the design decision to introduce perimeter insulation behind the PCC kerb, as delivered from original "6M" construction. (Note: for this particular size and shape of the A & E extension, this perimeter insulation was sufficient to satisfy current Building Regulations; under floor insulation was not installed).
The head of the cladding rail was cantilevered 725mm above the slab level (rather than stopping at underside of slab as in "6M").
This was in order to:
- conceal rooftop condensers and other minor M & E plant
- allow for adapted structure; at MK A & E the head of the perimeter beams and columns were designed to be left exposed, for ease of future steelwork connections as required for further extension. This precluded extending the slab edge to provide a head fixing point for the cladding rail
An incidental advantage of this design approach was that a substantial parapet upstand was achieved, rather than the low eaves pressing (less than 150mm) of the "6M" detail.
The hidden elevation of the parapet was sheathed in WBP ply and over-clad in a flat metal pressing.
Plasterboard lining
A lining of "Duplex Wallboard" was fixed to the inner face of the cladding rails, and taped to provide vapour resistance: this construction is generally equivalent to the original "6M" detail, except that the secondary horizontal supports to the plasterboard were specified as standard British Gypsum galvanised angles (GA2 25 x 25 x 0.9mm) rather than as the "H" sections used in "6M".
Thermal Insulation
Mineral fibre "Rockwool Cladding Roll U/F" was specified, in 120mm minimum thickness. This is paperbacked. No particular method was specified for securing this insulation, but nonetheless installation appeared to be satisfactory.
(Note that in some versions of "6M", insulation batts or rolls were secured using proprietary self-adhesive "stick-pins", bonded to the reverse face of the Plannja cladding; an equivalent detail might have been appropriate to MK A & E).
The insulation type and thickness was equivalent to that used in original "6M" construction, and was accepted by the BCO as compliant with current insulation standards.
In principle it would be simple to adapt the principles of 6M external wall construction to achieve greatly enhanced thermal insulation, but this was not a client requirement at MK
A & E.
Services distribution void/inner lining
This was generally consistent with "6M" construction, except that two layers of 9.5mm plasterboard were specified, rather than two layers of 12.7mm plasterboard at "6M".
This substitution of thinner plasterboard applied to internal partitions as well as to the inner lining of the external wall, and appears to have achieve some economies combined with reasonable performance.
(Note that HTM guidance generally describes a 100mm partition being constructed as nominal 20mm plasterboard/60mm metal study/20mm plasterboard; rather than 25/50/25 as in "6M" construction).
A design decision was taken to size RWP downpipes at nominal OD 75mm; this allowed downpipes to be accommodated fully within the services void, rather than at boxed-out columns as in "6M".
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