External Wall Construction
General Description: 6M construction
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; or in some instances joints may be lapped rather than butted.
Accessories such as cills and copings: purpose-made pressings from pre-finished flat Plannja sheet
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.
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.
External wall components: particular adaptations in matching construction
Cladding Sheets
Although the original Swedish supplier Plannja Ltd still manufacture 0.65mm sheet in PVF2 finish, they no longer manufacture profile reference 35, and will not set up cold-rolling facilities to re-create this profile, without considerable on-costs and delays. Other currently-available Plannja profiles do not provide a sufficiently close visual match to the original profile 35 appearance.
Plannja may be able to supply plain pre-finished coil into the UK, to be purpose-formed to a matching profile, using brake-press or similar methods. Forall Metal Products Ltd have been put forward by Plannja as a suitable fabricator, and detailed fabrication advice is available from Forall.
Plannja PVF2 colour reference "66 beige" is claimed to be close to the cream colour commonly used in the majority of original 6M buildings; this is a standard colour from Plannja's current range. However, there is some doubt whether this current colour reference does achieve a good match; but note that Plannja Ltd can supply non-standard PVF2 colours to special order.
Alternatively, experience with 6M-matching extensions at Milton Keynes Hospital by cladding supplier/subcontractor, Dialin Cladding Systems has shown that reasonably close matching appearance can be achieve using non-Plannja cladding materials.
Substitute cladding sheets may be sourced from British Steel Strip Products and 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 commonly used 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).
However, the BSSP Colorcoat sheets may be purpose-pressed from stock coil to a profile matching the original Plannja 35, 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 gradually come to show different dirt retention characteristics over time, when compared with the relatively smooth 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.
Cills and copings
These components may readily be purpose-formed using the HPS200 sheet, in an equivalent manner to the original "6M" components.
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.
A simpler fabrication method for matching corner claddings may be considered, whereby external corners are formed by cutting and bending the pre-finished fluted sheet, and by pop-riveting the resulting lap joint within the trough. Pop rivets will be needed at every trough, and a potential water penetration route may be left, unless each trough joint is individually mastic-sealed.
(A better approach to the design of the external corners could also be considered, possibly by introducing a positive "stop" profile at this point; although this would not match the original 6M appearance.)
Internal cladding corners
"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 can readily be achieved in new-build construction, but less easily in situations where a matching extension is to be made to a pre-existing original 6M building.
It will be difficult to strip-off and refix existing cladding sheets, necessary to achieve this detail; and to ensure that existing and new cladding profiles align horizontally.
A simple angled cover strip pressing at these abutments will be easy to achieve, will be appropriately weatherproof, will not match 6M details elsewhere, but will nonetheless present a reasonable appearance.
Subframes to doors, windows etc.
Purpose-made pressings may be specified, generally to match the original "6M" specification. In original 6M contracts, a named specialist fabricator was usually specified, but in matching construction no problems should arise in sourcing these purpose-made components, directly through the main contractor, in accordance with the original detailed drawings and specification. A reasonable colour match can be achieved in polyester powder coating finish.
Windows, external doors, louvres
The original Alcan Ltd "Weatherguard Mark 2" windows, and original Viking "Elan" doors, are no longer commercially available in the original 6M format.
In new matching construction, specification for these components may be "open" in the form of a general performance specification for powder coated double glazed aluminium windows, with no particular manufacturer or range identified at the tender stage.
Alternatively, Glostal Ltd "System 320" windows and "System 202" external doors, may be specified, which will meet current performance criteria, and generally appear satisfactory and somewhat more substantial than the original "6M" windows and doors. These components may be supplied and installed by Messrs. Deane & Amos of Northampton, or other approved domestic supplier/subcontractors.
Services distribution void/inner lining
Some economies combined with reasonable performance may be achieved, by substituting two layers of 9.5mm plasterboard, rather than two layers of 12.7mm plasterboard at "6M"; or by the use of alternative materials as noted below.
- 65 x 50 RSA continuous angle, providing setting-out alignment and fixing point, to vertical cladding rails. This angle is fixed to the edge of the power floated slab using self-tapping "Tapcon" anchors
Ward Ltd "Multibeam" vertical cladding rails in galvanised pressed mild steel, overall depth 114mm, with flange width nom 65mm. The cladding rails have a bolted foot connection to the 65 x 50mm RSA.- External cladding in Plannja Ltd profiled pressed steel sheeting, in profile reference 35 horizontal trough profiles at 150mm vertical centre; terminated at the foot of the cladding by a purpose-made matching drip pressing.
- Primary wall lining comprising a single layer of 12.5mm plasterboard with vapour resistant facing.
- Thermal insulation formed between the primary wall lining and the external cladding. Various materials and insulation methods were used in 6M construction, including (in approximate historical sequence)
i) polyurethane foam in-situ
ii) blown mineral fibre
iii) mineral fibre insulation batts secured on self-adhesive "stick-pins" fixed to the inner face of the metal cladding - Services distribution void, nominal 150mm deep
- secondary/inner wall lining comprising two layers of 12.5mm plasterboard, fixed at foot to a continuous timber batten, and framed by vertical "jumbo" drywall studs
The plasterboard lining has tapered joints, filled and sealed for direct decoration (i.e. without plaster skim finish). (According to local conditions, moisture resistant or other specialised plasterboards may be used; and plywood may be substituted for the inner 12.5mm layer, to support local loadings and fixtures).
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