Oxford Method Research Programme

External Wall Components

Matching construction in replacing 6M components; and/or in extending 6M buildings in closely-matched appearance.

The general philosophy of 6M as a standardised approach to building, was that as
much as possible it would be an "open" building system.

In "open" building system, building elements are selected from commonly-available commercial products.

(This is distinct from "closed" building system, more typical of earlier versions of Oxford Method such as 2M/3M, and more typical of "CLASP" and other contemporary school building systems, in which a number of key components are purpose-designed as integrated components, and are purpose-made by a small number of specialist suppliers. 2M/3M cladding panels are an obvious example.)

Closed systems tend to give rise to considerable development effort and limited competitiveness; they require a considerable repetitive building programme in order to achieve the economies of large scale production.

In open building systems, the commonly-available building components are selected for dimensional compatibility with the co-ordinating dimensions of the system; for example, in 6M the layout of components/elements on plan are governed by the 600mm planning grid; and similarly the 150mm vertical spacings of the horizontal profile of the "Plannja" cladding, are coordinated with the typical 2400/2700mm room heights, and typical 900/1200/1500mm structure/services zone.

Repairs and extensions to system-built buildings will generally be easier to achieve in "open" building systems, than in "closed" building systems with their characteristic purpose-made components.

Nonetheless, problems of component matching and of general construction will arise over time, in "open" system buildings also, due to:

• specific products being amended or discontinued by manufacturers; or original manufacturers going out of business
• changes to general building methods and recommended good practice
• change to building legislation, such as thermal insulation or fire safety standards

The following apply specifically to the 6M ground slab edge and base of cladding: aspects of component matching and replacement

1) Pad foundations/column base plate/HD bolts/RHS columns

The standard 6M 150 x 150 RHS column (in varying thicknesses according to structural loadings) remains a standard commonly available steel section. Base plate and holding down bolt details may remain unchanged in principle from the original 6M details, subject to project-specific structural engineering advice.

Pad foundations in the majority of 6M projects were sized for the specific project loadings, with little structural redundancy, and little or no capacity for significant additional loadings (such as additional floors). This is a critical limitation on many existing 6M buildings. Structural engineers Roughtons may be consulted for details or original structural design and calculations.

In forming matching extensions to existing 6M buildings, consideration should be given to "oversizing" pad foundations and RHS column thicknesses, in order to build-in redundancy for additional loadings from future vertical extension. The recent A & E extension at Milton Keyns DGH (2000) incorporates such redundancy.

2) Ground-bearing slab(s)

The lean-mix insitu concrete base slab, with "Ventrot" or equivalent mopped-on DPM, remains an appropriate construction; as does the power floated upper slab (power floating was a relatively new technique at the time of 6M design development, but is now in the building industry well-established and commonplace).

In original 6M construction, the underside and perimeter of the ground floor slab did not incorporate any thermal insulation. This is not compliant with current insulation standards, nor proposed improvements to insulation standards, under Part "L" of Building Regulations, to be introduced in 2002.

"Retrofitting" ground slab insulation is not practicable but in new construction generally matching 6M building, the following elements of ground floor construction should be incorporated:

• building paper separating membrane laid over "Ventrot" or equivalent DPM
• 75mm rigid closed-cell horizontal insulation board (Dow Styrofoam "Floormate" or equivalent); with taped joints to minimise grout loss. 75mm "Floormate 350" and "Floormate 500" are available, for medium and high load bearing applications. Floormate "Avance 200A" has no ozone depletion effect.
• 50mm perimeter insulation (placed against perimeter PCC kerb)

The 150mm mesh-reinforced power floated upper slab may be formed over the insulation, as for standard 6M construction.

3) 100mm PCC Edge Kerbs

The 100mm wide PCC edge kerbs used in original 6M construction were to a non-metric standard profile under BS 340: 1979 (now superseded by BS 7263:1994). Current standard PCC kerbs are to minimum widths 125/150mm, with limited availability in "bullnosed" profiles appropriate to the Plannja cill profile.

From currently-available edge kerbs, Marshalls Ltd provide a non-BS bullnosed kerb, overall 150 x 305mm, which can appropriately be used in an "adapted 6M" edge detail (see drawing). Servicised Ltd "Bituthene" remains available for the local DPM dressed over the primed PCC kerb.

4) External wall/base of cladding components

• The continuous RSA setting-out/base fixing rail remains appropriate in principle. Because of the requirement for thermal insulation to the slab edge, and because of changes to available sizes of PCC kerbs, an increased RSA at standard section size 80 x 60mm is recommended; fixed to the PCC kerb rather than to the in-situ slab. "Tapcon" self-tapping anchors, used for fixing the RSA to concrete, remain available from suppliers TW Buildex. (see drawing).
• "Multibeam" vertical cladding rails remain available from Ward Ltd, but the profile depth of 114mm used in original 6M construction is no longer manufactured.

A section depth similar to the 6M original can be achieved using Ayrshire Ltd "C"-sections, 120mm x 65mm x 1.6mm thick. Alternatively and preferably (because of increased depth available for thermal insulation) the following currently available standard components may be used:

• Ayrshire Ltd "C" section 140mm x 65mm x 1.6mm
• Ward Ltd "Multibeam" section 145mm x 60mm x 1.3 - 1.7mm

The overall 6M perimeter wall zone should be maintained at 300mm deep, and consistent with the overall 600mm planning grid. Accordingly, the increased depth of vertical cladding rail results in a decreased perimeter services zone, reduced from nominal 150mm to nominal 120mm. (This reduced zone depth remains sufficient for all services except major drainage drops; these are conventionally boxed-in adjacent to column casings).

It was commonplace in original 6M contracts, for a large proportion of specialised areas of work, including cladding "packages", to be let as nominated sub-contract works. Subcontract nominations are currently less favoured as a contract procedure. Nonetheless, it is recommended that specifications define a single-point subcontract responsibility covering fixing of the base angle, vertical cladding rails, and profiled cladding itself; to ensure accuracy and quality of installation/

• Profiled pressed steel sheet cladding is still made by the original Swedish manufacturer Plannja Ltd; but is no larger manufactured to the original 6M profile with coordinating troughs at 150mm vertical centres. The closest matching standard Plannja product currently available is "Profile 60" with troughs at 160mm centres, and suitable for horizontal application and mitred corners, comparable to original 6M construction. The differing trough centres mean that this profile might be considered for a freestanding new development generally matching adjacent original 6M buildings; but that it is not appropriate for abutting extensions.

Plannja Ltd are able to manufacture purpose-made profiled sheeting to match original 6M profiles, but only in large quantities (in excess of 4000m²) to justify set-up and import costs.

In recent extensions in matching/adapted 6M construction - for instance at A & E Department extension at Milton Keynes DGH - where it has been necessary to achieve a closely-matching profile, it has proven practicable and economical to source purpose-pressed cladding sheets, pressed from standard Corus Ltd "Colorcoat 2000" PVF2 coated steel coil. A close colour match can be achieved, to the cream-coloured finish used in the majority of 6M buildings.

4) Mineral fibre insulation batts

These were fixed using self-adhesive "stickpins" as described, remain appropriate for the thermal insulation of the external wall. Rockwool Ltd high density mineral fibre "Thermobatts" with shiplap joints are available in 65mm and 75mm standard thickness, to make up the 140/145mm structural depth derived from the proposed "Multibeam" or "Ayrshire" vertical cladding rails.

Thermal Insulation Standards etc.

Original Plannja-clad 6M (external wall) construction calculated across the full depth of construction achieves U-values approximating 0-18. This was exemplary by the Building Regulations standards and building practice of the 1980s, and remains excellent at today's standards (2002 Building Regulations require a minimum U-value of 0.35 at the external wall of non-domestic buildings).

The slightly deeper insulation described above, arising from the proposed deeper cladding rails, allows external wall U-values approximately 0.16 to be achieved.

Recent local inspection of 6M external walls where partially stripped for extensions, at Milton Keynes DGH, has shown that:

• blown mineral fibre insulation appears to retain its integrity; i.e. does not slump over time
  there appears to be no occurrence of interstitial condensation causing damage or degradation to cladding components on to plasterboard linings
• there appears to be no occurrence of interstitial condensation causing damage or degradation to cladding components on to plasterboard linings.

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