Oxford Method Research Programme

Roof Construction

Original 6M flat roof construction (early 1980s) generally comprised:

• twin lattice beams, "Holorib" permanent formwork, reinforced insitu concrete slab etc., all as for intermediate floors described previously, except:
  i) overall slab thickness 125mm
  ii) increased pre-camber of lattice beams, so that finished roof drains with falls to outlets adjacent to column positions
• bituminous felt vapour barrier (BS 747 Class 1 type 1B)
• rigid polyisocyanurate foam insulation board, 25mm thick, with glass fibre facings both sides, and fully bonded in hot bitumen compound
• isolating membrane (to BS 747)
• 20mm mastic asphalt waterproofing, applied in two layers, and finished with limestone chipping with reflective paint to upstand etc.
• ancillary components including "Coxdome" proprietary rooflights with integral insulated upstands, "Harmer" rainwater outlets; promenade tiles, etc.

At eaves/roof edge construction, the "Multibeam" vertical cladding rail is taken past and fixed to the edge of the roof slab, and directly supports on uninsulated WBP ply upstand with "Expamet" lathing, with mastic asphalt dressed up the upstand. The "Plannja" cladding continues up the outer face of the eaves, finished by a purpose made matching pressed metal capping profile. The eaves upstand height is to a nominal coordinating dimensions 150mm above the top on insulation, i.e. the effective eaves height above all roof finishes is somewhat less than conventional building practice; nonetheless, examples of failure do not appear to have been experienced at this construction detail.

Later variants of 6M flat roofs used an inverted or "warm roof" construction, with the mastic asphalt waterproof membrane applied over a bonded isolating membrane and directly to the roof slab; and with loose laid thermal insulation over, with ballasted in the form of pebbles centrally, and insulation/concrete composite roof slabs at perimeters etc. In principle, the greater protection and improved thermal stability of the waterproof membrane, is expected to give rise to greater longevity.

Structural capacity of existing flat roof construction

Original roof slabs were thinner than intermediate floor slabs and were generally not designed with any significant structural redundancy, to allow for potential building expansion in the form of additional storeys. As noted previously, a further and more significant structural limitation on such expansion, arises from the lack of redundancy/capacity in the sizing of the column bases.

A feasibility study carried-out at Wexham Park Hospital, to investigate a potential first floor extension over single-storey 6M construction, noted the possibilities of stiffening the roof slab and increasing its load bearing capacity, by :

• inserting additional secondary beams under, at closer centres
• exposing and scrabbling the surface of the concrete slab, for fully-bonding additional thickness of proprietary screeds.

(Significant cost and disruption would arise from these procedures).

Rooftop extensions may become structurally justifiable in pre-existing multi-storey 6M construction, where it may be possible to demonstrate by calculation, the existence of appropriate structural capacity within existing column bases, columns and beams.

All example of such as roof top extension has been built at Milton Keynes Hospital, in lightweight glazed/panel construction. However, the large majority of 6M building are limited to one or two storeys, and such opportunities are not common.

Matching construction in replacing 6M components; and/or in extending 6M building

• in new/matching construction, simplified structural steel beams should be considered, with appropriate allowance within ceiling/roof voids for services distribution; as recommended for new/matching intermediate floor construction; also with consideration of "spare" structural capacity for potential rooftop extensions
• new/matching roof slabs should match intermediate floor construction, i.e. should be 150mm thick, again to ensure spare structural capacity. "Holorib" formwork remains appropriate. Slabs should be formed flat/level, if potential rooftop extensions are under consideration.
• the original 6M flat roof construction build-up, comprising mastic asphalt over insulation over roof slab, continues to be an appropriate form of construction, and details generally to match may appropriately be considered. Note that:
  i) glass-fibre faced polyisocyanurate insulation board is currently manufactured by as "Energy-lok) EL2 by Celotex Ltd; equivalent products may also be available from other manufacturers.
  ii) This product is available in a range of (non-tapering) thickness, 25 - 70mm. To meet current and future standards of thermal insulation, minimum 50mm and recommended 70mm thicknesses should be used. The original 6M eaves detail should be raised by one 150mm "module" to accommodate the additional building-up of roof construction
  iii) Blowing agents for "Energy-lok" polyisocyanurate board currently use HCFCs, and therefore do not meet environmental best practice
• a recommended alternative build-up of roof finishes would comprise:
  i) concrete roof slab laid flat at 150mm thickness, for potential future extensions
  ii) vapour barrier plus rigid insulation board manufactured with pre-cut tapers to falls and to minimum 75mm thickness. Appropriate materials include rigid mineral fibre (Rockwool Ltd); Foamglass (Pittsburg Corning); CFC-free phenolic or urethane foams (Kingspan). (If mechanically secured, the insulation boards can potentially be re-used.
  iii) Single-ply waterproof membrane, mechanically secured, (e.g. Sarnafil or equivalent PVC-based membrane)
• where maintenance access if required adjacent to roof perimeters, for rooftop plant or other items, current Health and Safety (CDM) Regulations require appropriate permanent guardings (these did not apply in original 6M construction). Free-standing guardrails with counterweights, (e.g. by Safesite Ltd or equivalent) resting on precast concrete paving slabs, will be appropriate with either roof construction described above.

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