News - Dec 2011 - Dealing with Double Skin Insulated roof systems in steel or aluminium

Generally metal roofing is usually built to the grade' desired quality, but occasionally things go wrong especially where site workmanship has not been up to standard.

What is a double skin insulated roof?

A double skin insulated roof is made up on site from separate components generally comprising, liner sheet, vapour control layer, spacer system, insulation, breather membrane and finished externally with top weathering sheets.

Top weathering sheets are generally secret fixed onto clips or standing seam sheets onto halters these being machine seamed once fixed. Pierce fix sheets are still widely used; these are fixed directly to the spacer system with external visible fixings.

Liner sheets can be solid or perforated to give an acoustic, sound deadening roof. Liner sheets are fixed directly to purlins and can act as a vapour control layer if a separate vapour barrier is not specified. If the liner is not used as a vapour control layer a reinforced vapour control sheet should be incorporated within the roof.

Insulation must be installed between the vapour control layer and the top weathering sheet, some systems may require ventilation above the insulation and others may not, it varies from manufacturer to manufacturer. Where there is no requirement to ventilate, the insulation should be compressed slightly to ensure there no air voids where condensation may occur.

Workmanship

As there are a number of components that are built up to form this type of roof, site quality is of paramount importance, and failure to build up the roof correctly substantially increases the risk of interstitial condensation and water ingress. Completing remedial repairs to this type of roof can be highly expensive and inconvenient to the building occupier.

To help with quality, a brief list of quality checks below is provided, although not conclusive:

• Top Weathering sheets
  • Ensure that the top weathering sheets are installed in accordance with manufacturer's instructions.
  • These must be long enough to discharge into the gutter correctly and to allow for an eaves angle if required by the system.

• Liner sheets/vapour control
  • Ensure there are no cuts or splits in the liner sheet.
  • Check for punctures of the vapour check by foot traffic or damage and patch as required.
  • The vapour control layer is sealed around openings and penetrations
  • The vapour control layer should be continuous and taped at junctions and joints.

• Insulation
  • Ensure that the correct insulation is used including thickness and thermal properties.
  • Dense insulation that can withstand load should be provided beneath areas where foot traffic is expected (e.g. plant access routes).
  • All packaging and rubbish should be removed from the void prior to insulating.
  • Ensure that the insulation is well packed and compressed where no roof void ventilation is required.

• Generally
  • Ensure that the surface finishes have no abrasions, dents or cuts.
  • The roof must not be used as a cutting surface for flashings or other metal. Hot swarf from angle grinders can burn into the coating of steel sheets and marks aluminium and rapidly turns to rust.

Flashings should have sufficient overlap or butt straps, 150mm wide, and be sealed and supported. Check frequency of fixings and that they are of the correct type.

Rob Clay Parker BSc (Hons) ICIOB; ABEng

Technical Services and Claims Director