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Over-roofing is not the answer to every roofing problem, far from it, but it can be a useful option when you have a leaking industrial roof that needs fixing. This is especially true if you have an old fibre cement roof. Profiled cement roof sheets were widely used on industrial buildings for many years, as they were cheap, fire-resistant, generally inert, vapour-permeable and hard-wearing.

In most cases, these laminated cement sheets were reinforced with asbestos fibres. However, artificial fibres were used for a limited time after using white asbestos-based products was banned in the UK in 1999.

It is highly likely that if you have a tired old grey fibre cement roof, it will contain white asbestos fibres, and it has provided at least 22 years of service. Getting fibre cement roofing and cladding sheets tested by a qualified company is always recommended, as a few very rare roof sheets contain the more toxic brown asbestos materials, and non-asbestos artificial fibres were used for a short period.

Fibre cement roofs were often installed with a single skin of roof sheets, and even the twin skinned version had little or no insulation within the construction. This is one reason why an over-roof often works well on old fibre cement roofs. An over-roof provides the opportunity to add insulation into the roof’s construction. A building’s thermal efficiency is becoming increasingly important as we look for ways to save energy and carbon emissions.

The biggest advantage of an over-roof is the existing roof stays in place. In most cases, this allows the normal operations within a building to continue uninterrupted as the roof is recovered and extra insulation introduced. It also saves on waste materials going to landfill, as asbestos is definitely not recyclable!

Installing an Over-roof

Welbro recently undertook a successful over-roofing project for a client running a printing business. This project provided an excellent opportunity to photograph the stages of an over-roof installation on an old asbestos cement roof.

Stage One – Saftey First

The old roof sheets are fragile and should not be walked on. If an operative was to fall off the roof stagings during the installation process, they are protected by the safety netting installed over the old roof. This netting remains in place and is left below the completed over-roof to ensure protection from start to finish.

Stage Two – Stable Support

Steel sections are fixed to the primary structure to provide a stable base for the over-roof. A vapour barrier is fixed over the supports to prevent condensation within the new construction before a proprietary support system is fixed to the new steel sections. This support system incorporates spacers and new purlins to prevent the insulation from being crushed between the old roof sheets and the new metal cladding.

Stage Three – New Insulation

New insulation is laid between the new support system and over the vapour barrier. Non-flammable mineral wool is the insulation of choice. The support system can be adapted to accommodate various depths of insulation, but at least 270mm is recommended to comply with current building regulations.

Stage Four – New Weathering Sheet

The final stage is to fix new profiled metal roof sheets to the support system to weather the new roof. 0.7mm thick plastisol coated galvanised steel sheets are used in this example project. Still, once the support system and insulation are in place, almost any metal roofing system can be fixed, including aluminium, standing-seam or GRP.

Bespoke flashing to the perimeter and ridge complete the system. Sometimes it is necessary to raise the height of parapet walls and other details to accommodate the extra height resulting from the extra insulation. It is prudent to line gutters during the over-roofing process, and there are several options for gutter liners.

An over-roofing system is not the answer to all roofing problems, but it can be a very efficient option worthy of consideration. A qualified engineer must undertake a structural assessment to ensure the existing structure can support the additional load imposed by an over-roof.

The reduction of waste materials to landfill and improved thermal efficiency by introducing insulation makes it an environmentally friendly option.

The choice of materials is also vital, so it is worth talking to a qualified and experienced company if you are considering a potential roofing project.

Over-roofing Pro’s

  • Operations often continue uninterrupted within the building during installation.
  • Insulation levels and thermal efficiency increase
  • Fewer waste materials to landfill
  • Usually quicker to install than complete roof renewal

Over-roofing Cons

  • Extra load on the structure
  • Little scope for improved aesthetics within the building

Over-roofing systems can be used for flat roofs, fibre-cement sheets and many types of metal roofing systems. Please call if you have a project that you think might be suitable for an over-roofing system.