RUBEROID BUILDING PRODUCTS - WHAT IS A COLD ROOF?

There are two primary types of cold roof with either a traditionally ventilated void space, where all types of underslating membranes can be used or with an unventilated void space where only certain certified breather membranes can be used.

Roof with Ventilated void space - insulation at ceiling level

A Cold - Ventilated Void Roof configuration is the traditional form of pitched roof build-up. The insulation layer is placed horizontally, directly above the ceiling of the upper story of the habitable section of the building, usually positioned between the joists in the loft space. The Rubershield-PRO breather membrane or any type of proprietary underslating material is placed at rafter level, either draped between the rafters, or supported on a sarking board (usually Scotland). In a traditional cold roof system, any void space below the underslating membrane has to be ventilated. In this roof type, a breather membrane or non-breathing underslating membrane would be equally suitable.

When an underslating membrane is installed draped in a cold roof configuration over open rafters, the use of counter battens is not required.

General Guidance For Cold Ventilated Roofs

• Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15 mm between the rafters is required to guide any rainwater penetrating the main roof finish away from the rafters to the drainage point.

· Provide for ventilation of the void space in accordance with BS 5534 (BS 5250: 2002), with the inclusion of a proprietary eaves and ridge (high level) vents, fixed in accordance with manufacturer’s instructions.

Cold Roof with Un-ventilated void space - insulation at ceiling level

In a Cold-Unventilated Void roof configuration the insulation layer is also placed horizontally, directly above the ceiling of the upper storey of the habitable section of the building, with the vapour control layer placed below the insulation at ceiling level. However, in this situation, only a breather membrane, certified for this configuration can be used. Rubershield-PRO breather membrane is approved for this purpose and should be placed at rafter level, with overlaps, penetrations and perimeters sealed with Rubershield Jointing Tape, draped between the rafters. However, there is no ventilation of any roof void space below the membrane; the only escape route for moisture vapour is through the breather membrane.

General Guidance For Cold Un-ventilated Roofs

• Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15 mm between the rafters is required to guide any rainwater penetrating the main roof finish away from the rafters to the drainage point. (The membrane must not be pulled tight against the underside of the tiling battens.)

• Using Rubershield-PRO in this type of cold roof requires no ventilation of any void space below membrane. Also, when using clay or concrete tiles or natural slates, air movement is usually sufficient in the void space above the membrane to negate the requirement for additional eaves and ridge ventilation. However, when using man-made slates (or some close fitting interlocking tile systems) additional ventilation above the membrane in accordance with BS 5534: 2003 (BS 5250: 2002) is required to avoid potential damage to battens and fixings caused by condensation. It is recommended that guidance be sought from the tile or slate manufacturers concerned.

• To minimise the risk of condensation in cold unventilated roofs, all penetrations into the roof space must be properly sealed to maintain the integrity of the vapour control layer or external seal, and loft hatches should be made convection tight. All water tanks in the loft space must be covered and pipework lagged. The rooms below must be ventilated in accordance with Building Regulations, and rooms that experience high humidity levels must have provision for separate air extraction.