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What is a cold roof?
There are two main types of cold roof:
- A traditionally ventilated void space, where all types of underslating membranes can be used.
- An unventilated void space where only certain certified breather membranes can be used.
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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 in a cold roof configuration draped between open rafters, the use of counterbattens is not required. |
In a Cold - Un-ventilated Void Roof configuration the insulation layer is also placed horizontally, directly above the ceiling of the upper story 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 unventilated roofs
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Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15mm between the rafters is desirable 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.
Use with sarking boards
As Scottish practice where using tiling battens, Rubershield-Pro must be laid beneath counterbattens of a minimum 25mm depth. In some instances tiles or slates can be installed by nailing through the membrane directly into the sarking board. However, the tile manufacturer’s instructions must be strictly adhered to.
Rubershield-Pro and all Ruberoid underslating membranes fully comply with requirements in BS 5534: 2003, for use with sarking board installations in ventilated cold roof configurations.
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Using insulation board above and between rafters |
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Where a roof underlay or breather membrane is to be laid over open rafters, a drape of between 10 to 15mm between the rafters is desirable 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 the 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 recommended 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 occupied building below must be ventilated in accordance with Building Regulations, and rooms that experience high humidity levels must have provision for separate air extraction.
Use with sarking boards
As Scottish practice, where using proper timber sarking planks (usually 150mm wide) laid with a 2mm gap between the boards, there is no need to provide additional ventilation below the membrane. However, where using sarking boards comprising
sheets of plywood or OSB, this type of cold unventilated roof configuration is not
recommended, and a traditional ventilated void approach should be adopted. |
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Typical cold roof design details
Some typical cold roof detailing examples are given in this section. Drawings should be used in conjunction with Building Regulations and British Standards. Further advice regarding the detailing of Rubershield-Pro and underslating membranes in general is available from Ruberoid’s Technical Services Department.
For cold roofs with ventilated voids, all of the Ruberoid underslating membranes can be used. |
Eaves (with soffit): Ventilated voids |
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Ensure fascia board is fixed at correct height.
Install Ruberoid Eaves Protection Strip.
Allow for eaves ventilation in accordance with BS 5250: 2002 |
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Abutments: Ventilated voids |
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Ensure ends of tiling battens are adequately supported on last counterbatten, install noggins if required.
Turn Underslating up against abutment wall by not less than 100mm, and seal to wall with Rubershield Jointing Tape.
Fix lead flashing Code 4 or 5, in accordance with current Codes of Practice and manufacturer’s recommendations. |
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Ridges: Ventilated voids |
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Lay underslating over ridge by not less than 150mm.
Allow for high level ventilation in accordance with BS 5250: 2002 |
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Valleys (with lead lining): Ventilated voids |
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Lay a continuous length of Underslating, at least 600mm wide, up the valley.
Cut main layers of Rubershield-Pro to a rake down centreline of valley or along sides of valley lining.
Cold roofs that are unventilated require the use of a breather membrane such as Rubershield-Pro. |
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| Cold roofs that are unventilated require the use of a breather membrane such as Rubershield-Pro. |
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Ridges: Unventilated voids |
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Lay Rubershield-Pro over ridge by not less than 150mm.
If using tightly fitting slates or tiles, then counter battens should be used. |
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Eaves (with soffit): Unventilated voids |
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Ensure fascia board is fixed at correct height.
Install Ruberoid Eaves Protection Strip.
If using tightly fitting slates or tiles, then counter battens should be used. |
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