Grafton plc
  www.buildbase.co.uk

  

 

Buildbase Limited,
Head Office:
Gemini One, 5520 Oxford Business Park South, Cowley, Oxford, OX4 2LL

Reg. in England No. 4725313 Member of Grafton Group plc. VAT Reg. No. GB 663 3617 30 Registered Office:
PO Box 1224 Pelham House, Canwick Road, Lincoln LN5 5NH

Meeting Your Standards

What is a warm roof?
Warm roofs are increasingly popular today as we aim to maximise our living space and the available accommodation in any construction. This will be the predominant form of pitched roofing construction in the UK.

A breather membrane such as Rubershield-Pro must be used in a warm roof configuration, to overcome the condensation risk. In a warm roof configuration, the insulation is positioned directly under the external waterproofing (e.g. tiles or slates), following the rafter line. A vapour control layer is usually placed beneath the insulation (on the warm side), to limit the passage of moisture vapour into the insulation.

The Rubershield-Pro breather membrane is placed on top of the insulation, with overlaps, penetrations and perimeters sealed with Rubershield Jointing Tape. Counter battens/tiling battens are used to install the tile or slate finishes.

Using Rubershield-Pro there is no need to ventilate the void space below the breather membrane, as this void space will always be at a similar temperature to the habitable portion of the building, minimising the possibility for condensation. The void space that would have previously been a storage loft, is now a usable space.

In addition, a warm roof contributes to increased airtightness, reducing heat loss by uncontrolled air movements, and aiding compliance with the revised Part L Building Regulations (April 2006).

Using partial fill insulation between rafters
When insulation is positioned between rafters but does not fill the whole rafter depth, an airspace must be maintained between the underside of Rubershield-Pro membrane and  the top of the insulation to allow a 10-15mm drape of the membrane, unless 38mm counterbattens are used. Other requirements are as for when the insulation is laid above or fully filled between the rafters.

Using insulation board above and between rafters

With rigid insulation boards, Rubershield-Pro membrane should be laid directly onto the insulation. Counterbattens (minimum 38mm deep) should be fixed according to insulation manufacturer’s instructions.

If insulation between rafters is ‘full fill’, the membrane should be laid directly over the rafters and insulation. Counterbattens (minimum 25mm deep) must be nailed directly into the rafters.

A vapour control layer should be installed on the warm side of the insulation as detailed in BS 5250: 2002 Code of Practice for control of condensation in buildings.

No ventilation of any void space is required below the Rubershield-Pro membrane.

With clay or concrete tiles or natural slates, air movement is usually sufficient in the space above the membrane (below the tiles/slates) to negate the requirement for additional eaves and ridge ventilation. However, when using close fitting fabricated slates (e.g. fibre cement) additional eaves and ridge ventilation is recommended above the membrane.

Insulation positioned at rafter level with timber sarking

As Scottish practice is for the use of timber sarking, which has a high resistance to the transmission of water vapour, it is advisable to conduct a condensation risk analysis.

Rubershield-Pro fully complies with requirements in BS 5534: 2003 for use with sarking board installations. 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 warm unventilated roof configuration is not recommended. In a warm roof where the membrane is supported on a plywood or OSB sarking board, ventilation of the mandatory 50mm airspace below the sarking board is required, in accordance with BS 5534 (BS 5250: 2002). Under these circumstances there is no requirement for further ventilation above the membrane.

Use of counterbattens above the membrane (installed on a sarking board) is required where tiles or slates are to be installed on battens. 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.

 

Typical warm roof design details
Some typical warm roof detailing examples are given here. Drawings should be used in conjunction with information available in Building Regulations and British Standards. Further advice, regarding the detailing of the Rubershield-Pro membrane is available from Ruberoid’s Technical Services Department.

Eaves

Ensure fascia board is fixed at correct height.

Install Ruberoid Eaves Protection Strip.

Abutments

Ensure ends of tiling battens are adequately supported on last counterbatten, install noggins if required.

Turn Rubershield-Pro 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.

Valleys (with lead lining)

Lay a continuous length of Rubershield-Pro, at least 600mm wide, up the valley.

Fit valley support boards to a height level with top of counterbattens.

Cut main layers of Rubershield-Pro to a rake down centreline of valley or along sides of valley lining.

Ridges

Lay Rubershield-Pro over ridge by not less than 150mm.