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Control of Condensation

Good roof design extends beyond the choice of roof covering. It is just as important to protect the supporting structure from the damaging effects of condensation. The most effective and proven method for preventing condensation is by providing appropriate ventilation, in accordance with the Building Regulations Approved Document C2 2004 and BS 5250: 2002. Significant changes have occurred in recent years and the introduction of low resistance, vapour permeable underlays and breather membranes has offered specifiers and designers the opportunity to look at pitched roof construction in a new way.

However, this has led to much heated debate regarding the relative merits and benefits of ventilated and unventilated roof systems, particularly the need for roof space ventilation in both warm (insulation at rafter line) and cold insulation at joist level) pitched roofs, especially when vapour permeable/breathable (type LR) underlays are used. Building Regulations Approved Document Part L 2006 and recent revisions to BS5250: 2002 Control of Condensation in buildings and BS5534: 2003 Code of Practice for Slating and Tiling have sought to resolve many of these issues.

Decra roof systems include products to meet these ventilation requirements at high and low level.

Ventilation requirements of Building Regulations C2 2004 and BS 5250: 2002:

Cold roofs with large voids above horizontal insulation, which incorporate the use of type HR impermeable underlays. Scottish practice: Cold roofs with large voids above horizontal insulation, which incorporate the use of type LR permeable underlays AND high vapour resistant sarking boards (e.g plywood, orientated strand board etc.)

Type of roof Pitch Insulation level Ventilation requirement at eaves (per metre) Ventilation requirement at ridge or top edge (per metre)
Duo pitched More than 15° Joist 10,000mm² 5,000mm² at each side of ridge*
  15° or less Joist 25,000mm² 5,000mm² at each side of ridge*
        * High level ventilation may not be required on low rafter pitches below 35º or at spans less than 10m
Lean to / Mono-pitched More than 15° Joist 10,000mm² 5,000mm²
  15° or less Joist 25,000mm² 5,000mm²
Duo / Mono-pitched 10° and above Rafter 25,000mm² 5,000mm² at each side of ridge

Cold roofs with large voids above horizontal insulation (which incorporate the use of type HR permeable underlays). Scottish practice: Cold roofs with large voids above horizontal insulation, which incorporate the use of type LR permeable underlays AND low vapour resistant sarking boards e.g open jointed sawn softwood boards (typically 150mm wide) with 2mm gap between.
Type of roof Pitch Insulation level Ventilation requirement at eaves (per metre) Ventilation requirement at ridge or top edge (per metre)
All dwelling size roofs 10° and above Joist 3,000mm² (well sealed ceiling)  
All dwelling size roofs 10° and above Joist 7,000mm² (well sealed ceiling)  
All dwelling size roofs 10° and above Joist   -  5,000mm² at each side of ridge (well sealed ceiling and not well sealed ceilings)
All larger dwelling size roofs or 10° and above Joist 5,000mm² (well sealed ceiling) 5,000mm² at each side of ridge
All larger dwelling size roofs 10° and above Joist 10,000mm² (well sealed ceiling) 5,000mm² at each side of ridge

 

Quick Links

Ventilation Requirement's
What are the ventilation requirements for your roofing project? 
Read more

Selection of Underlays
What is the best underlay for your roofing project , HR, LR roofing underlays or Breather Membranes. 
Read more 
 

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