Selection of underlay

BS 5534: Code of Practice for Slating and Tiling and BS 5250: Code of Practice for Control of Condensation in Buildings recognises the following types of underlay: Read more

Type HR Roofing Underlays

Type HR (High resistance, impermeable underlays with a water vapour resistance of greater than 0.25 MN - s/g).

Typical uses: cold roof structures with large voids above horizontal insulation that are ventilated using traditional, continuous eaves and ridge ventilation systems.

Decra 300 High Performance Underlay and Decra TwoF Economy Underlays comply with type HR specification performance requirements.

Where high resistance underlays are used:

• It is normal to ventilate between the underlay and the insulation.
• It is not normal to ventilate above the underlay (i.e between the underlay and the roof tiles).
  
  

Type LR Roofing Underlays

Type LR (Low resistance, permeable underlay with a water vapour resistance of less than or equal to 0.25 MN - s/g).

Typical uses: cold roof structures with large voids above horizontal insulation that are ventilated using traditional, continuous eaves and ridge ventilation systems OR breathable pitched roof systems (cold roofs that are ventilated above the underlay).

Decra 700 High Performance Breather Membranes comply with and exceed type LR specification performance requirements.

Where low resistance underlays are used:

• Without ventilation between the underlay and the insulation, it is necessary to ventilate above the underlay (i.e between the underlay and the roof tiles).
• Without ventilation between the underlay and the insulation it is a requirement of BS 5534 that it is necessary to carry out a condensation risk analysis.
• With ventilation it is normal to ventilate between the underlay and the insulation.

Scotland: low resistance underlays, laid fully supported on sarking boards, should for design purposes be treated as:

• LR low resistance underlay if installed on sarking that offers a low resistance to the passage of water vapour (such as 150mm open jointed sawn softwood sarking boards).
• HR high resistance underlay if installed on sarking that offers a high resistance to the passage of water vapour (such as plywood, orientated strand board or chipboard).
  
  

Breather Membranes

Breather Membranes (membrane with a water vapour resistance of less than or equal to 0.6 MN - s/g).

Typical uses: warm roof and cold roof structures.

Decra 700 High Performance Breather Membranes comply with these specified performance requirements