Decra Roofing Resource Centre, over 30 year’s experience of finding the right lightweight roofing solutions


Roofing underlay is required and should comply with the recommendations of BS5534: 2003: Code of Practice for Slating and Tiling, BS 8000-6: 1990 Workmanship on Building Sites, and BS 5250: 2002: Code of Practice for Control of Condensation in Buildings.

Unsupported (roofing underlays draped over rafters or counterbattens):


Underlay should comply with BS 747 2000 Type 1F or 5U

NOTE: Although 1F & 5U may be used with Decra roof systems, Decra roof systems recommends selection of an appropriate Decra underlay, which offer the best value with regard to performance and specification. 

Fully supported (roofing underlays laid directly to boarding or sarking):

Underlay should comply with the recommendations of BS 5534: 2003: Code of Practice for Slating and Tiling. It should also be vapour transmission tested in accordance with BS 3177.   

Decra Underlays

High performance underlays and breather membranes for use with the Decra Roofing system are available from Decra, and can be ordered with Decra Roof Tiles. These underlays have been selected by Decra because they offer the best value with regard to performance and specification when used with Decra roof systems. Underlays supplied by Decra also offer a one-stop roofing solution, maintaining the quality of the total roof envelope and consequently reducing risk for the roof client.

Decra's range of underlays and membranes 

Recommended laps for underlay
Pitch Minimum headlap Minimum sidelap
  Not fully supported Fully supported  
10° to 12° 300mm 200mm 100 - 150mm
12.5° to 14° 225mm 150mm 100 - 150mm
15° to 34° 150mm 100mm 100 - 150mm
35° and above 100mm 75mm 100 - 150mm

1. Any penetrations to the underlay should be suitably sealed to prevent water ingress.
2. Underlay laps to valleys should comply with recommendations of BS 5534.


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