Roofing Underlay
Roofing underlay, also known as roofing underlayment, is a protective layer installed between the roof deck and the outer roofing material. Its primary function is to act as a secondary barrier against water, protecting the roof structure and interior of the building from potential damage caused by moisture infiltration.
ITP’s Powerlon range of roofing underlays provides high-performance protection for all pitched roof types and walls in timber frame structures. Providing enhanced functionality, including high breathability and water vapour permeability to help prevent the build-up of condensation, they can be used on warm and cold, supported and unsupported, ventilated and unventilated roofs.
BBA certified and CE approved, our Powerlon roofing underlays are Independently tested for full compliance with BS 5534:2014 + A2:2018. Available in various grades, including an option for advanced UV protection, the membrane is engineered for high tensile strength and nail tear resistance.
Our Powerlon UltraPerm comes with pre-printed marking lines for both roof pitch and overlaps for supported and unsupported roofs make installation quicker and easier. Complementing the membranes is the Powerbond range of accessories which includes penetration and jointing tapes. Each roofing underlay roll is supplied with detailed installation instructions and a colour-coded geographical suitability guide per wind zone and membrane grade.
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Our range of Roofing Underlay
Powerlon UltraPerm
Available in four different grades, Powerlon UltraPerm is a range of high-performance breathable underlays for all pitched roof designs and wall applications (timber frame). Available with or without integrated self-adhesive strips, its non-woven composite membrane is tough, durable and tear resistant with exceptional water vapour permeability and hydrophobic (water repellent) characteristics.
The BBA-certified Powerlon UltraPerm range is fully tested to the latest BS 5534:2014 + A1:2015 zones 1-5.
Powerlon UV Roof
Powerlon UV Roof is a high performance, breathable roofing underlay with a superior UV resistance protecting the roof deck and insulation from harmful UV rays, which can cause degradation over time. Its resistance to UV damage ensures long-term performance and structural integrity. The technically advanced, engineered material is suitable for shallow pitched roofs with a pitch of 5 degrees and over.
What functions do roofing underlays provide?
Roofing underlay is a crucial component of any roof system, providing multiple essential functions to enhance the performance and longevity of the roof. Installing roofing underlay helps to reduce the likelihood of costly repairs over time. Acting as a secondary barrier beneath the outer roofing material, underlay offers critical protection against the elements. If the roof covering is damaged, cracked, or displaced, the underlay serves as a backup defence, preventing wind-driven rain, snow, or moisture from penetrating the roof structure.
In addition to weather resistance, roofing underlay safeguards the roof deck from exposure to direct sunlight, reducing the risk of warping or material degradation. It also helps control heat transfer, improving the roof’s energy efficiency by adding a layer of insulation. This contributes to a more comfortable and energy-efficient home or building.
ITP’s roofing underlay systems provide enhanced durability, with features such as tear resistance to ensure long-lasting performance, even under challenging conditions. Their ease of installation makes them a practical choice for contractors, while improved breathability minimises condensation, reducing the risk of mould or dampness within the roof structure.
Performance and installation requirements for roofing underlay
Building & Site Conditions (BS 5534: 2014 + A1: 2015):
Roofing underlays must adhere to the following conditions:
- Ridge Height: ≤ 15m
- Roof Pitch: 12.5°–75°
- Site Altitude: ≤ 100m
- Topography: No significant variations
- Underlay Uplift Deflection: ≤ 35mm
Wind Uplift Resistance:
Underlays should meet these resistance requirements (at a 345mm batten gauge):
- 1600 N/m²: Well-sealed ceiling
- 1900 N/m²: No or poorly sealed ceiling
- 2350 N/m²: No or poorly sealed ceiling with dominant openings
Installation:
- Use underlays only in wind zones where their wind uplift resistance meets or exceeds the design wind pressure.
- Installation guides, including wind zone suitability and overlap markings, are provided with each roll of roofing underlay supplied by ITP.
Condensation Control:
- Roofs with insulation between, above, or below rafters differ from those with insulation above a horizontal ceiling.
- Ensure a well-sealed ceiling and use an Air and Vapour Control Layer (AVCL), such as the Powerlon AVCL range, for effective moisture management.
Example Wind Pressure Calculation:
- Design wind pressure should not exceed 70% of the maximum wind velocity.
- Example: For a pressure of 1166 N/m², the design limit is 816 N/m².
Note: Follow all instructions provided with the product for optimal performance and compliance.
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