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A Guide to Selecting Underlay

Roof and wall underlays come in many forms. Here’s a guide to the considerations typical of a medium density project:

  • What’s the cladding? Direct or cavity fix? Absorbent (timber, brick, fibre cement) or non-absorbent (metal or plastic) cladding? Steel or timber framing? What properties are stipulated by the cladding manufacturer?
  • Compatibility and reduced risk: A growing concern and requirement across the industry — is documentation available to prove compatibility of the underlay with the sealants, tapes, cladding and frame? Is the underlay part of a wider system or solution that could be used? How long is the the warranty?
  • Roof underlay: What is the purlin span? Is underlay support needed? Can the underlay be used on pitches under 10 degrees?
  • Fire: Is the AS 1530 Part 2 Flammability Index of the underlay no greater than 5 in order to meet the NZBC Acceptable Solution C/AS2? As best practice, Thermakraft suggests fire retardant products be used across the entire build of a medium density build, even if minimum standards allow otherwise.
  • Wind zone: Does the underlay meet the Wind Zone requirements?
  • Air barrier: You may need an air barrier in specific areas only, but could it be more efficient to use an underlay with air barrier properties across the build?
  • Exposure rating: Consider the longer build times on Medium Density projects; how many days can the underlay be exposed before its no longer warranted?
  • Optimisation: Wide width rolls and cut-to-length could reduce labour and waste resulting in significant savings.
  • Supply: Delays in supply may compromise the integrity of other products. Is the underlay readily available?
  • For the client: Are they keen on the marketing potential of custom branding? Medium density projects present a big billboard opportunity.

What underlays meet all these considerations?

Thermakraft, manufacturing underlays in New Zealand, for New Zealand conditions and building code requirements, suggests the following compatible and approved solutions for a Medium Density build:

  • Watergate Plus Wall Wrap
  • Thermaflash Flashing Tape
  • OneSeal Pipe Penetration Seals
  • Covertek 405 or Covertek 407 Roof Underlay

If the build goes higher or rigid air barriers are required, Thermakraft recommends James Hardie RAB system with Watergate Plus flexible underlay if required, Thermaflash flashing Tape, Premium Joining Tape and OneSeal pipe and cable penetration seals.

View more information on Kingspan Thermakraft, including contact details.
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