DriStud FRU38 Roof Underlay 220gsm Fire Retardant Vapour Permeable Synthetic Self-Support Roof Underlay

DriStud FRU 38 is a white synthetic tri-laminate roofing underlay that is water resistant, vapour permeable and fire retardant. The FRU 38 is a self-supporting, roof underlay. It is intended for use on new and existing buildings in wind zones up to and including ‘Extra High’, as defined in NZS 3604:2011.

DriStud FRU 38 is a Heavier grammage self-support roof underlay, whereas DriStud FRU 36 is a lighter grammage self-support roof underlay.

Roll Sizes

  • 1250mm x 40.0m (50m2)


  • CodeMark™ Certified
  • Self Supporting
  • Can be installed over timber and steel roof framing
  • Fire Retardant (FR Index: 1)
  • Breathable & Absorbent
  • UV exposure: 20 days
  • Durability: 50 years
  • Direct fix with absorbent and non-absorbent cladding including metal cladding

Overlaps / support

DriStud roofing underlays must be installed with all writing and imagery facing out and with as minimal a number of overlaps as possible. Where overlaps are unavoidable a lap of 150 mm minimum must be used. DriStud FRU 38 must not span more than 1.2 m between adjacent supports.


If installing on a roof with a pitch greater than 10° then DriStud roofing underlays may be installed either horizontally or vertically.

If the roofs pitch is 10° or less it is recommended to be run horizontally with the higher layers sitting on top of the lower layers. It can be installed without supporting netting for horizontal installation.

When it is not practical to place underlay horizontally, DriStud FRU 38 can be run vertically on a minimum side lap of 150 mm. Self-support roof underlays must be installed on supporting netting or equivalent system.

The netting and underlays must be installed flat and taut to ensure condensation will drain to the gutter. If any doubts vertical laps are recommended to be sealed with 75mm DriStud Cool window flashing tape or DriStud Joining Tape to ensure it.

DriStud FRU 38 must have support on netting or safety mesh if used:

  • Under dark coloured metal cladding
  • On roof pitches less than 10° and it is vertically installed.
  • If support spacing > 1200mm

When DriStud roofing underlays are being used in conjunction with masonry tiles, anti-ponding boards should be used at the lower edges of the tiles.


TCL Hunt Ltd warrants that all DriStud roofing underlays will be free from manufacturing defects. Upon receiving of DriStud roofing underlays, it is recommended that a visual check is made. Where defects are observed, these will be replaced at the discretion of TCL Hunt, provided that they are returned to point of purchase.

If installed in accordance with TCL Hunt installation requirements, TCL Hunt warrants that DriStud roofing underlays will comply with all relevant provisions of the NZ Building Code. DriStud roofing underlays will have a serviceability equal to that of the roof cladding provided that:

  • the balance of the external wall is installed in accordance with the NZ Building Code, and,
  • all necessary maintenance is undertaken in respect of the external wall system.

Please refer to DriStud website www.dristud.co.nz for more information on warranty and disclaimer.

Technical Properties

Property Standard Value
Weight 220 ±5 g/m2
UV Exposure ASTM G154 20 days
Roll Sizes (m²) 1250mm x 40m (50m²)
Flammability index AS1530.2-1993 1
Fire Retardant Yes
Water Vapour Resistance ASTM E96 Procedure B Requirement ≤0.5 MNs/g; Pass
Absorbancy AS/NZS 4201: Part 6 Requirement ≥150 g/m²; Pass
Air Resistance BS ISO 5636-5:2003 ≥0.1 MN s/m³; Pass
Air Barrier Yes
Water Resistance AS/NZS 4201.4:1994 ≥100mm H₂O; Pass
Durability (years) 50
Supported Self Supporting
Roof Pitch ˃ 3°