Installing Underlay

Underlay can be laid vertically or horizontally. Side laps must be a minimum of 150 for roofs and 75 mm for walls; end laps must be a minimum of 150 mm for both roof and wall cladding. At the eaves, the underlay should terminate on the upper side of the eaves flashing or overhang fascia by no more than 20 mm.

The COP recommends that all underlay is terminated at the ridge, and if not it should be slit or slotted to allow passive ventilation of the ceiling cavity.

The COP allows roof and wall underlay to be laid either vertically or horizontally in all cases. However, that is in divergence with E2/AS1, so it is advised for buildings within the scope of E2/AS1 (ie, buildings designed within the scope of NZS 3604) that the requirements of the local TA is sought before diverging from E2/AS1.

Rips smaller than 75 mm on walls or roofs can be repaired using a compatible flashing tape, but roof underlay damage greater than this requires a new piece of underlay captured by the cladding fastenings.

Flue penetrations must have a minimum distance of 50 mm from the outer liner to any underlay or flammable material.

E2/AS1 requires self-support underlay laid horizontally on support to be used at pitches below 10°. That is not a requirement of the COP as underlay support is no substitute for good ventilation design and effective weathertight details.

When using vented battens, the underlay should be positioned on the upper side of the batten, directly under the roof cladding. Having the underlay directly under the roof allows the battens to vent the roof cavity directly and allows the underlay to perform its normal design function; putting roofing underlay under ventilated battens impedes roof cavity ventilation

With re-roofs in any material, it is not acceptable to lay a new roof over existing underlay or underlay support, unless the latter is in “as new” condition. See  Fixing Aluminium Sheeting.

Wall underlays must have a minimum side lap of 150 mm, and an end lap of 75 mm. Wall underlay on a drained cavity should be on the dry (inside) face of the cavity, and be rigid enough to restrain wall insulation from contacting the cladding, or have secondary strapping to achieve such.