What does roof underlay actually do?
Underlay is a sheet laid over the rafters or purlins, under the roofing. It's a second line of defence: it catches the condensation that forms under a metal roof on a cold night, and any wind-driven rain that sneaks past a lap or flashing, and sends it to the spouting (gutters) instead of your ceiling.
The Building Code's clause E2 requires concealed spaces to be built so outside moisture doesn't build up and cause condensation, mould or decay. E2/AS1, the acceptable solution that shows one way to meet it, says all metal tile roofing and all profiled metal longrun roofing must have a roof underlay.
Tile roofs are more varied. Under E2/AS1, Type I concrete tiles can be laid without underlay at 20 degrees or more, but underlay is required in Very High or Extra High wind zones and under spreaders, and tile roofs with underlay need anti-ponding boards at the eaves. Some tile makers insist on it anyway: Monier says its Horizon tiles must use sarking (underlay).
What a roofer's quote should say about underlay
- The product by name, and the type: E2/AS1 lists R1 heavyweight kraft or R2 self-supporting kraft, complying with NZS 2295
- If it's a synthetic underlay, the appraisal it relies on, and confirmation your roof space is lined
- Laps of at least 150 mm
- Laid horizontally on pitches below 10 degrees
- R1 underlay fully supported, or R2 self-supporting underlay at a span of no more than 1.2 metres
- Anti-ponding boards at the eaves on a tile roof with underlay
- The 25 mm air gap kept between the underlay and any insulation
- Whose warranty covers the underlay, since some roofing warranties don't
Paper or synthetic?
E2/AS1's underlay category for metal roofs is paper-based and excludes synthetics. A synthetic underlay is used on the strength of its own appraisal instead: Kingspan Thermakraft's Covertek 407, for example, is covered by BRANZ Appraisal No 651.
There's a recent catch. Kingspan Thermakraft says that from 1 July 2025 its synthetic absorbent underlays, such as the Covertek range, are limited to lined buildings only, after testing found UV exposure in unlined structures can cause early breakdown. It also cites the metal roofing Code of Practice as saying underlay isn't required in unlined structures at all.
One more detail worth knowing: Gerard's warranty for its pressed metal tile roofs doesn't cover underlay or other third-party products, so ask who stands behind the underlay on your job.
Insulation: what the Building Code expects now
H1/AS1 Sixth Edition, which applies to housing, took effect on 27 November 2025. It requires heated ceilings to reach a minimum construction R-value of R6.6 in all six climate zones, dropping to R3.3 for up to 500 mm from the ceiling perimeter where there isn't room for full-thickness insulation.
The climate zones run from Auckland in zone 1, through Hamilton (2), Wellington (3) and Taupō District (4), to Christchurch and Dunedin in zone 5 and Queenstown-Lakes and Invercargill in zone 6. These are the benchmarks for new consented work, and a useful target if you're topping up.
H1/AS1 points to NZS 4246 for how bulk thermal insulation should be installed. Ask any installer whether they work to it.
Topping up ceiling insulation in an existing home
MBIE's exemptions guidance says retrofitting insulation to ceiling spaces and subfloors of an existing building doesn't need a building consent. Insulating external walls does.
Mind the edges. Kingspan Thermakraft notes a mandatory 25 mm minimum air gap between insulation and roof underlay, and says bulk insulation can be squashed by up to 62% at the perimeter of a 10 degree pitch roof. On a low-pitched roof, the insulation near the eaves often ends up much thinner than the bag says.
Skillion and flat roofs, where the ceiling follows the roofline, have no ceiling space to fill. Insulating them properly usually means work from above at re-roof time, so talk to the roofer about it before the new roof goes on.
Rentals: the healthy homes insulation rules
Tenancy Services says rental homes consented before 3 November 2022 need ceiling insulation of at least R2.9 in zones 1 and 2 and R3.3 in zone 3, plus underfloor insulation of R1.3. Existing ceiling insulation installed before 1 July 2016 must be at least 120mm thick.
Insulation must be installed to NZS 4246:2016 and be in reasonable condition, with no mould, dampness, damage or gaps. A home that meets the 2021 Building Code insulation requirements also meets the healthy homes standard. Check with Tenancy Services which zone your rental falls in before you measure.
Before anyone goes into the ceiling
Only step on the ceiling joists, never the plasterboard between them, and watch for wiring and downlights buried in old insulation. If the roof might be asbestos cement, stop: Health NZ says the ceiling under an asbestos roof may hold high concentrations of asbestos dust, particularly if the roofing is weathered and brittle. Get the roof tested before anyone disturbs insulation or dust up there.
Stains on the ceiling: leak or condensation?
Brown rings on the ceiling aren't always a roof leak. Condensation dripping off the underside of a roof with poor or missing underlay, or warm moist air rising into a cold ceiling space, can leave the same marks. Our guide to condensation or a leak helps you tell them apart, and roof ventilation covers getting damp air out.
When to deal with each layer
At a re-roof: new underlay
Once the old roofing is off, the underlay is fully exposed. That's the time to replace it with a product that suits your roof, laid to the rules above.
With the roof off: check the ceiling space
Look at the insulation for gaps, damp patches or thin spots at the edges, and ask about ventilation while access is easy.
Any time: top up the ceiling
Adding insulation to an existing ceiling space doesn't need a consent. An installer who works to NZS 4246 is the right person.
Landlords: check against the healthy homes rules
Measure what's there against the Tenancy Services minimums for your zone, and keep a record.
Skillion or flat roof: plan it with the roofer
Insulation for these is usually part of the roofing job, so raise it when you're getting re-roofing quotes.
What you can check, and what to leave to the pros
You can
- Look into the ceiling space from the hatch with a torch, if you can do it safely
- Note roughly how thick the insulation is and whether there are gaps or damp
- Ask for the underlay to be named in any re-roofing quote
- Check a rental against the healthy homes minimums
- Keep product names and receipts with your house papers
Leave to a roofer
- Laying underlay, which is part of the roofing job
- Installing insulation to NZS 4246, by a roofer or insulation installer
- Anything around wiring, downlights or flues
- Insulating skillion and flat roofs from above
- Testing anything that might be asbestos before it's disturbed
What we'd do
If a re-roof is coming, we'd make sure the quote names the underlay and how it's laid, then use the open roof to check the insulation underneath. If the roof is fine, we'd simply top up the ceiling insulation now, using an installer who works to NZS 4246, and leave the underlay until the roofing itself needs replacing.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.