What underlay does
Think of it as the second line of defence. Wind-driven rain, condensation forming under a metal roof and the odd drip at a lap all land on the underlay instead of your ceiling insulation, and it carries that water down and out.
Because it sits under the roofing, it can only be properly replaced when the roofing comes off, so it rarely gets the attention it deserves.
When it's required
Under E2/AS1, all pressed metal tile roofing and all profiled metal longrun must have underlay. The Acceptable Solution's underlays are paper-based, R1 heavyweight kraft or R2 self-supporting kraft to NZS 2295, and it says metal roofs need paper-based underlay.
Tiles vary. Some double-profile concrete tiles may go without it at 20 degrees or more, but E2/AS1 requires underlay wherever a spreader discharges water onto a tile roof and in Very High and Extra High wind zones. Monier's flat Horizon tiles must have it, and a tiled roof with underlay needs anti-ponding boards.
Synthetic underlays such as Kingspan Thermakraft's Covertek range sit outside that paper-based category. Covertek 407 is covered by its own BRANZ appraisal and CodeMark certificate instead.
Kraft paper or synthetic?
| Kraft paper (E2/AS1) | Synthetic (Covertek 407 example) | |
|---|---|---|
| How it's accepted | Named in the Acceptable Solution | Its own BRANZ appraisal and CodeMark |
| Low pitches | Laid horizontally below 10 degrees | Not suitable below 3 degrees |
| Support | R1 fully supported; R2 spans up to 1.2 m | As its appraisal sets out |
| Exposure before the roof goes on | Cover it promptly | No more than 30 days of weather or UV |
| Unlined sheds and garages | The metal roofing Code of Practice says underlay isn't required | Kingspan limits its absorbent synthetics to lined buildings since 1 July 2025 |
Laying it properly
E2/AS1 wants underlay lapped at least 150 mm. Under Gerard pressed metal tiles, BRANZ says it's laid tightly across the roof, and a skillion roof needs a ventilated air gap of at least 25 mm under the tiles.
Insulation shouldn't touch it. Kingspan Thermakraft notes a mandatory 25 mm minimum gap between insulation and underlay, and says bulk insulation can be squashed by up to 62% at the edges of a 10-degree roof. A roofer and insulation installer who talk to each other avoid that. E2/AS1 also notes that solvent in freshly LOSP-treated timber can affect the bitumen in paper underlays.
DRIDEX, the roof with its own fleece
COLORSTEEL DRIDEX has a fleece on its underside that soaks up condensation as it forms and lets it go later. NZ Steel says it replaces separate underlay in many applications, and its warranty actually requires it to go on without underlay. NZ Steel accredits DRIDEX installers, and the warranty excludes UV damage to the fleece and poor ventilation.
What makes underlay work bigger or smaller
On its own, underlay is rarely a job; it's part of a re-roof. What changes the work is whether the roof needs plywood (some tray profiles do), whether a skillion needs a ventilated gap, how the insulation sits, and whether the old underlay has let water damage the framing.
Gerard's warranty doesn't cover underlay or other third-party products, so the underlay maker's terms and the roofer's workmanship are what stand behind it.
Questions for a re-roof quote
- Which underlay are you using, and is it accepted for my roof type and pitch?
- Kraft paper or synthetic, and why?
- How will you keep the gap between underlay and insulation?
- For a tiled roof, are anti-ponding boards included?
What you can check
You can
- Look in the roof space with a torch for sagging, torn or stained underlay, stepping only on the joists
- Check for drips or damp insulation on cold mornings
- Mention any roof-space damp to whoever quotes
Leave to a roofer
- Replacing underlay during a re-roof
- Fixing laps and support
- Ventilation for skillion roofs
Our advice on underlay
On a re-roof, we'd treat new underlay as part of the job, not an extra, and ask exactly which product is going on and why. If there's damp in the roof space now, we'd find out whether it's a leak or condensation before anyone quotes, because the fixes are different. Condensation or a leak? helps you tell.
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.