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What is roofing underlay supposed to do?
MRM describes the role of absorbent roof underlay as temporarily absorbing condensation and later releasing that moisture back to the atmosphere. It also contributes to pressure equalisation and moisture management within the assembly. Sources: NZ Metal Roofing Manufacturers.
What happens to underlay during a reroof?
Current MRM guidance says that with reroofs, it is not acceptable to lay the new roof over existing underlay or underlay support unless the retained material is in as-new condition. Its reroof bulletin also recommends no more than a single compliant layer of underlay, apart from laps, under profiled metal roofing. Sources: NZ Metal Roofing Manufacturers.
That means a quote that simply says "new roof over existing paper" deserves clarification. The condition, type and intended build-up should be explicit. Sources: NZ Metal Roofing Manufacturers.
Can underlay solve a condensation problem by itself?
No. If the moisture load exceeds what the assembly can manage, underlay can become saturated and other roof-space materials can remain wet. MRM says the response is to reduce moisture entering the cavity and/or improve the relevant ventilation pathway, depending on the design. Sources: NZ Metal Roofing Manufacturers; NZ Metal Roofing Manufacturers.
BRANZ similarly frames roof-space ventilation as moisture management above the ceiling insulation and below the roof cladding, not as a generic add-on for every roof. Sources: BRANZ.
What should an underlay line in a roofing quote tell you?
- Named underlay product or at least the required performance/type.
- Whether the existing underlay/support is removed or retained, and why.
- How the underlay interfaces with eaves, ridges, penetrations and any ventilation strategy.
- Whether added insulation or a changed roof build-up affects moisture design.
- Any fire, support or project-specific requirements that apply.
Why product and project requirements still matter
Underlay requirements vary with cladding systems and the project compliance path. RoofHub explains the general role; the designer, roofer, supplier or consent documents need to confirm the actual product and installation requirements for the building. Sources: NZ Metal Roofing Manufacturers.
Related questions
Does roof underlay drain condensation into the gutter?
That is not its primary role. MRM specifically warns against treating underlay as a drainage plane or a second roof; it is intended to absorb moisture temporarily and release it when conditions allow. Sources: NZ Metal Roofing Manufacturers.
Can a roofer put new underlay over the old underlay?
Current MRM guidance says a reroof should not be laid over existing underlay or support unless the retained material is in as-new condition, and recommends no more than one compliant underlay layer apart from laps. Sources: NZ Metal Roofing Manufacturers.
Sources and review notes
Prepared by RoofHub editorial team. Reviewed . A source check confirms what a page states, not an independent inspection or invoice audit. Published project accounts are attributed to their publishers.
- primaryNZ Metal Roofing Manufacturers: Code of Practice v26.09: Underlay - Current MRM guidance describing underlay as temporary moisture absorption/release rather than a substitute drainage plane, and recommending no more than one compliant layer on a profiled-metal reroof except at laps. Page checked 2026-10-21. Publication date not stated. · industry-technical
- primaryNZ Metal Roofing Manufacturers: Code of Practice: Ventilation Pathways - MRM guidance on roof-space ventilation pathways, moisture movement and cases where additional ventilation may be needed. Ventilation is a design response to moisture conditions, not a generic cure for every roof problem. Page checked 2026-10-21. Publication date not stated. · industry-technical
- primaryBRANZ: Roof space ventilation (2018-12-01) - BRANZ bulletin explaining that roof-space ventilation in NZ is primarily about moisture management above ceiling insulation and below roof cladding. Retained as technical background alongside the current MRM Code of Practice. Page checked 2026-10-21. Publication date shown above. · industry-technical
Original calculations are labelled as examples. Historical prices stay dated, missing fields stay unknown, and conflicting specifications are flagged. Sources are not endorsements or paid rankings. Read our methodology or report a correction.
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