The New Zealand Metal Roofing Manufacturers (NZMRM) Code of Practice provides detailed guidance for the design and installation of metal roofing and wall cladding systems. In 2026, important technical provisions on roof drainage, downpipes, and penetrations have brought renewed attention to how these elements should be designed and detailed.
For commercial roofing in particular, these are not minor considerations. Large roof catchments, internal gutters, mechanical plant and multiple service penetrations can all increase the consequences of inadequate drainage or poorly designed flashing details.
So, what do the 2026 MRM COP downpipe updates and MRM COP penetration updates mean in practice? We look at some of the most important considerations for new roofs and roof replacements for metal roofing systems.
Of course, you can’t consider a downpipe in isolation from the rest of the roof drainage system. Catchment area, rainfall intensity, gutter capacity, outlet location and the number and position of downpipes all influence how effectively water can be removed from a roof.
The current Code of Practice specifically notes that downpipe placement significantly affects gutter and downpipe calculations. Depending on outlet placement, individual downpipes may need to serve very different proportions of the overall roof catchment.
This becomes particularly important on large commercial roofs, where considerable volumes of rainwater may need to be managed during short-duration, high-intensity rainfall.
One of the most important points in the current 2026 MRM COP downpipe updates is the distinction between a downpipe’s nominal size and the capacity of its dropper.
The Code states that downpipe capacity is almost always governed by the dropper capacity. Because the dropper fits inside the downpipe, its internal diameter is smaller, and manufacturers can vary that measurement.
The NZMRM calculator therefore uses the smallest internal dropper diameters measured from commonly available products when performing its calculations.
In practical terms, specifying a nominal downpipe size alone does not necessarily establish how much water the complete outlet can handle. You need to consider the limiting component in the drainage path. For commercial projects, where roof catchments can be substantially larger, correctly calculating these capacities is particularly important.
Not necessarily. The appropriate downpipe arrangement depends on the individual roof. Catchment area, gutter design, rainfall intensity, dropper capacity and downpipe positioning all contribute to the calculation.
The important takeaway from the 2026 guidance is therefore not simply that downpipes need to become larger. It is that drainage capacity must be established using the relevant dimensions and conditions, rather than relying on nominal pipe size alone.
For a new roof, you can incorporate these considerations during the design and coordination stages. During a roof replacement, you may need to assess existing gutters, outlets, and downpipes to determine whether they still suit the new roofing system.
Penetrations are one of the more complex areas of metal roof design because every opening interrupts the normal path of water down the roof.
The current COP makes clear that no single flashing arrangement suits every penetration. Instead, penetration design depends on factors including its width, position on the roof, expected debris, and the catchment above it.
The Code identifies three principal curb configurations: level, arrowhead and cricket. It also differentiates between over-flashings, under-soakers, and hidden gutters based on the penetration’s proximity to the roof apex or eave.
Level back curbs are commonly used and comparatively straightforward to fabricate and install. However, because they have little or no transverse fall, debris can collect behind them.
The COP recommends considering a freer-draining arrowhead or cricket arrangement for penetrations wider than 600 mm, in aggressive environments, or in locations where ongoing maintenance is difficult.
Arrowhead curbs incorporate a diverter to move water around the penetration, while cricket curbs divert water with less turbulence and eliminate the flat areas behind the penetration where debris can accumulate. This is particularly relevant to commercial roofing, where larger penetrations and substantial upstream catchments can make effective water diversion critical.
Commercial roofs frequently need to accommodate HVAC equipment, plant supports, electrical services and other building infrastructure.
The COP specifically addresses mechanical services penetrating metal roof cladding. Where plant-room supports pass through the roof, it recommends Circular Hollow Sections over Rectangular Hollow Sections or other hot-rolled sections because they can be more readily weatherproofed with E.P.D.M. flashings.
Importantly, coordinate the support framing before installing the roof cladding. This lets the roofing installation proceed without trying to weatherproof multiple structural penetrations at once. Cable penetrations also require careful detailing. The Code states that they must be goose-necked and should not simply exit through a vertical flashing where sealant becomes the only barrier against water leakage.
For designers, builders, and roofing contractors, the MRM COP penetration updates send a clear message: treat penetrations as part of the roofing design, not an afterthought after installation.
The technical requirements reinforce the importance of considering the roof as a complete weatherproofing and drainage system.
Downpipes need to work with the gutters, droppers and roof catchment they serve. Penetrations need to account for water travelling down the roof, their position, their width and the likelihood of debris accumulation. Coordinate commercial plant and services with the roofing system before installation wherever possible.
For new construction, early collaboration between designers, builders, services contractors and roofers can help resolve these details before work reaches the roof.
For replacement projects, existing drainage and penetration arrangements should not automatically be assumed suitable simply because they have been in place previously. The new roofing system provides an opportunity to assess these details against current requirements and the building’s conditions.
The current Code of Practice recognises that a downpipe’s nominal diameter does not necessarily reflect the capacity of the complete outlet. Because the dropper fits inside the downpipe and has a smaller internal diameter, it can govern capacity. The NZMRM calculator uses internal dimensions based on commonly available droppers.
Not automatically. Downpipe requirements depend on the individual roof drainage design, including roof catchment, rainfall intensity, gutter configuration, outlet capacity and downpipe placement. The correct arrangement must be calculated for the specific project, not determined from nominal downpipe diameter alone.
The appropriate flashing detail depends on factors including the penetration’s size and position on the roof, the catchment above it and potential debris. The COP uses level, arrowhead, and cricket curb configurations, alongside over-flashings, under-soakers, and hidden gutters, to provide different solutions for different conditions. Flush penetrations such as roof windows also have specific flashing requirements.
The 2026 NZMRM Code of Practice shows how much technical consideration goes into a well-designed metal roofing system. Downpipe dimensions, dropper capacity, catchment, penetration position, flashing configuration and services coordination can all influence how the roof performs once exposed to New Zealand conditions.
Aspect Roofing stays on top of MRM COP downpipe and penetration updates, and the entire Code of Practice v26.06 for new roofing and roof replacement projects throughout Auckland. If you’re planning a project and need advice on roof drainage, penetrations, flashings or the wider roofing system, contact our team to discuss your requirements and stay compliant.