Flat Roofs: The Risk and the Reward
Flat roofs — or more accurately, low-slope roofs — have a mixed reputation in New Zealand because of the weathertightness failures associated with some flat-roofed construction of the 1990s and 2000s. But flat roofs, when designed and built correctly, can perform well for many decades. The problems arise when flat roofs are built without the design discipline, material quality, and installation care that they require — and getting those things right is more demanding than for a pitched metal roof.
The demand for flat roofs has grown with the architectural preference for contemporary design — the horizontal lines, usable roof decks, and clean aesthetic of flat-roofed architecture are popular in the current market. Meeting this demand while managing the performance risk requires both competence in the technical requirements and clear communication with clients about maintenance expectations.
Membrane Systems
New Zealand flat roof construction uses a range of membrane systems: torch-applied modified bitumen membranes; cold-applied liquid membranes; TPO and PVC single-ply membranes; and traditional built-up bitumen felt systems. Each has different installation requirements, performance characteristics, and maintenance needs. System selection should be based on the specific project conditions — roof configuration, drainage, traffic, UV exposure — rather than cost alone.
Falls and Drainage
Flat roofs are not truly flat — they have a designed fall to drain water to gutters or deck drains. A minimum fall of 1:80 (sometimes expressed as 12.5mm per metre) is typically required by membrane manufacturers to ensure water drains rather than ponds. Achieving this fall requires careful attention to substrate framing or the use of tapered insulation systems, and verification that the completed roof achieves the design fall before the membrane is applied.
Parapet Flashings
Parapets — the upstanding walls at the edges of flat roofs — are among the most common failure points in flat roof systems. The junction between the horizontal membrane and the vertical parapet is a complex detail that needs to accommodate thermal movement, remain watertight over the long term, and be protected from UV degradation. Proprietary parapet flashing systems that are part of the membrane manufacturer’s warranted system are the most reliable approach.