Roof Pitch Calculator for the UK and NZ: Degrees, Tiles and Falls

Why the UK and NZ use degrees, not x-in-12

The American rise-over-run convention (e.g. '6 in 12', meaning 6 inches of rise per 12 inches of run) is rare in the UK and New Zealand trade. Both countries use metric measurements and express roof pitch as an angle in degrees from horizontal, matching the way building regulations, manufacturer technical data sheets and structural calculations are written. A '30 degree roof' or 'pitched at 35°' is the standard way to describe a roof on a UK or NZ building consent or tile manufacturer's datasheet.

This matters practically because tile, slate and membrane manufacturers publish minimum pitch requirements strictly in degrees. Converting an imperial rise-over-run figure (common in older pattern books or imported literature) into degrees is often the first step before checking it against UK or NZ compliance tables.

Pitch conversion: degrees, ratio and percentage

Note the percentage grade column is not the same as the degree value — they diverge quickly above 20°, which trips up anyone assuming 'percent' and 'degrees' are interchangeable. Always check which unit a spec sheet or building control officer is asking for before quoting a number.

UK minimum pitch for tiles and slate (BS 5534)

In the UK, BS 5534 (the code of practice for slating and tiling) sets the framework for wind uplift and exposure-based fixing specifications, but the actual minimum pitch for a given product comes from that manufacturer's BS 5534-compliant test data, cross-checked against exposure zone, rafter length and the underlay type used. As a general guide across common UK products:

NZ minimum pitch and weathertightness (E2/AS1)

New Zealand ties roof pitch to weathertightness performance under the New Zealand Building Code Clause E2 (External Moisture), with E2/AS1 as the most commonly used acceptable solution for residential roofing. E2/AS1 sets minimum pitches for roofing types based on the roof cladding, the length of the roof plane, and the exposure (wind zone) of the site — steeper pitch or shorter roof planes are required in higher wind zones.

Because New Zealand has strong regional wind zone variation — from 'low' to 'extra high' under NZS 3604 — the same roofing product can have a different minimum pitch in Wellington than in Auckland. Always check the manufacturer's published Appraisal or BRANZ technical statement for the specific wind zone before finalising a design, as this is what building consent authorities will check against.

Pent roof pitch and mono-pitch design

A 'pent roof' (also called a mono-pitch or skillion roof) is a single sloped surface with no ridge, common on extensions, garages, sheds and increasingly on contemporary UK and NZ new-builds. Pent roofs typically run shallower than dual-pitch roofs — commonly 5° to 15° — because a single long slope at a steep angle would be structurally awkward and visually heavy on a small extension.

• Below about 5°, a pent roof is functionally a flat roof and should be detailed with a waterproof membrane rather than tiles or slates.
• Between 5° and 10°, standing seam metal, well-lapped corrugated sheet, or certain low-pitch tile systems can work, but check the manufacturer's minimum pitch carefully.
• Above 10°, most standard tile, slate and sheet products become viable, subject to the exposure checks above.
• Pent roof drainage usually runs to a single gutter along the low eave — size that gutter for the full roof catchment area, not half as on a dual-pitch roof.

A common design mistake is specifying a pent roof pitch purely for aesthetics (e.g. a very shallow 3° 'modern' look) without checking the chosen covering's certified minimum — this is one of the most frequent building control rejections on UK and NZ extensions.

Flat roof fall: ratios instead of degrees

Once a roof drops below roughly 5°, the angle becomes too shallow to describe usefully in degrees, so UK and NZ practice (and BS 6229 in the UK) switches to describing it as a 'fall' — a ratio or a rate of fall expressed in millimetres per metre.

The key design rule in both countries: specify the fall steeper than the bare minimum, because construction tolerances, deck deflection and insulation settlement typically remove some of the designed fall over time. A roof designed exactly to 1:40 often performs closer to 1:60 once built, which is why BS 6229 recommends designing to 1:40 as the minimum, not the target.

Frequently asked questions