COP v3.0:roofing; roof-pitch

7.1 Roof Pitch 

The pitch is the angle between the horizontal and the roof line. It is also the relationship between the rise and the horizontal span of the roof. See 18.2 Roof Pitch Tangent for the tabulation of these values and a calculation tool.

7.1.1 Minimum Roof Cladding Pitch 

 

7.1.1A Minimum Recommended Pitch

ProfileRib HeightMinimum PitchRise per metre of Span
Trapezoidal asymmetrical20 – 35 mm52 mm
Trapezoidal asymmetrical and symmetrical36 – 60 mm52 mm
Trapezoidal symmetrical20 – 35 mm70 mm
Secret-fix>30 mm52 mm
secret-fix<30 mm141 mm
Standing seam fully supported flat sheet metal>30 mm52 mm
All other types of fully supported flat sheet metal 87
Corrugated and other profiled sheeting16.5 – 20 mm141 mm
Corrugated and other profiled sheeting21 – 35 mm70 mm
Horizontally lapped metal tile25 mm upstand12°213 mm
Minimum pitches quoted in this table refer to roof cladding pitch and not the building design roof pitch.

Buildings designed with widely spaced purlins and widely spaced portal frames may require an increased design pitch to comply with the minimum recommended as-laid pitches.

Low pitched roofs require greater attention to flashing details. The ability of side laps or end laps to withstand water penetration also becomes more critical at low pitches, but the good design of flashings can ensure weathertightness in extreme conditions.

Water backup against vertical faces caused by high velocity, localised wind eddies, especially inside parapets and at the bottom edge of walls, are all vulnerable details. Pressure equalisation-designs and wind baffles are more effective in preventing water ingress than increasing the flashing cover width.

7.1.1B Exceptions to the Minimum Recommended Roof Cladding Pitch requirements:

  • Curved roofs, where by design the minimum pitch at the crest is always less than the prescribed minimum pitch. In these cases, the pitch at the eaves must comply with the profile’s minimum pitch, and the pitch at the upper end of a terminated arc must be a minimum of 3°.  (See 15.1 Curved Roofs).
  • The back curbs of penetration flashings where the minimum pitch is 1.5°. (See 9 External Moisture Penetrations)

    7.1.2 Runoff 

    Runoff is the ability of the roof cladding to discharge maximum rainfall without water penetrating through side laps, end laps or flashings and depends on rainfall, the catchment area, the roof pitch, and the profile geometry.

    The pitch determines the rate of flow. Steep slopes shed water faster than shallow slopes. However, at minimum falls, with a maximum rainfall intensity of 100 mm/hour, trapezoidal rib profiles with a height greater than 26 mm can have a maximum run of at least 40 m, and trapezoidal rib profiles with a rib height 20-25 mm can have a maximum run of at least 30 m.

    The roof gutter capacity calculator in 5 Roof Drainage  can calculate capacities for any known profile, rainfall and pitch, by treating the pan width and rib height as a gutter and the rib to rib dimension plus any discharge from adjacent surfaces as the catchment. When the sheeting length exceeds the above limitations, the capacities should be checked.