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 roof pitch determines the rate of flow; steep slopes shed water faster than shallow slopes. The rib height and spacing of trapezoidal profiles also affects its shedding ability.

        For example: At a rainfall intensity of 200 mm/hour, a five-rib trapezoidal profile at 3°, with a rib height of 27 mm can have a run of 90 m.

    The 7.1.4 Maximum Run Calculator can calculate capacities for any known profile.

    7.1.3 Roof Drainage Capacity 

    The capacity of a roof profile to drain water is determined by its geometry and the roof pitch. The catchment area is the distance between rib centres times the length, and the effective cross-section area and the wetted perimeter is taken to the height of the overlap on corrugate, or capillary bead on trapezoidal and trough section profiles.

    The 7.1.4 Maximum Run Calculator gives the maximum length that a roof can drain at a given pitch and rainfall intensity. The manufacturer’s data can be accessed from the drop-down box; for other profiles, the data can be entered manually into the worksheet.

     

    Where the flow of water is concentrated by penetrations or spreaders, go to the 9.4.4 Maximum Area Above Penetration Calculator.

     

     

    7.1.4 Maximum Run Calculator 

    More information about 5.2 Roof Drainage Design can be found in this PDF Document. A responsive online tools for calculating maximum run of any given profile, pitch, and rainfall intensity is available at  https://www.metalroofing.org.nz/maximum-run-calculator.

    Before using this calculator, please read 5.2 Roof Drainage Design.

    The Maximum Run Calculator calculates the maximum roof length to achieve effective roof drainage for any profile, pitch, and rainfall intensity. Insert the values in the designated fields.

    For an explanation of each element, please click on the corresponding question mark.

    For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

    Note that this site address is used only for convenience if printing calculations to attach to documentation.
    This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
    The address is not recorded or shared with any other parties.
    Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
     mm/hr
     
    Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
     
     
    The minimium Short-Term Intensity Multiplication Factor determined by the application type.
    You can increase this manually for critical applications.
     
    Enter 1:X or mm per metre- the calculator will automatically convert
    Minimum Fall 1:500, Maximum Fall 1:100
     
    1: =  mm per metre
      rads
     bends
     
     m
     
     
     °
      rads
     m
     
    Select whether runoff will drain on both sides of penetration or just 1;
     
     m
     
     each
     

    For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
     
     
     
    You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
    Illustration is for explanatory purposes only and is not to shape or scale.
     
    Wetted Perimeter=49Pitch=76Cross Section Area=272Free Surface Width=42Depth=17Wetted Perimeter=85Pitch=76Cross Section Area=585Free Surface Width=76Depth=17
    Illustration is for explanatory purposes only and is not to shape or scale.
     
    DepthPitchCrestCapillary DepthPanDepthPitchCrestPan
    Illustration is for explanatory purposes only and is not to shape or scale.
     
    Wetted PerimeterPitchCross Section AreaFree Surface WidthWetted PerimeterPitchCross Section AreaFree Surface Width
     mm
     
     mm
     
    Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
     mm²
     
    Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
     mm
     
     °
      rads
     °
      rads
     °
      rads
     
     mm
     
     
     mm
     
    Must be less than the upstand, D
     mm
     
     
     mm
     
     x   mm
     
     mm
     
    Select Manufacturer (if applicable) and Profile
     
    Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
     mm
     
    Width of the pan.
     mm
     
    Calculated result from (Pitch) - (Crest).
     mm
     
    Width of the crest (top of rib).
     mm
     
    Total depth of profile.
     mm
     
    Depth of profile from the pan to the height of the capillary tube.
     mm
     
    Data provided by a manufacturer, especially for irregular profiles.
     mm²
     
    Data provided by a manufacturer, especially for irregular profiles.
     mm
     
    Data provided by a manufacturer, especially for irregular profiles.
     mm
     
    Data provided by a manufacturer, especially for irregular profiles.
     mm
     

     
     
     
     
     
     
     m
     
     m
     
     mm
     
     m
     
     mm
     
     mm
     
     mm
     
     mm
     
     mm
     
     mm
     
     mm
     
     
     
     
     
     
     
     
     
     m/s
     
     m³/s
    mm
     
     
     
     
     
     
     
     
     
     
    This result is the maximum capacity that can be drained by an element of your selected configuration.
    Be sure to consider all relevant elements when assessing a roof area.
     
    This result is the maximum length of roof that can be drained by your selected configuration.
    Be sure to consider all relevant elements when assessing a roof area.
     m
    This result is the maximum area that can be drained above a penetration by your selected configuration.
    Be sure to consider all relevant elements when assessing a roof area.
    This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
    Be sure to consider all relevant elements when assessing a roof area.
     

    Conditions and assumptions for flat gutters:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions.
    2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
    3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

    Conditions and assumptions for downpipes:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions
    2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
    3. Gutters must have fall for downpipe sizing to be valid
    4. Calculations consider weir, orifice and friction effects
    5. Orifice discharge coefficient of 0.61 assumed
    6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
    7. Minimum pipe gradient of 20% assumed for friction conditions

    Conditions and assumptions for valleys:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions
    2. Minimum height of Type A valley returns to be 16 mm
    3. Minimum freeboard of 20mm mm for valleys below 8°
    4. Minimum freeboard of 15mm for valleys 8° and steeper

    Conditions and assumptions for maximum run:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions
    2. Only valid for supercritical flow (most roofs)

    Conditions and assumptions for penetrations:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions
    2. Only valid for supercritical flow (most roofs)
    3. Where Both Sides selected, assumes an even split of flow to either side of penetration

    Conditions and assumptions for level spreaders:

    1. Mannings n assumed to be 0.014 to represent long term friction conditions
    2. Only valid for supercritical flow (most roofs)
    3. Corrugate Profiles
      1. No discharge to lap row
      2. One discharge hole per second trough
      3. Assumes flow to top of profile (no freeboard)
    4. Trapezoidal or Trough Profiles
      1. May discharge to lap row
      2. One discharge hole per trough
      3. Assumes flow to capillary groove of profile