COP v3.0:roofing;

External Moisture Roofing 

This section should be read in conjunction with 6 External Moisture Overview.

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

    7.2 Fastening Roofing 

    All fastenings that pierce the sheeting should be provided with adequate sealing washers to prevent leakage. Sealing washers should be made from Ethylene Propylene Diene Monomer (EPDM).

    Fastenings should be tightened only enough to form a weatherproof seal without damaging the sealing washer or deforming the sheet profile. Deformed sheeting will cause water to pond around the seal.

    Swarf should be removed from under the sealing washer as it will not only cause staining but also interfere with the seal.

    7.3 Thermal Expansion And Contraction 

    All metal cladding and flashings are subject to expansion and contraction caused by changes in temperature, and their design should allow for this movement. The energy produced should be absorbed without damage to the cladding, fixings or structure. The recommendations in this section are specific to preventing damage and leaks through thermal movement. Thermal movement can also cause disturbing noise levels in dwellings with shorter member lengths than those recommended in this section. (See 12.1 Roof Noise.)

    The ribs of metal trapezoidal or corrugated roof and wall cladding absorb expansion across the width of the sheets, but special provisions are needed over the sheets' length.

    Much of the longitudinal expansion is taken up by the bowing of the sheet between fastened supports. The extent to which this happens depends on the profile strength and support spacings.

    Failure by thermal expansion normally results in shearing of the fastener. Fasteners into lightweight steel purlins up to 3 mm in thickness are less vulnerable as they tend to rotate rather than be subjected to repeated bending resulting in fatigue failure. Fasteners into hot rolled steel sections or timber are far more vulnerable to this mode of failure and in all run lengths over 20 metres provision for expansion must be made when fastening into such supports.

    Where overlapping sheets are fastened through the ends, they must be considered as one length to calculate thermal movement. Unfastened end laps are not recommended.

    Wall cladding does not require the same provisions as roof cladding, because of solar radiation angle.

    Oversized holes and washers give some room for expansion and contraction, but it is not enough to allow movement without stress or distortion over long spans. In such cases, a step joint should be used. (See 8.4.4.3B Stepped Roof Flashing)

    7.3.1 Temperature Range 

     

    Ranges of temperature likely to be experienced in NZ by different steel cladding are:

    7.3.1A Steel Cladding Temperature Ranges

      Max/Min Roof Temp °C No Wind
    InsulatedLight colour+60° -15°=75°
    InsulatedDark colour+80° -15°=95°
    UninsulatedLight colour+50° -10°=60°
    UninsulatedDark colour+65° -10°=75°

    Aluminium and zinc, which have twice the expansion rate of steel, do not necessarily expand to this degree because of the different characteristics of mass, emittance, and radiance which affects their temperature range. Copper expands one and a half times as much as steel, and stainless steel can expand up to 1.5 times as much as steel depending on composition.

    The theoretical expansion of steel roof cladding in mm is 12 x temperature change x length in metres/1000.

    Steel expansion rates can be calculated as follows:

    Given a length (e.g., 30 m) and that the material (e.g., a light-coloured uninsulated roof) moves through a 60°C range (e.g., + 50°C -10°C), the theoretical increase in length is 12 x 60 x 30/1000 = 21.6 mm.

    This amount of movement of roof cladding and components does not have to be provided for in practice, because:

    • The building also expands with the ambient temperature, although to a lesser degree.
    • Fasteners into light gauge purlins will roll rather than bend. The purlin flange may also roll to a degree.
    • The roof cladding bows between purlins when it is constrained. Sighting down a corrugated steel roof on a warm sunny day will show an undulating line compared to a straight line when the roof is cool. The forces created by expansion and contraction are self-levelling, i.e., each component moves under load until the resisting force is more than the expansion force.
    • When a length of sheeting is solid fastened at the centre and unconstrained at either end, the movement is towards the ends of the sheeting; meaning the actual expansion or contraction movement is only half that of a full length of roof or wall cladding fastened at one end. Special design of the ridge or head barge flashing is required in these cases to allow free movement. Alternatively, sheets can be solidly fixed at the upper region, so all expansion takes place in the lower part of the sheet towards the eaves.

    7.3.2 Roof Cladding Expansion Provisions 

     

    The expansion of roof cladding depends on the materials, the constraints imposed by the fixing, the heat paths in the building and the actual temperature. The following graphics are indicative of favourable and unfavourable conditions for thermal expansion and suggest what these are. They show the lengths under both sets of conditions above which special provision needs to be made to accomodate thermal expansion.
     
     

     

    Notes:

    1. Where a roof requires oversize holes, only the portion of the roof outside of the recommendations require provision for expansion.  Normally this takes place towards the eaves
    2. These are guidelines only and special engineering of the roof, fixing or ventilation may allow greater spans to be used.
    3. These diagrams refer only to roof cladding screwed through the top. Secret or clip fixed roofs can move more freely if installed correctly and allow for using greater run lengths.
    4. The recommendations are based on preventing damage to the fasteners and are not a recommendation to prevent roof noise.

     

    7.4 End Laps 

    End laps should be avoided if possible when installing metal roof cladding as an incorrectly sealed end lap may entrap water and cause corrosion. When the sheets are too long to be transported or exceed the longest recommended length (see 7.3.2 Roof Cladding Expansion Provisions), the transverse or end lap joint can be avoided by using a waterfall step. (See 8.4.4.3A Step Apron Details)

    When long lengths outside the capacity of available transport are required, secret-fixed roof cladding can be supplied by using an onsite roll-forming machine.

    Where end laps are unavoidable, a sealed joint should be made using sealant at both ends of the lap. The upper seal is critical as condensation entering the upper side of the lap from underneath can cause rapid corrosion. (See 14.12.1 Sealing End Laps.) Rivets are used to fix the sheets together and should not be fastened to the purlin. The sheets are fixed to the purlin using screw fixings.

    The two lengths should be regarded as one length for expansion provisions.