The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
This section should be read in conjunction with 6 External Moisture Overview.
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.
| Profile | Rib Height | Minimum Pitch | Rise per metre of Span |
|---|---|---|---|
| Trapezoidal asymmetrical | 20 – 35 mm | 3° | 52 mm |
| Trapezoidal asymmetrical and symmetrical | 36 – 60 mm | 3° | 52 mm |
| Trapezoidal symmetrical | 20 – 35 mm | 4° | 70 mm |
| Secret-fix | >30 mm | 3° | 52 mm |
| secret-fix | <30 mm | 8° | 141 mm |
| Standing seam fully supported flat sheet metal | >30 mm | 3° | 52 mm |
| All other types of fully supported flat sheet metal | 5° | 87 | |
| Corrugated and other profiled sheeting | 16.5 – 20 mm | 8° | 141 mm |
| Corrugated and other profiled sheeting | 21 – 35 mm | 4° | 70 mm |
| Horizontally lapped metal tile | 25 mm upstand | 12° | 213 mm |
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.
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.
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.
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.
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.
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)
| Max/Min Roof Temp °C | No Wind | |||
| Insulated | Light colour | +60° -15° | = | 75° |
| Insulated | Dark colour | +80° -15° | = | 95° |
| Uninsulated | Light colour | +50° -10° | = | 60° |
| Uninsulated | Dark 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:
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.
