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Disclaimer

Although the information contained in this Code has been obtained from sources believed to be reliable, New Zealand Metal Roofing Manufacturers Inc. makes no warranties or representations of any kind (express or implied) regarding the accuracy, adequacy, currency or completeness of the information, or that it is suitable for the intended use.

Compliance with this Code does not guarantee immunity from breach of any statutory requirements, the New Zealand Building Code or relevant Standards. The final responsibility for the correct design and specification rests with the designer and for its satisfactory execution with the contractor.

While most data have been compiled from case histories, trade experience and testing, small changes in the environment can produce marked differences in performance. The decision to use a particular material, and in what manner, is made at your own risk. The use of a particular material and method may, therefore, need to be modified to its intended end use and environment.

New Zealand Metal Roofing Manufacturers Inc., its directors, officers or employees shall not be responsible for any direct, indirect or special loss or damage arising from, as a consequence of, use of or reliance upon any information contained in this Code.

New Zealand Metal Roofing Manufacturers Inc. expressly disclaims any liability which is based on or arises out of the information or any errors, omissions or misstatements.

If reprinted, reproduced or used in any form, the New Zealand Metal Roofing Manufacturers Inc. (NZMRM) should be acknowledged as the source of information.

You should always refer to the current online Code of Practicefor the most recent updates on information contained in this Code.

Scope

This Code of Practice provides requirements, information and guidelines, to the Building Consent Authorities, the Building Certifier, Specifier, Designer, Licensed Building Practitioner, Trade Trainee, Installer and the end user on the design, installation, performance, and transportation of all metal roof and wall cladding used in New Zealand.

The calculations and the details contained in this Code of Practice provide a means of complying with the performance provisions of the NZBC and the requirements of the Health and Safety at Work Act 2015.

The scope of this document includes all buildings covered by NZS 3604, AS/NZS 1170 and those designed and built under specific engineering design.

It has been written and compiled from proven performance and cites a standard of acceptable practice agreed between manufacturers and roofing contractors.

The drawings and requirements contained in this Code illustrate acceptable trade practice, but recommended or better trade practice is also quoted as being a preferred alternative.

Because the environment and wind categories vary throughout New Zealand, acceptable trade practice must be altered accordingly; in severe environments and high wind design load categories, the requirements of the NZBC will only be met by using specific detailing as described in this Code.

The purpose of this Code of Practice is to present both Acceptable Trade Practice and Recommended Trade Practice, in a user-friendly format to ensure that the roof and wall cladding, flashings, drainage accessories, and fastenings will:

comply with the requirements of B1, B2, E1 E2 and E3 of the NZBC;
comply with the design loading requirements of AS/NZS 1170 and NZS 3604 and with AS/NZS 1562;
have and optimised lifespan; and
be weathertight.

COP v25.06:Roofing;

External Moisture Roofing

7 External Moisture Roofing

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

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 Pitch & Rise Calculator for the tabulation of these values and a calculation tool.

Minimum Roof Cladding Pitch

7.1.1 Minimum Roof Cladding Pitch

7.1.1A Minimum Pitch for Generic Metal Roofing

Profile	Min Height	Min Pitch	Check Capacity after run of...
Trapezoidal Asymmetrical	20	4°	20 m
	27	3°	40 m
	36	3°	65 m
Trapezoidal Symmetrical	20	4°	15 m
Trapezoidal Symmetrical	30	4°	25 m
Secret Fix	30	3°	60 m
Secret Fix	25	3°	30 m
Standing Seam	30	3°	85 m
Standing Seam	25	3°	35 m
Corrugate	16.5	8°	15 m
	21	4°	20 m
	35	3°	65 m

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 a good design of flashings can ensure weathertightness in extreme conditions.

Water buildup on vertical surfaces due to intense localised wind patterns, particularly around parapets and along the lower edges of walls, creates vulnerable areas. 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).
Reroofing trough section roofs at less 3°, where satisfies the requirements and considerations of Building work that does not require a building consent (Exemptions Guidance for Schedule 1 of the Building Act 2004) Fifth edition August 2020. More information is available at Building work that does not require a building consent, Section 1.1 General repair, maintenance and replacement of building parts (page22).

Runoff Capacity

7.1.2 Runoff Capacity

Runoff capacity 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.

Roof Drainage Capacity

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.

7.1.3A Trough Capacity

Trough Capacity_v1.3.

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

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

7.1.3B Penetration Gutters

PenetrationGutter_v1.0.svg

Water concentrated by penetration gutters.

Fastening Roofing

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.

Thermal Expansion And Contraction

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 8.5.5.3B Stepped Roof Flashing)

Temperature Range

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
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:

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.

Roof Cladding Expansion Provisions

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.

7.3.2A Favourable Circumstances for Controlling Expansion

Favourable Circumstances_v1.1

7.3.2B Unfavourable Circumstances for Controlling Expansion

Unfavorable Circumstances_v1.1

Unfavourable circumstances for controlling thermal expansion

Notes:

Roofs requiring oversized holes are solid fixed towards the ridge and provision for expansion is made towards the gutter line.
These are guidelines only and special engineering of the roof, fixing or ventilation may allow greater spans to be used.
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 greater run lengths.
The recommendations are based on preventing damage to the fasteners and are not recommendations to prevent roof noise.

End Laps

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 8.5.5.3A 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 14.11.1 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.

Wall Cladding End Laps

7.4.1 Wall Cladding End Laps

As with roofing, wall cladding sheets should, wherever possible, be laid in a single continuous length. Where this is not feasible due to excessive length or other constraints, there are options for lapping.

When the internal environment is dry and the wall is unlined, sheets can be simply end-lapped by approximately 150 mm. In dwellings, lined buildings, and buildings with moist internal environments, the laps should be sealed at both ends as per roof laps. The downside of end lapping is that there are four layers of material at the side lap/end lap junction. This often leads to an untidy looking join.

The preferred option where aesthetics must be considered is not to end-lap the sheets, but to have a zed flashing under the upper sheet and over the lower, with cover as shown in 8.3F Barge Flashing Cover — Trapezoidal and Secret Fixed Table and 8.3J Transverse Apron Flashing Cover Table. This is both tidier and allows the sheets to thermally expand individually.