Which system is used depends on the design of the structure and profile selected. Spring curving can be used on wider radii, while pre-curving can generally be done down to a 300mm or 400mm radius.
Where a barrel curved roof is below the specified minimum roof pitch recommended for the profile, side laps should be sealed over the crest of the arch with lap tape or silicone sealant until the minimum pitch is reached. Avoid double lapping because condensation can become trapped in the lap, which can cause accelerated corrosion with all metal products.
If the width and height of the roof are known, this information can be used to obtain the radius of curvature and subsequently the sheet length and the length of seal required for any profile.
IMPORTANT NOTE: The seal length (s) is required on each side of the crest (b).
The Code of Practice Online provides an interactive tool for these calculations. This tool is only available online at 15.1.1A Spring Curving Calculator
N.B. This length of seal is required on each side of the crest.
| Profile | Pitch |
|---|---|
| Corrugate | 8° |
| Symmetrical Low Trapezoidal | 4° |
| Asymmetrical Low Trapezoidal | 3° |
| Secret-fix Tray | 3° |
End laps must not be placed in the region of the curve where the roof pitch is below the minimum pitch for the profile in 7.1.1A Minimum Pitch for Generic Metal Roofing.
Because of limitations in manufacturing pre-curved sheets, end laps are often required. Transverse laps should be sealed at both edges to prevent the ingress of both rainwater and condensation, in accordance with 14.12.1 Sealing End Laps.
Penetrations should be placed at the apex, or where the pitch is greater than the minimum for the profile.
Barrel curved roofs have no ridging to allow water vapour to exit at the ridge, so other means of venting the ceiling space should be considered. (See 10.10 Ventilation Pathways.)
Provision for expansion should be provided in the same manner as required for straight lengths, but the configuration of curved roofs means that some expansion will be taken up by a springing of the profile further up, which results in less movement. When the total sheet length is considered for expansion, positive fixing should be used at the crown, and provision for expansion accommodated on the slopes. Sealed and fastened end laps do not act as an expansion joint.
When curved roofs are unlined and used as canopies or exposed eaves, the underside of the sheeting becomes an unwashed area. Therefore, it needs to be washed and regularly maintained to comply with the durability requirements of the NZBC and the supplier's warranty. Because pre-painted cladding is not intended for use in this type of micro-climate without regular maintenance, the underside of the soffit should be lined in all severe and very severe environments. (see 16 Maintenance )
Spring curving, also known as draping or arching of roofs, is a method of providing continuous lengths of roof cladding over a curved roof structure without pre-curving the sheets. It is best suited to corrugated profiles, or symmetrical roofing profiles of low rib height and narrow pan width, which can follow a curve without excessive panning or distortion. Asymmetrical rib products have greater minimum radii, which are limited by their pan width and rib height.
Because symmetrical profiles do not have a large rain-water carrying capacity they are limited in maximum radius and length. Maximum radius is limited by the need to provide adequate drainage at the top of the curvature and minimum radius is limited by the need to avoid distortion without pre-forming.
Continuity over a minimum of three purlins is required for successful spring curving and therefore any interruption such as a penetration may affect the ability of the sheet to drape curve.
Purlins must be accurately positioned with the top faces tangential to the radius of the arch and should be within a 5 mm tolerance to avoid purlin creasing. Roof traffic should be restricted to avoid damage, particularly in the low pitched region or in highly visible areas, particularly in the low-pitched region or in highly visible areas. Some purlin creasing or canning is to be expected with stronger profiles at minimum radii.
For convex roofs, the minimum radii should be adhered to because the pans are in compression, whereas with concave roofs the pans are in tension and the panning or distortion of these roofs will be less.
The tables above for recommended radii assume the cladding is draped over an arc where the base chord is parallel to the ground. When the base chord is on an incline, the maximum radius can be increased.
The two top purlins should be spaced to enable the sheeting to follow an arc that minimises purlin marking.
Draped curved roofs or curved ridges should be fixed by fastening each sheet first to one side of the roof and then pulling it down to be fixed on the other side. Where sheets are end-lapped, alternate sheets should be laid in sequence to avoid cumulative errors and be laid from opposite sides of the roof to ensure squareness is maintained.
Because extra uplift load will be imposed on the end fasteners of convex spring curved roofs, through the torsional action of the sheets on the fasteners, screws and load spreading washers should be used on the penultimate and the last purlins. This torsional uplift must also be considered when designing purlin to rafter connections and other structures below.
Low tensile metals and G 300 coated steel can be formed into a number of simple and compound shapes. Generally, roll curving is applied to corrugated profiles, and crimp curving to trapezoidal profiles.
A curve can be rolled in the centre of a straight length of roof cladding to provide a ridge, but for ease of fitting and transport, a lap is often required at the first purlin down from the ridge, and an end lap is formed at that point. This should be sealed in the same manner as is required for any transverse lap.
As pre-curved sheets are typically formed from medium-strength (G300) material, the lower strength of this material must be considered when considering purlin spacings and roof traffic.
Fitting curved sheeting requires considerable care to ensure a satisfactory and aesthetically pleasing job. Setting out requires first checking that the materials delivered on site are within specified tolerances, and before commencing work the building should be checked for squareness.
Both roll curving and crimp curving will produce a curve with the effective cover dictated by the forming process. It is critical that the material is run to the correct width before being presented to the curving tool, or the effective cover of the straight portion will not match that of the curve. A variation of only 2mm between the straight and curved portion of a sheet will make it very difficult, or impossible, to install.
The curving process can cause dimensional changes, which can lead to misalignment, so the sheets should be kept square with the building. Some minor saw-toothing at the gutter end is to be expected when fitting curved sheeting. When multiple curves are required that cannot be provided on one sheet, the sheets should be fixed in the order shown below.
Roll curving is typically used for corrugated profiles. Pre-curved corrugated roof cladding is used for bull-nosed veranda roofs, ridges, or roofs where the radius is less than the minimum required for sprung or draped curved roofs.
The sheets are passed through offset curving rolls, which progressively form curves in a wide range of radii down to 300 mm. There is, however, a straight portion of about 80 mm at each end of the sheet which may need to be trimmed off if a true curve is required.
Circular barns have been successfully clad with 0.4 mm steel for many years, but 0.55 mm steel or 0.9 mm aluminium should be used for roll-curved roofs subject to foot traffic. As roll curving is normally done with G300 steel or H34 aluminium, the lower yield strength of these materials must be taken into account when setting purlin spacings.
If the edge of the sheet is too flat or long, and at tight radii, rippled edges may result, and these should be dressed out using a dressing tool, or trimmed off before the sheet is installed.
Crimp curving is applicable to all profiles, but it is most suited to trapezoidal profiles.
Crimp curving is produced by pressing a small crimp in either the tops of the ribs or the pans of the sheeting, progressively shortening it at these points and thereby causing it to bend. The radius can be altered by the spacing of the crimps and the angle of the bend by the number of crimps.
As crimp Curving is normally done with G300 steel or H34 aluminium, the lower yield strength of these materials must be taken into account when setting purlin spacings.
