COP:other-products; curved-roofs

14.1 Curved Roofs 


There are two main methods to clad curved buildings.
  1. Draped sheets, known as spring curving.
  2. Pre-curved sheets, either roll-curved or crimp curved.

For compliance with the requirements of the NZBC, designers should abide by the limitations of profiled metal cladding for curved roofs.

The curving process or crimping does not produce any strength enhancement for point or wind load. Curved roofs usually have maximum purlin spacings to avoid distortion.

Designers and contractors should be aware that light gauges such as 0.40 mm steel and 0.70 mm aluminium are likely to show distortion when used for curving.When asymmetrical-pan trapezoidal cladding is used for curved roofs and appearance is paramount, a heavier gauge cladding should be specified.

They are 'Restricted Access' roofs, which means that walking traffic should be restricted to within 300 mm of the purlins, and in the pan or over two ribs if they are adjacent to the vertical lap. Because of the changing pitch, edge protection must be provided, or a safety harness used when installing curved roofs. See 12.1 Safety.

All side laps of curved sheets below the minimum pitch for the profile must be mechanically fastened and sealed.
Curved flashings are described in 7.4.6 Curved Flashings


14.1.1 Spring Curving 

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 from eave to eave without a ridging. It is suited to symmetrical roofing profiles of low rib height, which can follow a curve without excessive panning or distortion.

Because these profiles do not have a large rain- water carrying capacity they are limited in radius and length.

Maximum radius is limited to provide adequate drainage at the top of the curvature and minimum radius is limited to avoid distortion without pre-forming.

Asymmetrical and tray roof cladding can be draped , but only to a large radius before panning or distortion occurs and they are , therefore , unsuitable for all except large radii. They do not have the same restrictions on rain- water carrying capacity as symmetrical claddings. Because corrugate cannot be satisfactorily turned down into a gutter, wind pressure can drive rain up the corrugations, causing 'blow back' and allowing water ingress. Spring curved corrugate should not terminate below 8°.


Roof cladding must not terminate at a lower pitch than that permitted for the profile , unless the designer can demonstrate compliance with the NZBC by detailing an alternative method of weathering and durability.

All trapezoidal and tray roof cladding below 8° must have the pan turned down into the gutter.

All roof cladding at all pitches must have either a pull-up or a dog-eared stopend.


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.



Only G550 MPa grade (HS) coated steel is recommended for drape curving.

Tables 4.9.1.A & B 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.

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.


14.1.1B Spring Curving Calculator

Width = w=w
Height = h=h
Radius of curvature = r=r
Minimum pitch = p=p°
Sheet length = l=l
Length of seal = s=s

Enter width and height to calculate:
Width and Height not valid - please re-enter 

Full Calculation Details and Example
Start with : w = Width of roof
=  12
Start with : h = Height of roof
=  5
To find r the radius of curvature
r  =  
4h²+ w² 8h
(4 x 25) + 400) 40
=  12.5
To find l the sheet length
Find the length y
y = r - h
=  12.5 - 5
=  12.5
Find the length x
x  =  
w 2
20 2
=  10
To find the tangent of angle A
tan A  =  
x y
10 7.5
=  1.33
To find angle A
A  =  aTan(
x y
=  aTan(1.33)
=  53°
Find the arc length c b
c b  =  
2 π r A° 360
2 x 3.1412 x 12.5 x 53 360
=  11.56
Find the sheet length l
l  =  cb x 2
=  cb 23.12 + 100mm
 =  23.12
To find the length of seal
p = Min Pitch for corrugate = 8°
s  =  r x (tan 8°)
=  12.5 x 0.1405
=  1.76


N.B. This length of seal is required on each side of the crest.
It is recommended that all profiles be sealed to 8°.

If the sheets are lapped laterally they must be sealed.

14.1.2 Laps 

By definition, a curved roof is flat at the crest of a curve, and because it is below the specified minimum roof pitch required by the NZBC for unsealed laps, side laps should be sealed over the crest of the arch until the minimum pitch is reached.

All vertical laps should be sealed if the pitch is less than the allowable minimum as tabulated below:

14.1.2A Curved Roof: Sealed Lap Pitch

Minimum pitch below which vertical laps should be sealed
Symmetrical Low Trapezoidal
Asymmetrical Low Trapezoidal
Secret-fix Tray


When the pitch of the roof is below the minimum, the side lap is required to be sealed over the crown, and lap tape or silicone sealant should be placed on top of the rib and firmly held down while fixing takes place. Intermediate side stitching is required at the midpoint of all side laps using self-sealing rivets or stitching screws.
The side lap of profiled sheeting is designed with anti-capillary provisions to be self-draining.
Before the continuous manufacture of corrugate from coil, symmetrical corrugate sheets were often laid with two nesting laps, which commonly corroded due to condensation, even when the laps were primed. All metal profiles now produced in NZ have capillary grooves. Trapezoidal profiles are designed for one lap only and corrugate used for roofing is designed for 11/2 laps with an under and an over.
Avoid double lapping because condensation can become trapped in the lap, which can cause accelerated corrosion with all steel products, including pre-coated steel. Lap priming should not be used as the permeable paint surface can retain moisture and accelerate corrosion.

14.1.3 Transverse Laps 

To avoid a transverse lap, or if the sheet is longer than can be transported or safely handled, a step in the roof structure should be provided. See Step Apron.

At a step or a lap, the end span must be reduced. See 3.5.1 Point Load.
If a transverse lap cannot be avoided, it must be mechanically fastened and sealed and must be made watertight from the inside by lap tape or sealant.

The sealant should ensure that the condensation flows past the joint and either be absorbed by the underlay or drain to the eave.

Severe corrosion problems have been caused on curved roofs by condensation running down the inside of roof cladding and into the laps. This was a common mode of failure when short lengths of galvanized corrugated sheeting were used in the past, but long run roofing without end laps has significantly reduced this type of failure.
Do not assume that the paint coating would provide barrier protection. The manufacturers' and industry requirement, since 1995, is to seal all transverse laps.
The time of wetness, which is a major factor of corrosion, is increased when unsealed metallic coated steel cladding and flashing laps are subjected to a continuously damp environment. This situation is also detrimental to pre-painted metal cladding, which are attacked through the permeable paint coatings and at cut edges.

Where a draped roof is regarded as too long to transport or too difficult to handle as a drape curve in one sheet, the crown sheet should be as long as practical and the transverse lap should be placed as far down the roof as possible to increase the pitch at this point.

At the termination of curved sheets at minimum pitches in exposed areas, additional weathering is required at the turn down. Ventilated filler blocks and/or baffles should be used to prevent blowback, which can cause corrosion because the underside of the sheeting becomes an unwashed area.

Penetrations or 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 6.5.1A Minimum Recommended Pitch.
Additional timber or steel supporting structure must be installed upside and downside of any penetration hole greater than 300 mm x 300 mm to provide fixing for the sheet and a reduction of the end spans.
Support must be provided to resist the uplift on sprung curved sheets at all penetrations.
All side laps of curved sheets below the minimum pitch for the profile must be mechanically fastened and sealed.

Continuity over a minimum of three purlins is required for successful drape curving and therefore any interruption, such as a penetration or other cutting of the sheet, may require machine curving to ensure the curvature is maintained.

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 pitch region or in highly visible areas. Damage as a result of walking traffic can be seen as creasing at purlin lines or canning in the profile pans.
Some purlin creasing is to be expected with stronger profiles, and at low pitches this can cause corrosion due to ponding. 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 , although it depends on the profile.



Only vented profiled filler blocks should be used at the eave on curved roofs so that some air movement is provided within the ribs. See 9.4 Ventilation.
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 using oversize holes, should be made at the crown.

14.1.4 Durability 

When draped curve 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. The underside of pre-coated roof cladding is provided with a primer and backer coat only; it is not as weatherproof or UV resistant as the top-coat.

Because pre-painted cladding is not intended for use in this micro-climate without regular maintenance, the underside of the soffit should be lined in all severe and very severe environments.
(see 15 Maintenance )

14.1.5 Purlin Spacing 

When the purlin spacing is close to the maximum allowable for the profile and ease of curvature, the roof cladding is more likely to be damaged by foot traffic and distortion between the purlins. When the radius of curvature is close to the minimum, the purlin spacing should be reduced to the end span distance for each gauge and profile. See 3.14.5 Wind Load Span Graphs.

Access on curved roofs should be restricted and be regarded as Type B, and extra care should be taken during installation because of the changing pitch. Because some profiles used for curved roofs are close ribbed, it is not possible to walk in the pan. The walking pattern should be restricted to within 300 mm of the purlin and the load spread over two ribs. This is more important when low strength steel is used for pre-curved sheets.

Avoid using 0.4 mm G300 steel or 0.7 mm aluminium for roof cladding subject ed to walking traffic.

The designer should consider the radius of curvature, profile, thickness, grade, and purlin spacing as these are all related parameters of curved roof design.

Maximum purlin spacings should be adhered to and any sheets damaged by foot traffic in the area below the minimum pitch for the profile should be replaced.

All curved roofs must have end spans reduced to two-thirds of the intermediate span, as required for straight roofs because the kl load - factor requires a reduction in purlin spacing at the roof edges. Where translucent sheets are required to be curved, the normal purlin spacings should also be reduced.

If the design loads are high, or where the eave is not lined and the roof cladding is exposed, extra fixings and load spreading washers are required.

It is important that the radii limitations and water drainage characteristics for specific products are considered at the building design stage, so that water runoff over the low pitch region will not exceed the maximum for the profile used. The maximum radius of curvature permissible for corrugate and symmetrical profiles is limited for this reason.

Bull-nosed verandah or lean-to roofs, which are simply supported spans and do not have the continuity required for point load, should have their purlin spacings reduced to less than normal end spans.

Because the sheeting is continuous over the top of a curved roof and the wind dynamics are different, purlin spacings do not need to be reduced at the crest, as is normally required at the ridge on gable or hipped roofs.

The two top purlins should be placed to enable the sheeting to follow an arc that minimises purlin marking.

Draped curved roof or curved ridges should be fixed by fastening each sheet first to one side of the roof and then pull it down to be fixed on the other side. 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. Shift the two top ridge purlins to provide an even radius.


Because extra uplift load will be taken by the end fasteners, screws and load spreading washers should be used on the penultimate and the last purlins and screws are the preferred fastener for curved roofs, although nails may be used on intermediate purlins.

Rafters and purlins must have additional fixing to the structure to resist the additional uplift load at the eave caused by curved sheeting.
Any assumed increase in span due to an increase in strength of the roof curve should be discounted, as concave and convex draped curved roofs are limited to the maximum purlin spacing allowable for the particular profile, and are dependent on the wind design load.



14.1.6 Concave Roofs 

Roofing can be spring-curved into concave shapes however designers should be aware of the limitations on the minimum pitch where the curve is terminated, the extra uplift load that will be taken by the fasteners at the centre of the curve, and take into account the catchment area of the roof.

The pitch for concave roofs must not be less than 8° for corrugated profiles, 4° for symmetrical trapezoidal profiles, and 3° for other profiles. Screws and load spreading washers must be used for fixing cladding on all sprung concave curved roofs. The purlins must have additional fixing to the structure to resist the extra uplift load on sprung curved sheets.

The additional load produced by draping concave and convex metal roof cladding depends on the radius of curvature and the thickness of the metal. The induced load has two forces:

  • additional load on the fastener; and
  • additional load on the purlin/rafter or truss connection.

Although the former is the responsibility of the roofer, the COP recommends that the purlin connection is inspected for adequacy. The connection prior to any additional load imposed by the draped roof will be determined by table 3.6. An economical solution to the increased connection load is to use a proprietary purlin strap.





14.1.7 Pre-Curved Roofs 

Low tensile metals and G 300 coated steel can be easily roll-curved in a pyramid rolling machine to small radii and can also be crimp curved into these shapes. See 14.1.9 Crimp Curving.





















14.1.8 Roll Curving 

Pre-curved corrugated roof cladding is used for bull-nosed verandah roofs, ridges, or for roofs where the radius is less than the minimum required for sprung or draped curved roofs.
Corrugated (symmetrical sinusoidal) G300 roof cladding is easily curved or bull-nosed. The sheets are passed through matching curving rolls, which progressively form curves in a wide range of radii. If G300 and G550 steel sheets are to be used together, because these two materials will not have matching profiles, adjustment of the roll-forming machine setting is necessary.

Circular barns have been successfully cladded 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. G300 coated steel of 0.4 mm and 0.7 mm aluminium are only suitable for roofs without access or for wall cladding.

G300 steel can be curved to a radius as small as 300 mm using pyramid curving rolls. There is, however, a straight portion of about 80 mm at the end of the sheet which may need to be trimmed off if a true curve is required.
If the edge of the sheet is too flat or long, rippled edges may result, and these should be dressed out using a dressing tool or trimmed off before the sheet is installed.


A curve can be rolled on one end of a straight length of roof cladding to provide an over or cranked ridge, but for ease of fitting and transport, a lap is usually made at the first purlin down from the ridge. This should be sealed in the same manner as is required for any transverse lap.
An alternative ridge detail can be used with straight sheets, without any lap, by roll curving or draping the cladding over the ridge, where the ridge purlins are extended their maximum span.
For safety, roofs which are often used as a means of access to or onto a verandah should be provided with an intermediate support. Simply supported roofs cannot withstand foot traffic to the same degree as continuously supported sheeting.

14.1.9 Crimp Curving 


Crimp curving is applicable to all profiles, but it is most suited to asymmetrical sections that cannot be rolled or drape curved.

Crimp curving is produced by pressing a small crimp in either the tops of the ribs or the pans of the sheeting. The profile is progressively shortened at these points causing it to bend. The radius can be altered by the spacing of and the number of crimps.

Some machines are capable of forming high-strength steel by a combination of compression and tension in the die design, and some machines require the use of strippable film as a lubricant to avoid coating damage. Where sheets are to be end lapped and different strength materials are used together, machine adjustment is required to ensure an acceptable fit because their profiles are not usually consistent.

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.

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 in 14.1.9A Fixing Order: Curved Sheets.



All transverse laps of crimped curved roof cladding must be mechanically fixed and sealed.

Some paint checking and microcracking is likely to occur at the crimps on metallic coated steel cladding and these may show a white bloom. This is more readily seen in unwashed areas, such as when crimp curved sheets are unlined as a canopy or over a walkway roof. This area is required to be washed regularly under the maintenance provisions of the supplier's warranty.

The underside of colour coated roof cladding is provided with a primer and backer coat only and if this is exposed in an unwashed area and can be seen, it should be post-painted with two coats of Acrylic paint. These areas are subject to maintenance as an unwashed area. (see over-painting section 13.7) Because the top of crimped sheeting is also subject to the collection of dirt and debris, particularly at the low pitched area, it is subject to maintenance requirements.

All side laps of crimped curved sheets below the minimum pitch for the profile must be mechanically fixed and sealed.

14.1.10 Timber Fixing 

When attached to timber purlins, the longitudinal wires of the safety mesh must be either bent down and fixed to the sides of the purlins or fixed to the tops of the rafters by 40 mm galvanised steel staples with a 3.15 mm diameter and spaced at 150 mm.

Staples must be driven so that a cross-wire is between the end of the wire and the staple, or the end of the wire is bent back and twisted four times around the same wire so that individual wires cannot be drawn from a staple.


The longitudinal wires must be fixed to the purlins or rafters by galvanized steel wire loops of not less than 3.15 mm diameter. Place the centre of the tying wire around the longitudinal wire at an intersection, so that a transverse wire is between that point and the end of the longitudinal wire.

The tying wire must be passed once completely around the rafter, and then drawing the two tails of the tying wire in opposite directions over the two strands of the tying wire and twisting together with at least three complete turns.



When joining rolls or sections, the two transverse wires must be placed together and the longitudinal tail wires must be twisted around each other. One longitudinal tail wire must be twisted four times around the main portion of the same wire. The other longitudinal tail wire must be twisted once around the main portion of the same wire and then four times around the two transverse wires