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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.

Recommended Radius for Concave and Convex Curves in Symmetrical Profiles

 

 

 

Recommended Radius for Convex and Concave Curves in Asymmetrical Profiles

 

 

 

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

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.

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.

 

Spring Curving Calculator

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

The Code of Practice Online provides an interactive tool for these calculations. This tool is only available online at www.metalroofing.org.nz/cop/other-products/curved-roofs#spring-curving-calculator


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



 
Full Calculation Details and Example
 
Formula
Example
 
Start with : w = Width of roof
w  
=  12
 
Start with : h = Height of roof
h  
=  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.

/cop/other-products/curved-roofs#spring-curving
Revision Category: 
1 - Minor Errata
Revision Detail: 

Updated reference to the tables in this clause.

Draft Clause: 
014_001_001_000_000_000_000_000_000