COP v3.0:fitness-purpose;

12 Fitness For Purpose 

In addtion to Corrosion (NZBC: B2 – Durability), other issues which may affect the lifespan or perceived quality of metal roof and wall cladding, include:

  • Oil Canning.
  • Purlin Creasing.
  • Colour Differential.

12.1 Roof Noise 

It is impossible to prevent expansion, but it can be controlled by reducing the surface heat of the roof cladding by using lighter colours and ventilating the roof space.

Transverse expansion is accommodated by the concertina action of corrugation or rib of metal cladding and does not usually give rise to any noise. However, because flashings are stiffened at 90° to the cladding, there is differential movement between them, which requires expansion provisions for fastening, and to minimise noise associated with this movement.

All materials expand or contract with changes in temperature, but those with a greater mass usually move less or more slowly than thin sheet materials. Metals expand more than other building materials, except plastics which can expand more than steel. Green or wet timber contracts on drying, producing shrinkage but it also expands or contracts with temperature fluctuations.

 

12.1A Poor Purlin to Rafter Connection

Poor purlin to rafter connection-tightness often causes noisy roofs.

 

Roof expansion noise can be caused by the energy released when the roof expands relative to its support and sliding occurs at the purlins, clips, or fasteners. Friction between the roof cladding and its support controls the sliding; surfaces with a lower coefficient of friction, e.g., metal to metal, would slide more easily than metal to timber.

When the friction is exceeded, and the metal roof cladding moves, it creates noise. Further temperature increase will cause a stress build-up, until the limiting static friction point is reached again, and the cycle repeats. During each cycle the thermal energy is released impulsively, and the higher the friction the louder the noise.

The noise can be reduced if expansion can take place uniformly by using sliding fixings, or interposing low friction material (e.g., PVC noise tape) between the roof and its support.

Where the roof is rigidly fixed, the purlins will likely tend to rotate, and this can also produce noise.

When using long length roof cladding, oversize holes or other suitable expansion fixings are essential to avoid noise.

The many other factors that determine if a roof will produce undue noise, include:

12.1B Roof Noise — Fixing Issues

  • Secret-fixed roofs, where the clip fits too tightly over the rib or is misaligned.
  • Over-nailed roof cladding, i.e., too many fasteners.
  • Over-tightened roof cladding, i.e., nailed or screwed too tightly. The 'ticking' or creaking noise heard when the sun goes behind a cloud is usually caused by metal against metal, or at the fastener hole.
  • Crest fixing produces more noise than pan fixing, because of the movement of the fastener at the shank hole.
  • Noise can be caused by inadequate timber nailing, causing differential movement at timber joints.

12.1C Roof Noise — Structural Issues

  • Gutters and valleys should be free to expand and move independently of the cladding and not have "wings", which preclude any movement.
  • Rigid framing and closely spaced purlins cause more noise than a flexible structure, e.g., steel portal frame construction is more flexible than laminated timber.
  • Roofs which are free to expand should be kept clear of concrete walls and other structures.
  • Specific problems are often due to structural detailing which requires special provision, e.g., where solid timber construction and a dark coloured cladding are combined.
  • Flashings should be limited to 12 m in length. Otherwise, noise is likely as transverse flashings expand to a greater extent than the roof cladding that they are attached to. Slip joints should be used in sheets longer than 12 m.
  • The edges of all flashings should be formed as shown in 8.3.2 Flashing Edges to avoid 'whistling', or a wind created noise known as "motor-boating"—a fast vibrating sound like the noise of an engine.
  • Flashings should be 0.55 mm steel or 0.9 mm aluminium and no wider than 300 mm.
  • Flashings should not touch the pan of roof cladding.
  • Insufficient clearance between a penetration and the cladding may cause noise.

12.1D Roof Noise — Material Issues

  • Using metals with a high rate of expansion, e.g., aluminium.
  • Dark coloured and unpainted, weathered metallic coated roof cladding absorbs more heat than light coloured claddings.
  • Impulsive energy release can give rise to 'pistol shot' noises that are very disconcerting to live with, but a dark coloured roof may only be a contributing factor, and often the cause may be a strong and rigid timber frame. Solid timber framing is well known for such noise.
  • Shrinkage associated with drying timber with a high moisture content.
  • Underlay that overlaps too far into a spouting or gutter can give rise to a noise known as 'flutter'.

12.1E Roof Noise — Ventilation Issues

  • When insulation is placed hard up to metal cladding, more heat is retained, and the metal surface temperature becomes higher than an uninsulated roof.
  • An attic space with insufficient ventilation increases the temperature within the roof cavity.
  • Roof cladding in an exposed position loses its absorbed heat more quickly than one that is in a sheltered valley.
  • Skillion roofs and curved roofs without provision for ventilation are subject to greater fluctuations of temperature than roof attic construction.

12.2 Purlin Creasing 

Due to improvements in colour coating technology, the level of reflection of new pre-painted roof sheeting is now considered to be higher. Overdriven nails or screws can produce visible distortion on the purlin line in the pan of trapezoidal profiles that cannot be easily remedied.

 

 

Trapezoidal profiles with a wide pan manufactured from 0.4 mm steel and 0.7 mm aluminium are particularly susceptible to purlin creasing, and although it does not affect performance, their appearance can be aesthetically unacceptable

It is the responsibility of the roofing contractor to ensure that nails are not overdriven. A nail or screw should only be driven into the purlin to produce a 50% compression of the sealing washer or until the roof is firm. Using too many fasteners should also be avoided, and nails should always be fixed at right angles to the roof.

Before fixing the roof cladding, the contractor should check the alignment of the purlins or girts. Purlins should be aligned within 5 mm tolerance of each other to avoid purlin creasing.

Purlins should be accurately positioned with their top face parallel to the rafter and should be fixed to a straight line.

When appearance is important or where wide pan trapezoidal cladding is close to eye level, heavier gauge cladding should be specified because light gauges such as 0.4 mm steel and 0.7 mm aluminium are likely to show distortion. Purlin creasing will happen on both concave and convex curved roofs if the recommended purlin spacings are exceeded, and great care should be taken to align purlins on such roofs.

Purlin creasing can be exacerbated by roof traffic. 14.6 Walking On Roofs

 

12.3 Oil Canning 

Distortion of flat metal areas is an aesthetic problem associated with the manufacture of metal roof and wall cladding and flashings. Flat pan architectural metal panels, wide flashings, and profiled metal cladding with wide pan configurations without stiffening ribs are all liable to show distortion in flat metal areas. It is known as oil-canning or panning.

Oil canning can be defined as visible waviness in the flat areas of metal roofing and wall cladding. It can also be referred to as panning, canning, stress wrinkling or elastic buckling, and is caused by differential stresses in the metal. As the metal tries to relieve these stresses in panels with high width to thickness ratios, material buckles out of plane producing the characteristic waviness of oil canning

It has an aesthetic effect and is not a structural or durability issue. Some highly reflective paint finishes and metals or different light conditions can exacerbate the visual effect of oil canning. Some distortion is inevitable in light gauges. It can become an issue of customer acceptance because customer expectations are often unrealistically high.

The degree of waviness can be hard to measure and is highly dependent on viewing angles, the position of the sun, and the reflectivity of the surface. Cladding installations with a high degree of visibility should be designed to minimise oil canning.

Oil canning is more common where the width of unformed sections is large. It can usually be avoided or minimised in normal rib and trough section profiles with a maximum pan width of 300 mm, and flashings that have a maximum unformed width of 300 mm. See 8.1 Flashing Materials

In standing seam roofs with pan widths of more than 300 mm, some oil canning is normally evident. Many designers regard oil canning in such profiles as inherent to the material and treat it as a desired effect accentuating the material's natural characteristics.

Manufacturers and installers should minimise unintentional non-flat conditions, and any visual waviness should be relatively even and regular.

There are various causes for oil canning:

  • material;
  • roll tool design and setting;
  • installation; and
  • expansion allowance.

 

 

12.3.1 Material 

All profiled metal roof and wall products begin in a coil form. Stresses induced during coil production can contribute to oil canning. Examples of these stresses are:

  • Full Centre: The coil is longer in the centre of the strip than near the edges. This creates buckles and ripples in the mid-coil area.
  • Wavy Edge: The coil is longer on one edge of the strip. That causes waviness on the long edge.
  • Camber: The side edge of the coil deviates from a straight line. The normal tolerance for a 1200 mm wide coil is a 2 mm deviation in a 2 m length, but some forming processes and end uses cannot tolerate that variation.
  • Uneven Material Strength: During the forming process material may tend to draw unevenly from the softer areas rather than evenly as designed; it leaves excessive material in the “harder” areas.
  • Slitting: Generally, coil for flashings and narrower products are cut by slitting from a single, wider master coil. Slitting of a master coil can release and redistribute residual forces. It can also mean that different qualities of the master coil are modified or changed in the slit coil, i.e., a full centre in a master coil can become a wavy edge in a slit coil and the slit coil may not retain all the attributes of the master coil or sister coils.

 

12.3.2 Tool Design 

By the nature of the process, many stresses are created during roll forming. These must be minimised and equalised as much as possible during manufacturing. Forming tools must be designed to form the material progressively.

Corrugated and ribbed profiles are most often formed from the centre and moved outward thereby “pushing” the differential stresses to the edges of the sheet. Generally, profiled metal rib and corrugated profiles, flashings, and most trough sections can be expected to provide finishes free of avoidable distortion.

Standing seam profiles typically need more forming on the edges of the feed material and little or none in the centre of the sheet, which tends to trap uneven stresses in the centre of the profile. Often one edge requires more forming than the other, meaning the stresses developed are not even in the sheet.

Some evenly distributed oil canning can normally be expected in standing seam products with a width of more than 300 mm, and it is considered acceptable.

12.3.3 Installation 

 

Oil canning can occur in fixed cladding, even though it does not fit accurately, when fixings are too far apart or when fixings are overdriven. It can also result from an uneven substrate, irregular bearing on the purlins or by the structural framing being out of line.

Thermal expansion can also increase oil canning. Longitudinal expansion should be accommodated by using sliding clips allowing movement. See 15.4.4.3E Expansion Clips. Transverse expansion is usually accommodated in the upstand of the profile, but this can only happen if adjacent pans are not in contact at the base. Wide perimeter flashings must be designed to allow for independent movement of the flashing and the cladding.

A convex curve in the roof structure can cause canning as it puts the pan of the profile under compression. Sometimes this curve is inadvertent. Concave roof cladding and flashings give rise to oil-canning because the pans are in compression. There are limitations on curved radii to avoid oil canning. See 15.1 Curved Roofs.

Commercially designed truss sections and rafters may have camber induced in their manufacture, anticipating deflection under load. The degree of curve that may be accommodated by any profile is largely determined by the width of the pan and is, also, affected by the material thickness and grade.

12.3.4 Minimising Oil Canning 

Good design and installation can minimise oil canning.

Materials:

  • Use thicker material.
  • Use low gloss paints or embossed surfaces.
  • Use natural weathering materials that dull over time.

Flashings:

  • Limit flashing width to 300 mm.
  • Limit the joined length of fixed flashings to 12 m.
  • Attach wide flashings with brackets that allow independent thermal expansion.
  • Manufacture a stiffening swage into flashings that have a face width greater than 200 mm.
  • Do not fix flashings to timber with a moisture content greater than 18%.

Cladding:

  • Limit cladding length.
  • Ensure the purlin alignment avoids convex curving.
  • Inspect for flatness before installation.
  • Avoid thin materials.

12.4 Colour Differential 

It is both the cladding manufacturer and the roof cladding contractor's responsibility to ensure that the same brands of pre-painted material are used on the same building.
Failure to do so could result in differences in colour, gloss and weathering, which quickly becomes obvious.
The differences come from different paint formulations and do not necessarily indicate that the materials will perform differently in service. All New Zealand manufacturers provide information about the manufacturer, the type of coating and the manufacturing date in the branding on the reverse side of uncoated and colour coated steel. Double-sided coatings are not branded.

12.4.1 Touch-Up 

Air-drying touch-up paints have different weathering characteristics to the baked-on finish of pre-painted coating systems and variations in natural light conditions will emphasise these differences, producing an unacceptable aesthetic appearance.

Spray cans should not be used for repairing scratches on pre-painted sheeting.

If the scratch is obvious from 3 m, the sheeting should be replaced, if not then it should be left alone. Minor surface scratches become less noticeable as the coating weathers and are best left as they do not appreciably affect the corrosion inhibiting properties of the material.

Widespread damage caused by rough handling or an accident, however, should not be corrected by repainting, but the affected material should be replaced.

12.5 Transport, Handling And Storage of rooflights 

All roof lights should be handled and stacked with care as film surfaces are easily scratched, and heavy stacks can damage lower sheets.
All roof lights should be stored flat, the right way up, on 75 mm battens not more than 1.2 m apart. Stacks should not be higher than 1 m and should be covered and protected from rain and sun. Thermoplastic sheets can overheat and deform in a stack, and exposed stacks can permanently discolour due to the effect of sun and water.

12.5.1 Maintenance 

First maintenance after 12 months requires cleaning any grime or debris using warm water and a stiff bristled brush. Every 2 - 3 years rooflights should be inspected for damage, the condition of the flashings, and sealants and the fixings should be checked for tightness.
Roof lights must not be painted over as this renders them hazardous to maintenance workers.

Because painted roof lights appear no different in place than metal sheets, this practice can be dangerous for any workers carrying out maintenance work on the roof. Painting can also cause heat distortion which can lead to premature failure.

As a warning, primary and secondary fasteners can be brightly coloured, providing a contrast with the remainder of the roof cladding surrounding the roof light areas. The area can also be marked around with a distinctly painted stripe.

Lichen will accumulate on plastic roof lighting wherever there is a source of nutrients, but it should be removed with care. See 16.7.1 Lichen And Mould.