In addition 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.
COP v24.12:Fitness-Purpose;
Roof noise refers to excessive intermittent sound emanating from the area of a roof surface or ceiling cavity.
Sound is usually created by movement and is carried by vibratory movement of the air. The movement that generates sound can be triggered by three main factors:
- Wind.
- Precipitation (rain, hail, or snow).
- Temperature (thermal).
Each of these can be broken down into sub-groups.
Roof noise triggered by the wind can take several forms:
- Underlay flutter,
- Banging of the roof itself, or things against the roof,
- Vibration of flashings.
12.1.1A Underlay Flutter
Underlay flutter will be perceived as a high-pitched noise. Depending on its origin, it can be minimised using various techniques, for example:
- If it is coming from the eaves, cutting the underlay back from the spouting and installing an eaves flashing.
- If it is coming from the body of the roof, installing a high front gutter.
- Installing spray-on foam insulation.
12.1.1B Wind Banging
Wind banging can be the pan drumming or banging against the sarking or purlin support. It normally only occurs with standing seam profiles and it is a function of the roof design, material thickness, and wind loads. These problems should be referred to the manufacturer or designer.
12.1.1C Flashing Vibration
Excessive movement or insufficient clearance between a penetration and the cladding may also cause noise.
Flashing vibration can be a whistling noise or a high-pitched sound like an engine, known as motor boating. The edges of all flashings should have a stiffening fold and the vertical face should be fixed to the structure or wall cladding to avoid vibration.
Most people find the sound of rain on the roof comforting, however, it may be intrusive.
The best way to attenuate precipitation noise is to increase the insulation levels in the ceiling space
Chip-coated metal tiles are likely to produce less precipitation noise than roll formed cladding. Fleece lined cladding has also been found to reduce precipitation noise.
Nearly all profiled metal roofs will exhibit thermal roof noise at times. Typically, this is a relatively minor and uncommon event, and attracts no attention. Sometimes, however, the frequency and/or volume of the noise can be to the point where it causes discomfort and distress to the occupants.
Thermal roof noise is caused by the roof expanding or contracting due to temperature fluctuation. The effect can be immediate as a cloud passes over the sun or delayed as the roof and roof structure cool down at night.
Not all materials in a roof structure absorb the same amount of heat, and not all materials have the same amount of thermally-induced movement. Differential movement may emanate from many places, including the layer of roofing exposed to the sun against layers underneath (i.e. a flashing or lap), the roof profile against the fastener, or the roof against the support structure. It can also happen when expansion of the roof causes one member of the support structure directly to move against another. Movement may also occur as sudden popping in the pan of a profile that releases static friction and causes sound.
The source of thermal roof noise can be difficult to identify in many cases. However, roof noise can be divided into three main categories by source. They are:
- Flashings,
- popping of pan, and
- other causes of thermal noise.
Flashings
Where flashings are solid fixed to a vertical surface and to the cladding, differential movement between them and the roof may cause noise. Using clip-fastened systems allowing flashings and roofing to move independently to each other will help minimise noise associated with this movement.
Pan Popping
Oil canning, or canning, is the term used to describe visible waviness of the pan of a metal roof. It is most noticeable in roofs with wide trays but may also occur in secret fixed or trapezoidal profiles.
Pan popping is accompanied by canning, but not all canning jobs result in thermal roof noise. Canning is most prevalent in tray profiles, but popping roof noise is most often found with trapezoidal or secret fixed roofs.
- Canning can be minimised by good rollformer tooling design and adjustment, it can be exacerbated by variations in mechanical properties and shape of the coil.
- Even steel coil well within standard tolerances can have characteristics that lead to high levels of canning in some profiles and tooling set-ups.
- Canning can also be induced by the building structure, a concave curve in the support structure will put the pan of the profile into compression and lead to canning.
To avoid canning problems, both the structure and the sheets should be inspected for straightness before installation.
Other Causes of Thermal Noise
Roof expansion noise can be caused by the energy released when the roof expands or shrinks relative to its support and sliding occurs at the fasteners, clips, purlins, or within the structure itself. The exact source of most roof noise is hard to identify.
A common observation of noisy roofs is poor contact between the purlin and the rafter due to insufficient pressure being put on the purlin when installing purlin screws, allowing the purlin to ride up on the thread. It may create a source of noise as purlins rotate under thermal expansion forces. Purlin screws should have a length of unthreaded shank below the head of at least the purlin thickness.
It is impossible to prevent expansion, but its effects can be minimised.
Thermal variation can be inhibited by using lighter colours or ventilating the roof space. Lowering roof temperatures minimises thermal noise. Increasing ventilation in the ceiling cavity is the easiest way to lower roof temperatures and may have the added benefit of lowering roof space humidity.
The accumulation of stress can be minimised by avoiding excessive run length. Friction can be reduced by using low friction underlay and using oversize fastener holes to allow materials to move freely. Fasteners should be checked for over-tightening and eased if necessary.
Many roof noise problems are typically associated with long, low dark skillion roofs, but some noisy roofs have none of those design features. To date, the MRM have not recorded any instances of thermal roof noise with roofs fastened to steel purlins or frames.
Thermal roof noise is a modern problem. Although often attributed to the roof material, steel and the profiles we form it into have not changed. In one case, a noisy trapezoidal roof was replaced with a corrugate roof manufactured from imported material, and the noise problem remained.
Changes that have occurred in the industry include:
- roofs are screwed on rather than nailed,
- roofs are longer and darker,
- purlins are screw fastened not nailed,
- houses are more airtight and far better insulated,
- trusses have largely replaced rafters, and
- framing timber is lighter and younger.
It is possible that a number of these aspects contribute in complex ways to produce varying thermal expansion noise levels, but we certainly don’t want to go back to nailed roofs and uninsulated houses.
The exact source of much roof noise is still a mystery. Auckland University is currently researching a scientific approach to identifying causes and solutions to roof noise.
Due to improvements in colour coating technology, the level of reflection and retention of gloss is higher. It will become much less obvious over time as gloss levels diminish and dirt accumulates on the roof.
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.
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
All trapezoidal and secret fix profiles will exhibit purlin creasing to some degree, the extent to which it is noticeable depends greatly on the line of sight and light conditions, which can change it from being immediately obvious, to almost invisible. Purlin creasing can be minimised by design by specifying 0.55 mm, rather than 0.40 mm material, or selecting a profile with a narrower pan. Minimising roof traffic on G300 tray roofing will also help, but the only way to ensure that purlin creasing will not be an issue is to lay a roof on solid sarking, or by using a corrugate profile.
As a rule, to minimise buckling, any dimension of a flashing should be a maximum of 300 mm between folds. Even so, transverse flashings such as ridges and aprons can exhibit some waviness due to the combined effects of thermal movement and timber shrinkage.
The only way to eliminate this is to fasten ridges on a hot day to dry timber. As this is seldom practical, light evenly spaced waviness in a ridge or transverse flashing is not considered a defect.
Being highly visible, waviness in barges and external corner flashings can be more problematic. In a situation where there is a low tolerance for waviness, flashings should be designed to keep the width of flat surfaces minimal and to be clip fastened on both edges to accommodate movement. Where possible, manufacturing from a heavier gauge material will also help minimise buckling.
Severe localised irreversible buckling of a flashing caused by compression wood is the result of a defect of the structure and should be rectified. See 8.9 Compression Timber.
Flashing Buckling
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.
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.
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.
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 E 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.
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.
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.
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.
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.
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.
Rainwater collected from roofs clad with steel and prepainted steel products will comply with the provisions of NZBC G 1 2.3.1, provided the water is not contaminated from other sources.
The first 25 mm of rainfall from a newly installed roof should be discarded before drinking water collection starts, and always disconnect downpipes when painting a roof. Spouting should be regularly cleaned to avoid the build-up of dirt and debris that can affect water quality.
Where a paint or paint system is applied to the roof, its suitability for the collection of drinking water should be established. When rainwater from pre-painted roof cladding is used for drinking, it is advisable to repaint the roof as soon as its surface has weathered.
Water collected from metal roof cladding, spouting or gutters made from aluminium, copper and stainless steel will not normally be contaminated by rainfall in suburban and rural areas. However, fallout from manufacturing plants, top dressing, and the contamination resulting from roof cleaning can affect the water quality, and in these cases downpipes should be disconnected.