The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
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.
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:
- 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.
- 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 8.4 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.
- 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'.
- 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.