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.
COP v3.0:installation; pre-empting-problems
Ponding will create a prolonged time of wetness, increased build-up of debris. Ponding will detract from a coated steel product’s life and will invalidate the product warranty.
The installation of penetrations must be done in such a way that they do not cause ponding. See External Moisture: Penetrations.
On low pitched roofs, careless or excessive foot traffic may cause rib damage and localised ponding. This can be minimised by installing temporary protection in critical areas, such as entry points, where the roof is accessed by other trades, or there is a step-down in the roof. It is the main contractor’s responsibility to ensure that other trades do not damage the roof.
It is recommendable using 0.55 mm minimum thickness material at purlin spacing guidelines for Heavy Traffic if roofs are highly visible or need to be accessed by maintenance personnel.
Gutters must be installed with adequate fall to ensure all water is transported to appropriately located downpipes. The installation and downpipe construction should allow the gutter to drain completely. Regular gutter cleaning and maintenance is required to remove leaves and other debris that may restrict water flow to downpipes.
Particular care should be taken at the entrance to downpipes and corners, to avoid blockages leading to water ponding. A gutter protection system (or any other product) that entraps debris or water between itself and any steel product surfaces, restricting coated steel’s ability to dry, is not recommended and is an exclusion in the product warranty.
Swarf is the term given to the metal debris arising from cutting or piercing operations when using friction saws, drills, or other tools on roofing and cladding products. In this context, swarf may also include any other discarded steel objects such as rivet shanks, nails, screws, and nuts which may come into contact with cladding products.
Steel swarf particles left on the surface will corrode and cause rust stains which will detract from the finished appearance of a project. These stains are often mistaken for early deterioration of the roofing and cladding itself. See 4.15 Swarf Staining and Cut Edge Corrosion.
The installer is responsible for preventing swarf staining, and it is strongly suggested that the recommendations contained in this COP be followed. Generally, swarf particles get on coated steel sheet products in three ways:
1. Loose particles left after cutting, drilling, and riveting operations.
2. Hot swarf particles from disc cutting or drilling operations which may adhere to the finished surface.
3. Loose particles which may be trodden in or become embedded in the surface film of pre-painted products under pressure from adjacent equipment or materials.
Note: Many swarf staining problems arise not from installers, but other contractors working in the vicinity. Main contractors need to be aware of this possibility and manage contractors accordingly.
Fresh swarf stains are characterised by small red-brown coloured areas with a central dark spot (the remains of the steel particles). The surface will feel like sandpaper, and the particle may be lifted with a fingernail. An old swarf stain will appear as a localised red-brown stain, the steel particle having corroded away, and the surface will be smoother.
- Power shears or hand snips produce the least amount of debris.
- Power nibblers give a clean cut but generate debris which if left is prone to corrosion.
- Metal friction blades produce fine hot particles which can embed deeply into the coating surface and corrode rapidly.
- Grinding of adjacent steelwork may also create such debris over a wide area.
Smooth soled or open tread pattern shoes should be worn when working on a roof; avoid the closely ribbed type which will carry swarf and other objects.
Swarf should be swept or hosed from the job progressively and certainly no less than at the end of each day. Swarf which has become stuck must be removed carefully, avoiding action which is likely to remove or change the appearance of the paint or metal coatings.
When sweeping or hosing into a gutter, clean out the gutter before leaving the job to prevent premature corrosion. On completion of the job, give a final wash or sweep down.
For critical applications, the project should be inspected after two weeks when rain or condensation will have caused any remaining swarf to rust. It will highlight affected areas.
The effect of swarf staining itself on pre-painted steel products is generally aesthetic and may not be detrimental to the performance of the product. The product life will, however, be severely affected where attached swarf particles have deeply penetrated the prefinished film and are in contact with the protective metallic coating, although this only occurs in severe cases.
Staining may take a long period to weather away as red oxides of iron are insoluble in water.
On metallic coatings, densely concentrated areas of swarf can cause corrosion to occur over a small area as the zinc in the coating sacrifices itself to prevent oxidation of the metallic coating. See 4.15 Swarf Staining and Cut Edge Corrosion.
Minimising the creation of swarf particles in the first place and regular removal of swarf is far superior to repair of excessive damage.
No cure will restore the surface to its original condition. However, damage can be reduced by prompt action.
- Brush the surface with a stiff bristle brush, or wet sacking, to dislodge particles which must then be removed, not just swept into the guttering. Do not use wire brushing or steel wool as it will mar the appearance of the sheet, remove much of the protective coating, and itself deposit more swarf on the roof.
- A household cream cleanser, used according to directions, will remove most mild swarf stains; however, it is likely to cause variance to the appearance of metallic coated and paint coated products.
- Take great care to remove the stain only and not to cause damage to the paint film. Minimise the cleaning of unaffected and unaffected material; it is often better to allow small amounts of residual swarf staining to remain, than damage the coatings with over-rigorous cleaning actions.
If the coating is severely damaged by swarf corrosion, the area should be painted or replaced. The whole visible area should be repaired, as air drying paints weather more rapidly and in a different manner to pre-painted roofing and cladding products. If swarf particles are painted over, rust bleed-through is likely to occur.
A common cause of swarf complaints arises because other trades have used grinding equipment in the vicinity of a newly completed roof. Wind carried swarf can contaminate large areas some distance from the cutting site. Main contractors should be aware of the likelihood of such damage, and project planning should include scheduling of all cutting or grinding work to be completed before laying the roof cladding.
Colour match paint is designed for matching accessories to the pre-painted material; it is not designed for repairing marks or blemishes. The term touch-up paint should never be used. Fasteners and accessories requiring colour matching should be painted before installation.
Air-dried paints used to disguise marks weather at a rate different from that of pre-painted material, sometimes dramatically so, and will often become more apparent than the mark they are intended to disguise. Minor scratches are best left alone, and they will not affect the performance of the pre-painted product due to the self-healing qualities of the primer and metallic coating. They become less evident as the coating weathers.
Minor scratches may be described as scratches that do not extend to the metallic coating, are less than 3 mm in width, and are not visually noticeable from a distance of 3 m. This characterisation will, however, vary with the concentration of the scratches, and the visibility of the area affected.
Extensive coating damage to any pre-painted steel product can only be rectified by replacement or repainting of the affected sheets.
Profiled metal roofing and wall cladding are readily paintable using good quality primers and water-based acrylic topcoats. Metallic coated roofs can be painted immediately after installation; however, dirt, grease, and any loose materials must be cleaned off, so the surface is clean and dry before applying the first coat.
An effective method for painting metallic coated roofing is to apply a good quality galvanised iron primer and two water-based acrylic topcoats, following the manufacturer’s recommendations.
Pre-painted products can be painted after exposure to weather. Normally, 6–12 months of exposure is required to achieve surface modification of the surface to allow the new coating to adhere.
Edge laps of unpainted metallic coated sheets steel do not require lap priming.
Sunscreen containing titanium dioxide or zinc oxide can accelerate the degradation of organic materials including auto finishes and pre-painted cladding surfaces. This damage is irreparable, so prevention of its occurrence is the only defence. For more information contact the material supplier.
Temperature dust and rainfall can create a good environment for lichens to establish and flourish, and this can occur on almost any surface. For more information contact the material supplier