For the architect, panel manufacturer, fabricator, installer or contractor considering a stainless steel roof, you can expect a beautiful, permanent roof system that will be very economical in the long run—if you rely on a competent architectural metal supplier and don’t make the assumption that stainless is like any other metal. Here are some do's and don’ts to guide you:
Recommend stainless steel where sustainability is important because it will last. On buildings that are designed to last longer than the anticipated life of other roofing materials, stainless steel is less expensive in the long run. While stainless steel may not make economic sense on a strip mall, for example, it is possible to install a permanent stainless steel roof that will save the owner repair and replacement costs in the future. When life cycle costing is conducted, including acquisition, maintenance, service disruption and disposal costs in present value terms, stainless steel is hard to beat.
Use a low glare stainless steel finish, like Contrarian Metal Resources’ InvaiMatte® where a highly reflective surface, as most stainless steel finishes are, will pose a problem. InvariMatte has been used on a number of airport terminals where glare poses a safety risk. In addition to promoting safety, a low glare finish will be less apt to show oil canning, or shape deviations in the panel work.
Use a uniform surface finish that is designed for architectural applications. Aside from having a low glare property, InvariMatte is also quite uniform, allowing for a good-looking bare metal installation.
Use a grade of stainless steel that is appropriate for the environment. Coastal environments and sites where deicing salt is used in close proximity will usually require Type 316 stainless steel. Type 304 is appropriate in most other locations. The low carbon versions of these grades (T316L/T304L) should be specified on parts that will be welded. Other grades may be considered, depending on the environment. If you follow our earlier advice with regard to selecting a competent supplier, you can rely on their expertise in grade selection. If you choose to work with Contrarian Metal Resources, a 30-year limited corrosion warranty will back up our material recommendation.
Detail the system so that it drains well. While stainless steel is very forgiving when it is exposed to moist conditions, good water and vapor management will promote the longevity of all of the affected materials in the system.
Use proper tooling for stainless steel fabrication. The mechanical properties of stainless steel are higher than mild carbon steel, aluminum and other metals that are used for roofing. The most significant difference the panel manufacturer or fabricator will notice during forming is a higher modulus of elasticity or greater “spring back”. In order to overcome this issue it is often necessary to have a combination of more horsepower and more tool sets in the roll forming operation. It is also important to have sufficient tooling harness so that the work rolls do not wear prematurely. Further, seaming tools need to be of sufficient capability to form stainless steel.
Monitor tool wear to ensure good results. Worn tooling bears the risk that free iron molecules will imbed in the surface of the stainless steel. If this occurs, it will cause the stainless steel to corrode, manifesting as red rust on the surface of the material. It is necessary in this case to remove the rust with a solution like dilute citric acid.
Remove heat tint from welding operations. Left untreated, this carbide-rich scale will propagate corrosion. Cryogenic blasting is an excellent method of scale removal. Pickling paste is also effective. Fabricators and installers often grind the scale off, but this method is not as effective and the repair area is much more reflective than the parent metal surface, particularly if a low glare finish has been used.
Strip the protective vinyl film at the earliest opportunity in the field, once other subcontractors are out of the area. First of all, a good architectural metal supplier will apply a strippable film that is capable of withstanding outdoor exposure for a period of weeks. This is not to say that you can leave the film on indefinitely. Assuming exterior-grade vinyl has been supplied, it is prudent to leave the film on the panel work throughout the installation process in order to protect the surface from contamination by construction debris, including wasted carbon steel fasteners, welding swarf, tar and paint. Strip the film as soon as the risk of contamination has passed. This will minimize the need for final clean-up activity upon owner inspection.
Abut dissimilar metals without evaluating the risk of bi-metallic, or galvanic corrosion. This is of particular concern where moisture can reside and where the metals are substantially different in terms of electrode potential. Fasteners should be stainless steel of equal or nobler grade.
Rely on solder for strength. While stainless steel is solderable, it is best to use solder for small connections that need to be made for water management purposes. However, in a shear condition, a stainless steel solder joint will fail.
Use muriatic acid, chloride-based cleaners or abrasives when cleaning stainless steel. Instead, you should use a mild detergent (ammonia –based is fine) and rinse for general clean-up. Acetone will address grease, tar and fresh paint. Use MEK on dried paint. Free iron contamination (rust) will sometimes respond to mineral oil, but imbedded iron particles will likely require dilute citric acid, followed by a rinse.
If you start with a competent architectural metal supplier that can help you select a grade and finish that is appropriate for the application, and follow these dos and don’ts, you can be assured of good results. While stainless steel costs more up front, in the long run the owner can be spared the cost and inconvenience of replacing the roof.