Is a roofing screw suitable for outdoor use?
I have followed sheet metal roofing construction from the late 1980s from the perspective of a mechanical and materials engineering professional. Quite often, I have had to marvel at low-quality and even non-compliant construction supplies. The painted installation screw used to fasten sheet metal roofing profiles is an excellent example of such a construction product.
When Pural® steel roofing coating was being developed at Ruukki in the 1990s, it was desired that the installation screws would also be as durable as the upcoming new product. Extensive corrosion tests were carried out to find a suitable screw, testing all installation screws available on the market. The result was quite a shock: even the best screws were nowhere near the corrosion resistance of Pural-coated sheet metal, and the worst ones were truly poor. When screw suppliers were asked for improvements, they had neither the enthusiasm nor the suitable technology to improve the corrosion resistance of roofing screws, so the necessary development was not carried out at that time.
For years, I have investigated the laws and regulations governing the use of installation screws and the corrosion resistance of roofing screws. At present, the roofing screws available on the market have four shortcomings.
Shortcoming 1. Painting roofing screws is not enough for corrosion protection
The studies I commissioned revealed that painting roofing screws is insufficient to protect the screw from corrosion. All corrosion experts know that two things are important in corrosion protection painting: pretreatment and a sufficient paint layer, preferably applied in more than one coat. Pretreatment increases corrosion resistance and improves paint adhesion, and the thickness of the actual topcoat should be at least 60 micrometres. The roofing screws available on the market lack paint pretreatment and primer, and the thickness of the topcoat is only 30–50 micrometres. In powder coating, a proper paint layer cannot even theoretically be achieved with such a coating thickness, let alone in practice.
Shortcoming 2. A roofing screw does not last as long as steel roofing
European standards, which must also be followed here in Finland, specify that connectors, fasteners and sealing washers must correspond in corrosion resistance to the assemblies being fastened1. When talking about sheet metal roofs, the material being fastened is galvanised and primer- and topcoat-coated steel, so the fastener, that is, the roofing screw, should also be galvanised and made of primer- and topcoat-coated steel.
Roofing screws that are inferior in structure and coating do not last as long as steel roofing, for which a warranty of up to 50 years can be given. In practice, the paint surface comes off roofing screws already during installation, and without repair painting, rusting begins immediately. Rusting screws create rust streaks on the sheet metal roof. In the worst case, the heads of the roofing screws come off, making the sheet metal roof no longer watertight.
1) SFS EN 1090-2 + A1 (Execution of steel and aluminium structures. Part 2: Technical requirements for steel structures. Section 5.6.1)
Shortcoming 3. A roofing screw made of stainless steel corrodes the roofing sheet
Good steel construction therefore requires that the fasteners use a material with corrosion behaviour as similar as possible to that of the material being fastened. This prevents the formation of a corrosion couple and unexpected rusting.
According to the standards, unintended contact between different metallic materials, such as stainless steel and aluminium or structural steel, must be avoided2. This is an old foundation of corrosion protection that seems to have been forgotten in steel construction. The aluminium used in the washer does not pose a corrosion threat to steel or zinc, because it is very close to both in the electrochemical series. The problem is a roofing screw made of stainless steel and its use as a fastener for painted steel roofing. Expensive roofing screws made of stainless steel are not suitable for fastening steel roofing, because when the corrosion risk materialises, they corrode the roofing sheet.
Screw sellers, however, are happy to sell roofing screws made of stainless steel, because they offer a better profit margin. And when the sheet metal rusts but the screw does not, any possible complaint concerns the sheet metal rather than the roofing screw, allowing the screw seller to escape responsibility. From the roof owner’s perspective, however, rusting of the sheet metal is a much worse and more expensive alternative than rusting of the screw.
2) SFS EN 1090-2 + A1 (Execution of steel and aluminium structures. Part 2: Technical requirements for steel structures. Section 10.4)
Shortcoming 4. Roofing screws are not suitable for cities or industrial environments
The durability of all construction materials is affected by climatic exposure, which, among other things, exposes roofing screws to corrosion. Climatic exposure is divided into six different classes. In Finland, the most common are urban and industrial climates, that is, climatic exposure class C3, which covers 80% of all buildings. The best roofing screws had a verified product declaration issued by the Finnish Association of Steel Structures, according to which they could only be used in a clean rural climate in climatic exposure class C2. Those who built in urban and industrial climates have had to build with non-compliant screws, since no roofing screw approved for climatic exposure class C3 or C4 had been available on the market, until now.
The corrosion resistance classification of a roofing screw can only be demonstrated in Finland by a European Technical Approval (ETA). The first roofing screw approved for climatic exposure class C4 came onto the market in spring 2026. The KINGI® roofing screw is the only one with ETA approval for climatic exposure class C4.
The world’s best and most durable steel roofing is made in Finland. Roofing screws must be able to withstand the same conditions as roofing sheet, outdoors and even in the climates of cities and industrial areas.