Only a sheet-metal roof has a lifespan of over 50 years
The most important task of a roof is to protect the building and keep wind, water and snow outside the building. The other tasks are secondary. The other parts of the building also remain in better condition when the roof does not leak. The old folk, who are sometimes even praised for their wisdom, say; “If the roof holds, the building holds”.
The durability of a roof can be assessed in many ways. A faded appearance mainly appears as an aesthetic nuisance, but it can also be a sign of reduced durability. Debris accumulating on the roof provides an excellent growing medium for moss that weakens the roofing material. When durability fails, the roof leaks. A roof starts to leak if a tile cracks, a hole appears in the felt or the sheet metal rusts through. The underlay often used today helps when new, but even it does not protect structures from moisture damage forever.
In residential construction, a durability target of 50 years is generally set for roofs, even though it may seem very short to many. Based on my experience, I claim that a 50-year target without leaking holes is most reliably achieved with sheet metal roofing.
Why is sheet metal popular among roofing professionals?
Sheet metal roofs began to be used in valuable buildings as soon as sufficiently thin sheet metal was learned to be manufactured. The products were naturally not exactly the same as today, but the end product was the same: a tightly formed steel roof with durable coatings.
Especially in the Nordic countries, where people have learned to live with snow and ice, construction professionals understand the strengths of a steel roof:
- Steel can be used to build a strong but lightweight roof that does not weigh down the building’s supporting structures together with the snow load.
- A well-designed and properly executed sheet metal roof cleans itself of snow, ice, debris and dirt.
- Steel makes it possible to create extensive roof structures with few leak-prone seams.
- The variety of products makes it possible to fulfil different appearance preferences.
- Installation is quick, which in Nordic conditions may be very important when moisture problems are to be avoided.
- Maintenance is easy and quick, so there is time to do it even during a shorter summer.
- By using the right coatings, roofing screws and other accessories, as well as correctly timed and performed maintenance, a sheet metal roof can last over 100 years.
What kind of sheet metal roofing lasts?
The steel must be strong structural steel so that the roofing can withstand both installation and later walking on the roof without changing shape. The sheet metal should be at least 0.6mm thick, or significantly stronger steel grades than the current ones should be used. The current practice of using the thinnest possible and relatively soft sheet metal is not the best possible in the long run. If the sheet metal is made thinner, its strength must be increased significantly so that installability and maintainability work.
The zinc coating protects the steel sacrificially in problem situations. A thickness of 20 microns, which is generally used today, is sufficient for galvanizing, but more attention than before must be paid to the paint coating, as its durability largely determines the roof’s maintenance needs and overall durability
Previously, painting was done on site after the roofing had been installed. When successful, it was a good solution. However, painting on the roof has a few weak links that unfortunately often ruin the entire job:
- the paint or the equipment used is not suitable for the site
- proper pretreatment cannot be carried out on the sheet metal surface on the roof, so the paint does not adhere to it properly
- the painting conditions or the paint’s drying conditions are unsuitable; the air is too hot or humid
- the quality depends too much on the painter’s skills and willingness to do a good job
When factory painting was developed for roofing sheet metal, most of the problems mentioned above were eliminated. Only the development of a paint well suited to roofs took time. Previously, paints made for wall use were used on roofs, but in the 1990s the first coating specifically intended for roof use was developed in Finland: a high-quality and fairly thick polyurethane coating. To improve durability, it was deliberately made with a normal gloss rather than a matte surface, so that dirt and debris would be easily cleaned from the roof.
A good polyurethane coating provides the roof with its first 20–30 maintenance-free years, if any installation scratches are repaired immediately after installation. Later, such a roof is easy to maintain with simple maintenance painting, allowing it to easily reach 50 years. With several maintenance treatments, even 100 years is achievable.