I often get asked about the different types of vertical seam metal roofs. Common questions surround the differences between them, and what would make one more appropriate for a particular application than another. I will explain the four different types of vertical seam and standing seam roofs here, in order of my perceived quality starting at “entry level” and ending with “premier”.
1. Exposed Fastener Panels
Also known a “through-fastened” and “corrugated” metal roofing, these panels are installed in large vertically-oriented overlapping sheets. They are commonly seen on agricultural applications and also self-storage units. They are best recognized by the high number of exposed screw heads visible on the completed roofs.
Exposed Fastener Panels perform very well in wind uplift tests because there is no interlock between the panels that could open up. The overlapping seams are screwed shut. However, the things that keep these panels on the “entry level” end of metal roofing are the exposed fasteners and the fact that the panels do not have any allowance for expansion and contraction.
The exposed fasteners are typically screws with a rubber grommet or washer of some sort and a metal cap covering much of the grommet. There are different quality screws available in terms of the size, the threading, the material the grommet is made of, the design of the cap over the grommet, the type of metal, and the type of coating on the metal. Higher end screws will be stainless steel and painted and have aggressive threading.
The differences in threading are in an attempt to get a screw that will not back out with expansion and contraction of the metal panels. However, with expansion and contraction, something has to give and, if the fastener doesn’t give, then the hole through the metal will begin to wallow out. It is not unusual to see the screws on these panels have to be tightened after 5 – 10 years and perhaps even replaced with larger diameter screws in years 12 – 15.
Exposed Fastener panels, being entry-level, are sometimes made more economical by employing lower quality metals and coatings in their production. Much of the metal is also from overseas where quality control practices may be less strict than in North America. In some cases, “secondary” metal – metal that was rejected for other purposes – finds its way into these panels. Most of these panels are steel as the greater coefficient of expansion and contraction with aluminum can be even more problematic for the fasteners.
Exposed Fastener panels generally can be used down to 3:12 pitch though some manufacturers do approve them for 2:12 roofs. In some cases, those panels may have sealant between the seams. These panels are sold in a variety of configurations. Some common configurations and generic names are 5V Crimp, R Panel, and PBR Panel. While Exposed Fastener panels are being used on homes, I often worry that homeowners are not aware of the limitations, risks, and possible inferior qualities of these panels when they purchase them. My recommendations on these panels for residential use are “buyer beware”. Make sure you know exactly what product warranty you are receiving, and who it comes from.
2. Nail Hem Panels
These panels do have a raised interlocking seam on the edge of the panels which typically range from 12 – 24” in width. The raised seams are usually in the height range of 0.75 “ – 1.25”. Nail Hem Panels are characterized by a hidden fastening channel running the length of one edge of the panels. While the actual fastening may be done with screws rather than nails, the “Nail Hem” name is derived from these panels being very similar in design to vinyl siding panels which also feature a hidden fastening strip, except they are oriented vertically on the roof rather than horizontally on the sidewall. The fastening strip often has elongated holes or slots for the fastener placement. Nail Hem Panels generally are recommended down to 3:12 roof pitches.
I have to be pretty blunt in regards to these panels and say that, while it is great that the fasteners are hidden rather than exposed, I really do not recommend these panels due to problems I have seen with them. These panel designs often have a very limited interlock at the seam. With uplift pressures, the panels bow in the middle and, while one edge of the panel is held tight to the roof deck by fasteners, the other edge is prone to kicking out and disengaging, causing wind failure.
Another potential concern with these panels is the fastener placement. In order to allow for the metal’s expansion and contraction, the fasteners need to be located exactly in the middle of the slotted holes. The fasteners can also be over-driven to the point of not allowing the panels to slide. If these things do not go perfectly well, the panels will not be able to expand and contract and instead will be caught in a bind that forces ripples into the panels, also known as “oilcanning”. I also sometimes see these panels being installed on top of insulation boards. That really concerns me as the “sponginess” of the insulation can make it even easier for fasteners to be over-driven, again causing problems with thermal movement.
Nail Hem Panels can be produced from aluminum or steel and may come in various quality grades based on metal thickness and quality as well as coating quality. It is not unusual for these panels to be formed on portable roll formers. Such roll formers must be kept in good adjustment to ensure the locks between the panels are properly fabricated.
3. Snap-Lock Panels
Snap-Lock Panels are installed using hidden clips and fasteners. The clips are secured to the roof deck and in turn, hold the panels in place. The clips allow for full thermal movement of the roofing panels. Snap lock panels come in various widths and seam heights. They are available in aluminum and steel of different thicknesses. Typical panels widths are 12” – 20” or even 24”. Seam heights usually range from 1.25” – 2”. Common steel thicknesses range from 22 gauge to 26 gauge while aluminum thicknesses range from .032” – .040”. Industry-leading PVDF finishes are common on these panels, as are the “second tier” polyester coatings.
Snap-Lock Panels are commonly used on roof pitches as low as 3:12 though many are approved down to 2:12 as well and will work very well. In some cases, they will be used on lower pitch porch roofs or as accent roofs, and sometimes even if metal shingles (requiring a 3:12 pitch) have been used on steeper roof portions. It is important with these panels to consider how much water individual panels may carry. Part of that analysis includes looking at water that may be shed from upper roofs onto these roofs. Snap-Lock Panels are pretty widely available and quite proven for a wide range of applications.
4. Mechanically Seamed Panels
These are standing seam panels with clips and concealed fasteners, similar to Snap-Lock Panels. The difference is that the metal roofing panels are mechanically seamed one to the next after they have been placed on the roof. The seaming is done by a machine or by hand crimpers. The concealed clips become part of the locking system. Mechanically seamed panels can often be used on very low slope roofs, even as low as 0.25:12. These panels are commonly used on low slope commercial and industrial roofs. In those cases, they often have just a clear coat on the metal rather than a colored surface. Mechanically seamed panels are most commonly produced of steel though on jobs with panel lengths are not extraordinarily long they can be aluminum as well. Jobsite formation of these panels is common.
Mechanically seamed metal roofs are the only metal option for roofs of less than 2:12 pitch. While they are used on steeper roofs, they can be “overkill” for many applications. One downside to this style of metal roof is that, with the panels being crimped together, repairs or alterations to the roof can be very difficult.