Are you considering a pre-engineered metal roof for your commercial building? This option is enticing for many who are shopping for a roof replacement, as it’s pre-made and, therefore, a more simple (and sometimes cheaper) choice. But, is this type of roofing a good purchase when you’re considering the long-term nature of a roofing investment? Not always. In this article, we’ll discuss the nature of pre-engineered metal roofing, the issues that can arise, and other insights and considerations to help you make your decision.
What Are Pre-Engineered Metal Roofs?
Pre-engineered buildings are distinct from conventional construction in that they require hiring a designer to have a building designed from the ground up. Pre-engineered buildings, on the other hand, are pre-built to a high degree. They can be manufactured in components that comprise that standard engineered shape, allowing you to build out from there.
So, what about the roofs on these pre-engineered buildings? Generally speaking, they are metal panels. The panels are the building’s waterproof membrane but also make up a structural element of the building, which are both outside of the roof’s insulation, and change with temperature shifts.
The Two Main Types of Pre-Engineered Metal Roof Panels
1. Exposed Fastener Design—In an exposed fastener design, the metal panels are fastened to the building’s purlins with screws.
2. Concealed Clip Design—In a concealed clip design, there are hidden clips between the panels in the seam that hook onto the first panel, with the next panel overlapping to attach. The clips are fastened to the purlin, and the panels are connected under the clips, so they’re concealed under the roof, leaving no exposed fasteners.
Where Issues Can Arise for Pre-Engineered Metal Roofs
1. Leaks Caused by Weather Changes
As we mentioned, the metal panels on these roofs are outside the insulation, so they will want to expand and contract. If someone were to drill holes in it—also remember that this is the roof’s water membrane—they will want to be careful not to create a leak. With several screws holding the panels down, they must place rubber grommets under the screw heads, making the screw compress the grommet against the metal to seal it. Assuming they’re all tight and square to the surface, they work well. But how does this hold up over time?
If those panels are screwed down on a hot day, for example, and then showers drop the temperature, those panels will contract and move. Over time, that panel saws back and forth under the screw deck and scrubs away at the grommet that was sealing it, and so some of those screws lose their seals, and minor leaks begin to form.
2. Leaks Caused by Deteriorating Sealant
Because panels are not long enough to go from the eave to the ridge, the panels are shorter and have an end lap. When the metal laps, there is the potential for water to get in. Generally, exposed fastener panels are 40 by 3 feet, and there is an end lap every 40 feet and a side lap every 3 feet, leaving several chances for water to leak in.
To prevent this, roofing contractors use tape to seal the laps. When the tape is new, and the pressure is good, there is a good seal. Although the sealant is sticky and waterproof, it degrades when exposed to UV light or the ozone, so it can’t be used where sunlight can get to it. Gradually, the sealant deteriorates. The expansion and contraction of the panels also help the deterioration process along.
3. Leaks Caused by Incorrectly Sealed Rooftop Penetrations
There are also penetration possibilities (e.g., A/C units, vents, exhaust stacks) that provide an interruption of the continuation of the roof’s metal panel so that it is no longer continuous. This requires a roofing contractor to seal up the connections—and those are tricky. Because of the detail and attention needed, they are expensive to construct properly.
Down the line, the occupancy of the building might also require a change. Perhaps, a change to the manufacturing line with a new ventilation system or an increase in occupancy that requires a new HVAC unit. New penetrations in a metal roof are very difficult to install correctly, and therefore are expensive. To install a new penetration properly, it may require removing a whole section of the roof to reconstruct the flashing of the penetration correctly.
It’s important not to cut corners here because when you cut a hole in the roof and just fill around it, it will eventually leak. Remember: It’s expensive and skill-heavy to do a penetration installation by the book. Always make sure that you have a trusted roofing partner to guide you.
For a concealed clip design, the roof is designed to move with the weather changes. This design eliminates most of the screws, allowing the roof to expand and contract because the clips can slide by design. The top is hooked into the roof panel, the bottom half to purlins, so this panel can expand and contract if installed correctly. The challenge is that A/C units don’t move, so the flashing around this curve has to allow the movement, or it will be torn apart. It is doable on the initial installation. However, it is tough to accomplish as a retrofit, so it is frequently overlooked, leading the penetration to leak until it is completely redone, which can cost upwards of 15 to 20 thousand dollars for one flashing to one A/C unit.
4. Drainage Issues Caused by Poorly Designed Gutters
Pre-engineered metal roofs have built-in gutters that can lead to drainage issues down the line. When built-in gutters have a problem, it’s usually a design flaw. In this chain of drainage construction, somewhere, a link is too small. So, we run into a problem with a costly solution. Usually, the owner will try numerous ineffective attempts before realizing they have to spend the money to fix it.
Are There Solutions for These Issues?
End laps, penetrations, and built-in gutters are the most frequent cause of leaks in pre-engineered metal buildings. Typical remedies include the application of a sealant compound and reinforcing fabric. If that’s done with reasonable care, it can be effective for a while—months or years, depending on the movement.
Another approach to remedy this is a single-ply retrofit metal roof system designed to be applied over the metal roof assembly. If they are designed properly, they can be a long-term solution to many of these issues. Why? Because we are going to insulate that previously uninsulated area. A single-ply membrane will protect the top, so the insulation dramatically reduces the expansion and contraction of that panel, solving flashing issues that are particular to metal roofs at these penetrations. They’re waterproof and work well, so, this way, you can reconstruct the whole roof in a way that is water-tight for decades.
As with any engineering victory, though, there is a compromise. That old metal roof was near hail proof, but membranes are not. This compromise isn’t terrible, though, as hail is somewhat rare and is an insurable event in this part of the country. In this situation where you have these untreatable leaks that happen in heavy rains, the prospect of hail damage sometime in the future isn’t much of an issue.
In a practical sense, if you own a metal building(s) and have heavy leaks somewhere, you need to have a professional look at this specific leak problem and design a permanent solution. It will likely not be based on sealant and fabric but rather a reconstruction of that detail. If you have several leaks, you should consider a retrofit system because it can be cheaper to retrofit a building rather than fix several of these details.
Built-in gutters are a unique challenge, though. If it’s a drip, it can probably be fixed, but if it’s a waterfall, then you have a capacity issue. If you found the problem under those conditions—heavy rain with a clog, for example—maybe it’s happening once a year, but it will continue to happen until you fix that capacity issue.
The Bottom Line
What began as a relatively simple, inexpensive, and effective design actually loses a lot of that simplicity due to wear and retrofitting. The simple design doesn’t allow for changes to be easily accomplished. Above all, you will want to ensure that you have a trusted roofing contractor with a good reputation helping you choose the best option for your building.
Are You Considering Your Options for a Commercial Metal Roof?
Our team of roofing experts is here to talk you through your commercial roofing options. If you’re ready to get started, contact Maxwell Roofing & Sheet Metal, Inc. today! Our team is experienced and ready to help.