When it comes to roofing, there are several types of attachment methods. But the two most common are mechanically-attached and fully-adhered. These are not just two alternate methods; there are consequences to either choice so it is important to make sure you choose the one that is right for your particular roofing situation. Learn how to be a part of that decision by learning more about this often overlooked, yet very important decision.
As we approach the winter season it is normal to start to experience a significant slowdown in building projects, whether retrofits or new construction. But it is also an opportunity to begin thinking about springtime when the construction industry comes back to life with fervor. If you’re an owner, architect, or specifier with a flat roofing need – whether that need is now or in the near future – one of the questions you should be able to answer is whether you will address your flat roofing needs with a mechanically-attached or a fully-adhered roofing system. This is not a trivial exercise. Knowing the pros and cons can allow you to make the best, most cost-effective decision. There’s also ballasted, which is explained in more detail in the footnote. But in this newsletter we’re going to focus more on the other two.
In the world of new construction, it seems to me that most of the flat roof roofing systems specified are fully-adhered black EPDM. This is what is most known and therefore, most used in the roofing industry. (Thankfully, this mentality is changing.) While a fully-adhered EPDM roofing system is a reasonable choice as compared to that of a mechanically-attached EPDM system, the real question addressed in this article is related more to the “attachment” type used, not necessarily the product itself, though that does have a bearing on the feasibility of the attachment type. For example, if a mechanically-attached roofing system is what is called for then choose a roofing product that is best suited for a mechanically-attached system. Or if a particular product type is desired, then determine if that particular product type is better suited for a mechanically-attached or a fully-adhered attachment type. (To find out more about the various roofing product types, please see our May 2011 newsletter Which Is a Better Roofing System – TPO, EPDM, or PVC?)
So as we move forward we’ll outline what a mechanically-attached and fully-adhered roofing system is as well as when their use is best suited. Finally, we will finish up with outlining some questions to ask yourself to help you determine which “system” is right for you. So let’s get to it…
So What’s the Difference Between Mechanically-Attached and Fully-Adhered?
A mechanically-attached roofing system is a membrane roof that is attached to the substrate; i.e., wood, concrete, steel, etc., by mechanical means. This mechanical attachment could be with screws, concrete nails, augers, etc. The fastener used is almost entirely dependent upon the substrate (type of roof decking). Any insulation, substrate board, or cover board that is a part of the roofing assembly would also be attached into the building’s roofing substrate, either directly or indirectly by means of the direct attachment of the other, adjacent components. Often these “intermediary” components are attached independently of the roof membrane and are done with sufficient fasteners and in a specific pattern so as to achieve particular wind up-lift ratings, if such are required. The membrane roofing system is then installed on top of and through those intermediary components and into the roofing substrate. Since the insulation in a mechanically-attached system is attached separately and then is further secured indirectly when the membrane roof is installed, the number of fasteners required to “fasten” the insulation to the substrate is much lower than a fully-adhered roof. For example, a typical mechanically-attached roof may only require 5 fasteners per each 4′ x 8′ insulation board, which is secured to the substrate. In this case, there would be one fastener in the middle of the board and one at each corner. (We actually use 6 – one at each corner and then one at the middle of both edges. We find this method keeps the insulation board flatter to the substrate and creates a better, smoother surface.)
A fully-adhered roofing system is a membrane roof that is attached to a roofing substrate indirectly using adhesives, which is just a fancy word for glue. Sometimes the roofing membrane is installed directly on top of the roofing substrate. But usually there is insulation and maybe even a cover Corrugated Metal Roof Deckboard that is also part of the assembly. If that is the case, then obviously the roof membrane is attached to the insulation (or cover board) instead of directly to the substrate. In such a scenario, the insulation (or cover board) may actually be mechanically-attached to the substrate first and then the roofing membrane is glued to the top of that. Let’s look at this a little more closely.
Let’s say we have a corrugated steel deck roof substrate with 5-inches of polyisocyanurate insulation on top of that, followed by a fully-adhered PVC membrane. In this case, while you could fully-adhere the insulation, it is often mechanically-attached. If it is to be fully-adhered then significant care must be taken to make sure that it is done correctly by fully utilizing the tops of the metal flutes. It is always good to keep in mind that if the insulation is not adequately attached to the substrate then it makes no difference how well the roofing membrane is attached to the insulation.
Therefore, the insulation is usually mechanically-attached to the substrate. But because the insulation is what is being used to hold the fully-adhered roof membrane to the roof, the insulation will require significantly more fasteners than that of a completely mechanically-attached system. The number of fasteners to attach the insulation to the roof substrate are typically anywhere between 16 and 24 per 4′ x 8′ board – all depending upon the wind up-lift requirements. Remember, in a fully-adhered system, the structural integrity of your roof is only as good as the weakest “attachment” layer.
One of the ways that the above scenario is addressed is to use what is called a substrate board. A substrate board – usually a 1/4″-, 1/2″-, or 5/8″-thick gypsum board of some kind – is laid over the top of the corrugated steel deck. This flat board ‘bridges’ the gaps in the corrugated decking. It is also typically mechanically-attached to the steel substrate for the same reasons as described above. All other components above the substrate board, including the roofing membrane, are then fully-adhered together. You will still need a significant number of fasteners to attach the substrate board to the deck because that then becomes the “critical attachment layer” for the rest of the roofing system. If insulation is used, then the substrate board is not needed. If it is still used, then it is usually not fastened directly to the substrate. Instead, it is usually attached when the insulation is attached. It gets fastened as a result of it being between the substrate and the insulation that is being fastened to it. In such a way, the insulation then becomes the “critical attachment layer.”Fully-Adhered Attachment
Another point to consider is the quality of the insulation. Most industry-standard insulation is created with a paper facer. The actual insulation is sandwiched between the top and bottom facers. This makes it much easier to handle and install. But because of this, architects and specifiers will often require a cover board in a fully-adhered system. Like a substrate board that is laid directly above the substrate, a cover board is used to “cover” all those intermediary components we talked about before. The roofing membrane is attached directly to the cover board.
A cover board’s use is often based on the belief that if you glue a roofing membrane directly to the insulation, you are technically attaching the roofing membrane directly to the paper facer of the insulation. It is not uncommon for those facers – which are basically just laminated to the insulation – to separate from the insulation. If the roof membrane separates from the insulation because the paper facer delaminates, then you’re left with a failed roof and it becomes subject to all sorts of problems, not the least of which is wind.
This seems like a good time to delve right into the pros and cons of mechanically-attached and fully-adhered roofing systems. Since we just detailed a problem that can occur with a fully-adhered system, let’s pick up where we left off…
A Word about Seams
Whether a roof’s attachment type is mechanically-attached or fully-adhered, another issue that needs to be address is the method of seaming. Though we will deal with this issue in more detail in a later newsletter, it is important to understand. Some roofing systems can be fully-adhered while the seams of that system are hot-air welded. Conversely, some roofing systems can be mechanically-attached, but the seams are glued or taped. Seams are typically the weakest link of any roofing system. Be sure you ask your roofing contractor about the seaming method’s and product types that are best suited for better, more reliable seaming. (Return to article)
Delamination of the paper facers to which a fully-adhered roof membrane is attached is a major reason why a cover board is required by many architects. (Another reason is to increase hail protection, but often that is just a side benefit.) If a cover board is installed directly to the insulation, then the roof membrane can be glued directly to the cover board, which doesn’t have the same facer delaminating issues that insulation is said to have. (By the way, this is not necessarily my belief, but it is a belief shared by many architects and specifiers.) This is thought to “address” the above-described problem. But to me, it is a silly reason. By installing a cover board, you are only moving the potential problem area down a layer. After all, what is the cover board adhered to? That’s right, the very same paper facer of the insulation board that is said to be subject to delaminating. So what difference does it make if your roof comes apart below the cover board or directly below the roof membrane itself? Either way, the roof would fail. To use a cover board in this way on a fully-adhered roofing system to address this delaminating issue is not addressing the issue at all. The only real way to address the issue is to use a cover board that is mechanically-attached directly into the substrate. This mechanical attachment would also include all the intermediary components below it such as the insulation and substrate board, if any. Then you could fully-adhere the roofing membrane to the cover board. But in my opinion, why do that? If you’re already mechanically-attaching the insulation, the substrate board, the cover board, etc., it makes no sense to then fully-adhere the roofing membrane. It’s silly. Why not just mechanically-attach the membrane as well?
Please don’t get me wrong or misunderstand me; there are times when fully-adhering a roof membrane is a legitimate way to go. It’s just that in the above scenario – when you’re already mechanically-attaching everything else – it makes no sense (to me at least) to fully-adhere the membrane. But there are times when a completely fully-adhered roofing system does make sense. The three main reasons that I can think of are:
A correctly installed fully-adhered system is uniformly adhered. There would be no gaps or hollow spots throughout the field membrane. So if a cut occurred to the roofing membrane and water began to pour into the cut, the water could not “run” everywhere because the cut would ‘theoretically’ be sealed on all sides. Unfortunately, this relies entirely on a roofing membrane that is in fact glued down on all sides with no gaps or hollow spots. This is easier said than done and is often impossible in a cold-temperature installation.
Fully-adhered systems can be done so that there are no penetrations made into a roof substrate. This is particularly important when the substrate below serves as a ceiling above. Unsightly fasteners poking through a ceiling is not often desired. Sometimes the substrate is concrete and in order to adequately fasten the roof to the substrate would require penetrating all the way through the concrete. This is spalling and can create a real mess. To avoid these types of situations, a fully-adhered system can be very helpful.
A properly installed fully-adhered roofing system can have very good wind up-lift ratings. This is because the roofing membrane is uniformly attached throughout. There are no gaps or areas to which the roof is not attached to what resides below it. As mentioned, however, the overall roof’s integrity is only as good as the weakest point or area of attachment.
A mechanically-attached system has the benefit of guaranteeing that the roof is attached directly into the structure of the building. This has obvious advantages over a fully-adhered system. There are other benefits as well…
Other Benefits of a Mechanically-Attached Roofing System
A mechanically-attached system is usually less expensive than a comparable fully-adhered system. The glues in a fully-adhered system are expensive. Couple that with the typical requirement of significantly more fastens used to secure the insulation, substrate board, or cover board, and a fully-adhered system can end up being a very costly one. A mechanically-attached system on the other hand uses little to no glue by comparison, and ironically often uses less fasteners than a fully-adhered system. Questions to Ask
A mechanically-attached system can be done in colder weather. A fully-adhered system, because of the use of glues, cannot be installed effectively in colder temperatures. Unfortunately, many roofers install fully-adhered roofs in temperatures below acceptable temperature thresholds. Not only is this completely ignoring good roofing practices, it will void any fully-adhered roofing warranty. With a mechanically-attached system, there are no such constraints.
Many times a mechanically-attached system can be done in less time. This is because there is no need to wait on glues to cure. Each sheet of a fully-adhered system may require 30 minutes or more (depending upon temperature and humidity) to cure. During that same time, a mechanically-attached roof is being installed.
What Are You to Do?
There is a lot more to this issue. For example, what about the seams? But it is boring and individual aspects of this topic don’t apply in every case. Therefore, you’re best bet is to contact your roofing contractor and let them help you through the process for your specific roofing needs. Hopefully what this article has done is put you into the mix. Instead of being a bystander and relying entirely on the knowledge and integrity of your chosen roofing contractor, you can now play a part in making sure your needs are being addressed. So to help you in that process, here are some questions that you can pose to yourself in order to help make sure you get the roofing product, system, and attachment type that best suits your particular situation:
- What time of year is my roof to be installed?
- What weather conditions and temperatures can I expect to exist during the proposed installation time?
- What is my roof substrate?
- Will fasteners penetrate all the way through and if so, will this be a problem?
- Are the individual members of the crew used to install my roof experienced installers of the particular product type and attachment type that I’m having installed? (Both require unique skills.)
- What attachment type is best suited for the particular roofing product I’m interested in?
- Are the seaming methods used for that particular product going to be effective for my particular roof’s design, or are they merely adequate? (Remember, seams are the weakest link.)
There is just too much to know in the world around us. We just can’t know everything. Nor do most of us care to do so. But that’s why we need to rely on the experts. As you go through this process, make sure that the roofing contractor and the specific representative you’re dealing with is actually an expert in their field. More importantly, make sure that they have your particular roofing needs in mind. There are a lot of hungry contractors out there, willing to cut corners to get work. Remember that you usually get what you pay for. In some cases you can get a great deal by a reputable contractor. But just be very cautious that the “great” deal isn’t too good to be true. High-quality contractors have high-quality employees. It takes more money to keep such talent. Doing a job right instead of fast also costs more. So getting that “great” deal may only mean someone is cutting corners with less than adequate installation crews who too often care more about getting a job “done” rather than getting it “done right.”
If you want to be sure that you get the best of both worlds – fair and reasonable prices by honest, high-quality personnel – then give RTN Roofing Systems a call at 970-593-1100. Whether you just want to ask a question or if you have major roofing needs, we’re here to help you. I hope that the above article helps you a little more to understand your roll in the process.
What About a Ballasted Roofing System?
Besides mechanical and fully-adhered attachments, many single-ply membranes can also be ballasted. This is when the roof membrane is held in place by aggregate. In a ballasted roofing system, the aggregate are usually stones at least an inch in diameter and applied heavily throughout the roof. In fact, the weight for a ballasted roof can range from about 15 pounds per SF (psf) to about 25 psf. The minimum allowed by code is 10 psf. That is very heavy. Increase that with snow loads, water loads (due to improper or clogged drainage systems), etc., and you can have a real problem.Ballasted and Green Roof
Ballasted roofs are still out there and are still being specified, but I don’t know why. It is a dying breed, however. Many specifications that would have normally called for ballast went to mechanically-attached, fully-adhered, or vegetative (using plant life to hold the roof in place.) Another variation of ballasted roofs that has gained some momentum is the use of concrete pavers. Though they can be installed in a more uniformly-consistent manner and typically at a slightly lighter overall weight than its rock counterpart, you still get the same negatives with a paver system as you do with a rock ballast roof. Besides the excessive weight of a ballasted roof, another major downside is repair and maintenance. If your roof springs a leak, how do you know where it’s coming from? How can you get to it to repair it? Those rocks (or pavers) are very difficult to move around as you are investigating leaks. They’re heavy – collectively anyway – and they mask the condition of the roof. I like to call a ballasted roof the nuisance roofing system. It’s very easy to put on, but very difficult to do anything with it after installation day. My advice: Stay away from ballast if at all possible.
So why use ballast? Think about it for a moment: If you could get away with putting a cheap roofing membrane on your building and then “protecting” and “holding” it in place with several truckloads of rocks or pavers, wouldn’t that be the cat’s meow? It’s cheap. There’s no other way around it. You get little of the energy benefits of a white roof, you add significant weight to a building’s structure, and create nearly impossible repair circumstances when the roof does spring a leak. Though we often encounter them when investigating leaks and doing reroofs/replacements, we stay away from the installation of ballasted roofs. They are typically not cost-effective and are often more trouble than they are worth. When there are viable alternatives out there, there is usually no reason for a ballasted roof.