What are the Best Attachment Options for PVC and TPO Membranes?
There are many attachment options for commercial membranes and as with most choices in roofing, it depends on the building design and use, local code requirements, and what the installer has the most experience with. This article will discuss the pros and cons of each approach to help you make the best decision for your project. Let’s start with the simplest system.
A big labor-saving advantage of single ply is the large roll sizes; for example, TPO is typically available in 10 x 100 ft. rolls. Once unrolled, screws and plates are used to fasten one edge down, as shown here:
The fastener spacing is determined by the wind uplift code requirements and will be specified by the architect or project designer and the manufacturer. Each sheet is only attached down one of the long edges and the other long edge is welded along the attached edge of the previous sheet. In that way, the screws are sealed into the roof system. Below is a look at a welder in operation:
The welder is using hot air to fuse the overlapping membranes together to form a single, monolithic surface. Some of the obvious pros of this approach are:
- It’s a fast way of installing a roof system. A typical 100 square roof, with two layers of polyiso insulation, can be mechanically attached and the roof fully closed up in five days with an optimum crew.
- It’s the simplest and lowest-cost way of applying a heat-welded single-ply system that can practically be installed year-round with warranties of up to 30 years.
But, as with all roofing systems, there are compromises. The membrane is only held down along the long edges. With a typical 10-foot wide sheet, that means there’s almost 10 feet of membrane between fasteners.
That leads to the cons for this approach:
As wind travels over a mechanically attached roof, it lifts the membrane up between the fasteners as shown below:
- Conditioned air gets pulled up under the membrane as it flutters, potentially causing some issues. Energy gets lost and, on a cold night, condensation can occur as the warmer, humid interior air meets that cold membrane surface. We strongly recommend always using two layers of polyiso insulation or a top layer of HD polyiso cover board, installed with staggered joints, to minimize these effects.
- Required use of half sheets around the perimeter can take extra time to install.
- In some situations, such as schools, fluttering membranes can be heard inside the building.
So, this brings us to the second method of attachment. To truly lock down a membrane, preventing uplift and fluttering, we must use fully attached methods.
Fully Adhered – Smooth Membrane
As the name implies, the membrane is not fastened down with individual fasteners, but is glued down with an adhesive. There are a few different approaches to this. The first uses normal smooth membrane, which gets coated with adhesive along with the substrate.
The membrane is rolled out and then folded back along its length. That allows the two surfaces to be coated with adhesive. Regardless of the type, the adhesive is allowed to dry or flash off. At that point, it’s really a contact adhesive and the membrane gets flopped back onto the insulation or cover board. The other half is then folded back and the process is repeated. Once the membrane is attached it is rolled in to make sure of adhesion, and then the seam is welded with the hot air automated machine.
If the building is occupied, consider using a water-based adhesive as the smell from solvent-based products, including low VOC adhesives, can be disruptive to building occupants. When selecting a fully adhered roof with smooth back membrane, some of the pros include:
- Smoother appearance compared to mechanically attached systems.
- Longer warranty durations — typically five years more compared to mechanically attached and up to 35 years on select systems.
- Acts as an air barrier, reducing the tendency for condensation to form under the membrane.
While there are many benefits to fully adhered jobs, there are also some cons including:
- It is labor intensive, as you have to apply adhesives to both sides of the membrane and wait for adhesive to flash off.
- Most adhesives have temperature limitations of only being able to be installed at 40°F and rising.
- All of your installers must know how to apply the correct application rate recommended by the manufacturer.
Fully Adhered – Fleece-back Membrane
The second approach uses a so-called fleece-back membrane. This is the same material as standard membranes except that the back surface has a polyester felt bonded across it.There is a smooth edge without fleece for welding. Adhesive is only applied to the substrate and then the membrane is unrolled directly onto the adhesive. In this instance, there are three types of adhesives that can be used:
- Water-based bonding adhesive is applied directly to the substrate and the membrane is rolled in while the glue is wet.
- This is a tremendous labor savings compared to traditional bonding adhesives. However, water-based bonding adhesives do take longer to cure and are even more sensitive in colder climates.
- Hot asphalt is applied directly to the substrate and the membrane is rolled into the asphalt
- This is a tremendous labor savings compared to traditional bonding adhesives. However, the smell can be quite bothersome and the number of contractors having the appropriate kettles has seen a decline.
- Low Rise Foam (LRF) is applied directly to the substrate in a spatter pattern or ribbons (depending upon application) and the membrane is rolled in shortly after application of the adhesive.
- This is a tremendous labor savings compared to traditional bonding adhesives, and there are a variety of types of applicators for LRFs. Some may require several thousands of dollars; however, some require minimal investments, so take a look at all of the options available.
- Typically all LRFs are also VOC compliant and have minimal odor
While adhered fleece systems can have quite the sticker shock when looking at material costs, in many instances the labor savings these systems provide can offset the higher material cost. Just like adhered smooth back systems, adhered fleece-back systems can qualify for warranties up to 35 years.
There is no flash-off required—in fact, we don’t want the adhesive to dry before closing up the membrane. The wet adhesive penetrates into the fleece, making for a very strong bond.
A very viable alternative is the self-adhered system. In this case, the membrane is shipped pre-coated with adhesive and covered with a release liner.
In a self-adhered installation, the membrane is kicked out over the insulation or cover board. Similarly to a fully adhered application, the membrane is then folded back, exposing the release liner. The liner is split in two along the length of the roll; the exposed half gets pulled off and the membrane is flopped back into place. This is repeated for the other half, and then the membrane is rolled to ensure adhesion and then the seam is hot-air welded. Typically, self-adhered system warranties only qualify up to 20 years.
There is another attachment option that offers some of the benefits of both mechanically attached and fully adhered. This uses induction heating to lock the membrane down to special plates used for the insulation attachment.
Induction Heating/Metal Plate Attachment
Imagine that the metal plates used to attach the polyiso insulation are coated with a TPO resin. Next, picture the TPO membrane being welded to each of those plates. That’s exactly the situation with this method of attachment. The membrane is rolled out and seamed. Then a special induction heater, placed over each polyiso attachment plate in turn, heats the plate for five seconds. A weighted magnet placed above the plate for 20 seconds ensures the membrane and plate become welded together.
In this way, the membrane is point attached with typically no more than 30 inches between any two attachments. The picture below shows how this works in practice, with the operator fairly quickly heating one plate while a series behind him are weighted. After each plate is heated, a magnet is taken from the line behind him and moved up to the newly heated plate.
Overall, the pros of using an induction welding system are that they:
- Install in approximately half the time of fully adhered jobs.
- Can qualify for the same length warranty as fully adhered roofs.
- Easy-to-use machine minimizes installation error.
While there are some great benefits of using an induction-welded system, there are some cons:
- Uses fasteners that still put holes in the roof deck.
- Requires a capital investment.
So next time you’re debating which product would work best for an upcoming roofing job, consider some of the options discussed in this blog post. If you have questions about the installation of these various systems, check the videos on gaf.com or call your local GAF sales representative.