Fully Adhered or Mechanically Fastened?

Yet another roofing debate that we find ourselves coming across here at Chase Construction NW, Inc. is whether to install TPO using the mechanically attached (MA) or fully adhered (FA) method. While both are fully warrantied, and everyone has a preference, it is still a subject that comes up quite often. So which one is better?

Let me give a brief overview of each method using the most basic assembly components (membrane & insulation). Keep in mind typical specified assemblies will often include other components such as a vapor barrier, substrate board (fire rating), and or a coverboard (traffic, hail, protection).

Mechanically Attached (MA)

Mechanically Fastened is the most common, and cheaper of the two methods, though price may be the reason for its popularity with contractors. A typical MA assembly often includes multiple layers of rigid board (4’x8’) insulation with the base layer being installed loose and the top layer being secured with plates and screws that are driven through both layers of insulation into the roof deck.

The bottom sheet of membrane is mechanically attached using screws and plates before the next sheet overlaps the fasteners and is hot air welded to the sheet below concealing the fasteners.  The hot air weld creates a monolithic sheet with a water tight seal. Some contractors swear by this method stating that it’s faster and more economical.

Although the MA system is cheaper to install, it comes with some inherent issues.  For instance, when a strong wind blows across a MA membrane roofing system, the uplift from the wind will cause the membrane to flutter, or even billow at times depending on the strength of the wind.  This can cause issues as conditioned interior air from below is drawn upwards into the roofing assembly potentially causing condensation, especially in winter months.  This scenario can be exaggerated if an Air/Vapor Barrier isn’t included in the assembly.

Leak tracing is another potential issue with a MA system. If the membrane is punctured, the water can travel between the membrane, insulation and roof deck before manifesting below. These leaks are more difficult to locate which makes them more expensive to repair.

Fully Adhered (FA)

Much like the MA assembly, the typical FA assembly often includes multiple layers of rigid board insulation with the base layer being installed loose and the top layer being secured with plates and screws that are driven through both layers of insulation into the roof deck.

Fully Adhered insulation assemblies over concrete roof decks are typically installed using Low Rise Foam (LRF) adhesive instead of mechanical fasteners.  Using LRF eliminates the need to pre-drill the concrete before fastening which is a production killer.

After the insulation is secured, the membrane is rolled out, overlapped at the seam, and then fully adhered to the insulation using a solvent based bonding adhesive.  After the membrane is overlapped and adhered, it will be hot air welded at the seams to create a monolithic sheet over the entire roof.

Although this method is more expensive (adhesives) and takes longer to install, promoters claim the benefits outweigh the cost difference.  Not only is a fully adhered system more aesthetically pleasing (no wrinkles), it’s much more resistant to wind uplift.  The FA system has the benefit of spreading the wind uplift over the entire roof as opposed to point loading the seams as in a MA system.  Additionally, a fully adhered system is much more resistant to drawing the air from below into the assembly like a MA system.

So Which is Better?

Although each method can achieve great wind ratings and are fully warranted, the majority of roof consultants prefer a fully adhered single ply assembly over mechanically attached.  They like that fully adhered systems offer symmetrical wind loading, don’t draw conditioned air into the assembly, prevent condensation and potential fastener corrosion, and better aesthetics.

Ultimately, the determining factors are the project specific building characteristics, budget, building location, and the building structure itself.  A professional roofing contractor or design professional is best equipped to determine which approach to use given this criteria.

Regardless of whether the membrane is fully adhered or mechanically attached, the most important factor is choosing the right contractor to perform the work.  It’s critically important to engage a contractor who is certified with a membrane manufacturer.

Certification signifies the contractor has the training and manufacturer support to design assistance when needed.  Additionally, a certified contractor’s work will be inspected to ensure manufacture guidelines and specification details are followed, it’s like an insurance policy.

About Us:

We at Chase Construction NW, Inc. are passionate about roofing! Whether you are looking to have a TPO system installed or repaired, or you are looking for a different roofing system all together, we can do it for you! We have been Master Elite® Roofers with GAF for 8 years and have been installing quality roofing systems for 16 years. Let our passion turn into a quality roof for your building or home! Contact us today by filling out our free estimate form or calling us at (253) 544-3651.