In the realm of commercial flat roofing system products; there are several system options to choose from such as TPO and EPDM. In addition; there can be varying specifications on how each particular product may be installed.
The specifications determined for a flat roof system can also vary based on the field conditions encountered, and with design considerations for other building envelope circumstances in mind.
To add further complexity, there are different manufacturers of specific product types; and each manufacturer may have a different history of longevity or reliability where their particular product is concerned.
Here is another dynamic to add to the equation. The roofing contractor or specifier of the roofing assembly will often have a bias for one product brand or system type, versus competitor suggested option.
So what does someone responsible for deciding on a commercial roofing investment to do? How can that group or individual be confident that they are making the right decision for their particular circumstances?
The best answer is to become educated on the subjects of the products themselves, the manufacturer of the product, and the roofing contractor organization being considered.
Two of the most commonly selected commercial roofing systems include TPO (Thermoplastic Polyolefin) and EPDM (Ethylene Propylene Diene Monomer). Both of these water-proofing membranes fall into the category of single-ply membrane systems.
Single-ply membranes can also include PVC (Poly Vinyl Chloride), and liquid-applied coatings. Single-ply systems are often compared with multi-ply systems such as BUR and Modified Bitumen.
Single-ply systems have gained popularity for a variety of reasons. One reason involves solar reflectivity, such as is a characteristic of TPO. Another significant factor in reducing emissions as a result of burning asphalt or bitumen into the atmosphere.
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Further considerations can be the weight of the assembly; as single-ply systems are typically lighter and make an excellent choice as a recovering option, when advisable.
To compare EPDM and TPO systems; one should understand the basic installation differences and similarities of the two products.
- Both EPDM and TPO come in bales (much like buying a roll of carpet).
- Both TPO and EPDM can be installed using an adhesive (similar to contact cement).
- Both systems can be installed loose laid, and or ballasted, or fully adhered.
- Both TPO and EPDM are fully recyclable products.
Differences between the two single-ply systems are as follows:
- TPO is manufactured using lamination of rubberized materials over a scrim material. It further has a highly reflective white surface.
- EPDM is similar in appearance to a bicycle tube material, as is it comprised of rubber, and has characteristics of being highly elastic.
- TPO seams between field sheets are hot air welded, producing a seam four times as strong as the membrane itself.
- TPO’s white reflective surface can help lower a building’s temperature, whereas EPDM has a black surface that can contribute to retaining heat in a building.
- EPDM has a history of over forty years being successfully used in Canada and the U.S.
- TPO has a lesser installation history in Canada and the U.S. of about twenty years.
- TPO is promoted as being superior in terms of algae resistance.
- TPO is an excellent choice for extensive green roof assemblies as it does not permit root growth through the membrane.
- EPDM material is typically less expensive to purchase vs. TPO
- TPO is typically less expensive than PVC membranes; which places it in the centre of the spectrum on a material per square foot cost basis; when comparing the major single-ply products.
- The average expected lifespan of EPDM is typically 25 years.
- The average expected lifespan of TPO ranges from 15-30 years based on the thickness of material purchased.
- 60 mil is most commonly used by contractors for TPO systems.
When discussing any single-ply roofing assemblies; proper detail work is the key in terms of workmanship. Areas such as drains, inside and outside corners, roof appliances, and field seams are the most common failure points contributing to leaks.
As both EPDM and TPO are excellent for dealing with thermal expansion and contraction, particularly on larger buildings, they make an excellent choice for regions where radical climatic change can occur between winter and summer.
All single-ply assemblies benefit from drainage being incorporated into the slope of the roof. Single-ply systems are not designed to sit under large volumes of ponded water for long periods of time.
An advantage of TPO is that it’s hide is tougher than EPDM in terms of puncture resistance from footfall traffic. (Example: someone stepping on a nail or screwdriver and puncturing the membrane).
It is wise to incorporate walking pads or pathways on either of roof systems to limit foot traffic over the membranes, when allowing for service of HVAC equipment, etc.
Using either single-ply membranes can increase the snow load capability of a roof structure if they are not ballasted systems being used.
The advantages of either single-ply system is that they can be easily repaired, and leak sources are usually easier to identify.
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EPDM is best installed by gluing down the field sheets over an insulation board which has been placed over an existing roof, or in the case of a new roof deck; over a continuous vapour retarder, and thermal insulation over the bare substrate.
In the case of a recover specification; any underlying gravel should be removed first, and wet spots cut out prior to any re-roofing activity beginning. Mechanical attachment of the insulation is achieved by using discs and screws which are installed in a prescribed manner to resist wind lift. The screws need to be of sufficient length to secure into the steel or wood deck beneath.
Concrete decks can have the insulation installed using adhesive applied methodology, for secure attachment.
Where EPDM is concerned, the overlapping seams are cleaned using a primer solution which removes the factory talc and provides for a clean bond. EPDM seam tape is then used between the seam laps, and the seams are rolled to ensure full contact of the membranes.
Similarly the field sheets are also rolled with a heavy roller and broomed to eliminate wrinkles and blisters in the membrane. Important is that the adhesive is allowed to become tacky before the membrane is bonded. Laying EPDM is similar to laying carpet, in that skilled technicians cut around curbs, protrusions, etc.
The target and reinforcement flashings are then completed at the appliance areas of the roof, in a similar fashion as the field sheets. The contacting surfaces are cleaned to prepare the surface areas using a primer solution. The bonding is achieved with adhesive and or tape, the critical joint areas using such items as corner and “T” patches.
By comparison; TPO is installed in the field of the roof, and can be loose laid in the middle portions of the roof field. Perimeters of the field sheets are mechanically attached at a prescribed rate to accommodate wind uplift standards.
Around the perimeter of the field sheets are installed further TPO panel sheets to effectively “picture frame” the perimeter of the field of the roof. Such sheets are most often fully adhered using an adhesive similar in nature to contact cement, but designed specifically for use with TPO.
Perimeter walls, parapets, and appliance details are also fully adhered using a 0.45-inch thickness of the material. 0.60-inch material is usually specified in the field. 0.80-inch material is also an available product.
Where TPO is concerned; the overlapping seams of the material are thermo-fused to one another using both a handheld heat gun for smaller detail work; and robotic hot air welders that do a similar job on the field seams.
The advantage of robotic welding provides for clean, uniform seams which can thermo-fused at a consistent temperature.
Welded seams are deemed to be four times stronger than the field membrane itself.
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On both TPO and EPDM systems, a field inspection throughout the course of the project and upon conclusion of the project; should include probing of the seam work to reveal if any bonding issues exist.
As TPO or EPDM systems age, and arrive near the end of their lifecycle; the following failure points can occur:
On EPDM, the material can balloon or overstretch as a result of fatigue and or long term UV degradation.
On EPDM, seams can separate as the adhesive can begin to fail after a number of years and as a result of weathering. Further the rubber membrane can begin to crack as the rubber begins to become degraded from long term UV exposure.
TPO as it reaches the end of its life cycle can crack or craze due to breakdown and weathering over decades.
Seam failures are not as typical with TPO unless as the result of workmanship; but details such as drains, and walls which can be reliant on adhesive bonding, can be more prone to become common failure points.
It is important to note that EPDM is also available with a white surface; however, the material is reported to be considerably more expensive. EPDM can also be applied using hot asphalt (felt backed EPDM), which can make it an economical re-roofing option for particular circumstances.
It is important to mention that where TPO is concerned; the product has evolved through the decades in the form of chemical modifications designed to mitigate early isolated failures of the assembly.
To consider is that most flat roofing products at one stage or another of their developed use, have also had to modify their products and processes to handle similar issues.
As examples; one-ply modified bitumen systems evolved into two-ply systems as we know them today.
Older single-ply “mods” were supposed to be coated with silver reflective paint to resist UV degradation and now the cap sheet of the assembly, utilizes ceramic granules to resist UV breakdown.
Asphalt in older BUR assemblies once contained a high concentration of coal tar pitch; which as a carcinogen; was phased out by the late seventies. While the old BUR’s lasted for decades, the newer asphalt versions are typically a twenty to the twenty-five-year system today.
The subject of commercial flat roofing is a large one with many other worthy water-proofing assemblies to be discussed.