When to use TPO and when to coat?

by Jennifer Wilson | February 12, 2016 12:13 pm

by Lee Martucci
Building owners, commercial roof specifiers, architects, and contractors have some decisions to make when it comes to the material they should use to repair or replace a low-slope roof. Three popular choices include:

• coating over the existing roof;
• thermoplastic polyolefin (TPO) recover (putting a layer of TPO over the existing membrane); or
• TPO replacement (tearing off the existing membrane and installing a new TPO membrane).

Whether one is using a coating or a TPO solution, it is integral the roof is sound and no moisture is trapped within the assembly. It is important to determine which option will be most effective for the property, and the present and future needs of the building. Further, the underlying roof needs to be dry; it cannot have residual moisture.

TPO has been specified as a roofing solution for more than 20 years. Its benefits can include:

• highly reflective surface;
• good performance in ponding situations;
• resistance to mold, dirt, and punctures;
• durability; and
• ease of installation.

Coatings are also a viable option as they:

• have a highly reflective surface;
• are easy to install;
• require no tear-off;
• are typically considered a maintenance item and can be written off as an expense in the first year; and
• are monolithic and seamless.

There are many factors to consider when deciding between a TPO roof or a roof coating for low-slope commercial building spaces. Both have their own advantages and disadvantages; generally, neither is better than the other. However, before making this important decision, one must consider the many factors surrounding roofing specification.

Budget
Every commercial project has a planned cost estimate, so it is important to consider budget when deciding which material to specify. However, one should consider future expenditures as well as the present budget. If cost is a major factor, TPO and coatings have some overlap on price depending on the system, material, and thickness option. These roofing options may be treated differently for tax purposes. One should consult a tax advisor to determine which system is more beneficial from a tax perspective.

TPO is considered a long-term investment with its extensive warranty. Therefore, it is important to look at the potential future savings. Even if the initial installation price is high, it may be worth paying more upfront for the right system and not worrying about the roof, since it is guaranteed to last more than 20 years.

Time of year
When a roof repair is needed, it cannot be delayed too long. TPO induction welding or mechanically attached TPO can be installed in colder temperatures. Fully adhered temperature limits are 4 C (40 F) and up. Most coatings must be installed at 10 C (50 F) and above. Since water-based coatings can freeze, it is important to ensure the temperature does not drop below a certain point before the coating fully cures (typically in 24 hours).

TPO can also be installed in hot weather. Most coatings require the roof surface to be 48 C (120 F) or less so the coating does not blister, which can easily occur on a hot summer day—especially with a dark roof.

Wind and rain
Weather is a large factor when dealing with roof repair—wind and rain can be an enemy to a roof repair job. If it is windy, TPO has an advantage over coatings because the spray application can be challenging to use on gusty days.

Additionally, a coating must have time to cure and become rain-resistant. Most acrylics take approximately 24 hours to cure, but there are other coatings, such as silicone and solvent-based, that will cure faster and are more resistant to an early rain.

Rain is not as big a problem for a TPO installation. With many TPO systems, workers can continue until just before the storm starts and resume when the weather improves—allowing for greater flexibility on the project and with the contractor’s time. Regardless of the system, contractors should always follow best practices by checking the weather forecast and planning appropriately.

Ponding water resistance
Potentially a big problem on commercial roofs, ponding is often not covered in the warranty. The ponded water serves as a breeding ground for fungus, mold, and algae, the buildup of which can more readily break down a soft, water-based coating like an acrylic than a substance like TPO, which has heat-welded seams. Most acrylic coatings are not recommended in ponding situations due to a relatively high absorption rate. Silicone and polyurethane are better in these applications due to their low absorption rates. Regardless of the system, proper drainage is key to minimizing ponding water and designing a lasting roof.

Hail resistance
In areas prone to hail, it is important to select a commercial roof material that can survive its impact. TPO has been studied extensively and offers good puncture resistance. Fully adhered TPO is a good option to protect against punctures. A fleece-back TPO—adhered in conjunction with a high-density polyisocyanurate (polyiso) cover board—provides enhanced resistance from punctures in areas prone to hail and has advanced protection against heat aging and UV weathering.

Coatings have also been shown to help protect a metal roof from damaging hail. This is due to its elastomeric properties, which protect the roof from the dings and dents hail can cause.

Speed
If the timeline is tight, TPO can be particularly versatile as it can be installed quickly by rolling out a 3-m (10-ft) wide sheet and welding all the seams. TPO also has many prefabricated accessories available to reduce the time spent field-fabricating details. However, if many penetrations are on a commercial roof, welding still needs to be done.

Coatings are self-terminating and have no seams. Therefore, if a roof has more penetrations, a coating can be put down quickly and be easier to install.

Investment/equipment
To install TPO on a commercial roof, a heat welder is necessary. This is a $10,000 to $15,000 investment. For small coating jobs, a roller will work, but a serious coatings contractor may want to invest in a sprayer, which typically costs $8000 to $12,000. One should account for expensive installation equipment or extra time when special equipment is not used. Each can increase the overall cost of the job.

Reflectance
Reflective roofs can help save energy costs by reducing the transfer of heat going into a building. It can also improve aesthetics and extend the life of the roof. Both TPO and coatings are similar in their reflective values, as they become a layer of UV and waterproofing on the roof. The difference is apparent when the roof needs insulation. If it does, it is best to put TPO over the insulation. For a solar installation, TPO is more advantageous, as it has a longer guarantee. One can get the most life out of both the roof and solar system in this case. Using a coating with a solar installation is possible if the system is mounted—allowing the roof to be recoated at any time.

Aesthetics
On a visible roof requiring some aesthetic work, an easy solution is using a cover board, TPO, and installing polypropylene honeycomb core sheets over it. If it is visible and aesthetics are a priority, coatings are a good option. If the roof has an existing coating and is visible, it should be recoated.

Fundamentally, more new roofs are going to be TPO—which is the better solution for longer-term warranty. A 35-year warranty can be reached with thermoplastic polyolefin, while coatings typically qualify for a 10- to 20-year warranty.

Lee Martucci is a senior product manager responsible for coatings and asphaltic products for GAF in Parsippany, NJ. He has more than 25 years of experience in marketing and product management where he has been focused on the development, launch, and management of building material products for both the business-to-business and business-to-consumer markets. Martucci holds a bachelor of science in business and economics from Lehigh University and earned the New Product Development Professional (NPDP) certification from the Product Development and Management Association (PDMA).

Source URL: https://www.constructionspecifier.com/when-to-use-tpo-and-when-to-coat/

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