RoofViews

Building Science

The Benefits of 12-Foot TPO — What You Need to Know

By Thomas J Taylor

November 10, 2020

RhinoPlate TPO installation

How Designers & Super Crews Can Maximize Roofing Efficiency

This case study looked at how a roof system designer, working with an expert crew, can both increase roofing efficiency and offer performance advantages to the building owner.

Case Study

Roof Project

Description of Building — The building used for this evaluation was single story big box type, less than 40 feet in height, with a roof area of 125,000 ft2 in a rectangular configuration 290 × 431 ft. The roof was assumed to be a new installation, i.e. new construction or a total roof replacement.

Roof Membrane — Two membranes were evaluated; 10 foot and 12 foot wide TPO. The 12 foot wide sheet has lower wind uplift resistance versus the 10 foot sheet for equivalent mechanical fastener patterns. In general, for wind uplift resistance of I-105 or higher, either induction welded or adhesive attachment would likely be required given common installation components and methods.

Membrane Attachment — Five scenarios were examined:

  • Case 1: 10 ft TPO mechanically attached
  • Case 2: 12 ft TPO induction welded attachment (RhinoBond)
  • Case 3: 10 ft TPO adhered, using solvent based adhesive
  • Case 4: 12 ft TPO adhered, using solvent based adhesive
  • Case 5: 10 ft TPO induction welded attachment (RhinoBond)

Polyiso Attachment — In each case, the polyiso was mechanically attached.

Fastening Patterns and Membrane Layout — For buildings with widths greater than 200 feet, regardless of height, the following is used to calculate the perimeter size:

The width of the perimeter region is defined as the least of the following two measurements:

0.1 x building width or 0.4 x building height

For this building, 0.1 x 290 = 29.0 ft and 0.4 x 40 = 16 ft. Therefore, the perimeter was set as 16 ft, meaning two boards of polyiso and three TPO half sheets.

The fastening patterns per 4 x 8 ft polyiso board varied by case as shown in the table below from GAF's Architectural Details Handbook:

handbook


Membrane Use — Every roof has its own unique challenges when laying out membrane. Good estimators, crews, and designers are skilled at arranging the membrane sheets to avoid unnecessary waste. For this case study, it was assumed that waste membrane is minimal and that differences between the 10 and 12 ft cases can be ignored. Membrane-use calculations were based on the roof area, and didn't include any parapet walls. All welds were assumed to have 6" overlap.

Fastener Use — For the mechanically attached Case 1, it was assumed that the screws and plates are 6" OC. For the RhinoBond and polyiso attachment, the table above was used.

Results

For each Case, the following were calculated for this 125,000 s.f. roof:

  • Total area of TPO required. This was the roof area plus the area required for edge and end seams.
  • Seam length, i.e. the total number of linear feet for all the seams.
  • Cost of screw and plate fasteners, including insulation fasteners, membrane fasteners, and RhinoBond plates when used.
  • Adhesive cost, based on a traditional solvent based adhesive.

Note that the material costs represent an average and will vary depending on region and job size etc.

Material Cost — The RhinoBond attachment combined with a 12 ft sheet is only slightly more expensive than a 10 ft mechanically attached (MA) sheet and is lower cost than the 10 ft RhinoBond system as shown here:


The difference in material usage is due to the reduced number of seams.

Labor Cost — While labor costs in monetary terms are hard to estimate, due to regional differences in labor rates, quantity of installers, experience levels of crews, etc, comparisons of the number of fasteners and seam lengths can indicate where savings are to be had.

The number of fasteners for each case is shown here:


It's clear that the traditionally low cost mechanical attachment process requires significantly more fasteners than any other approach.

The topic where a 12 ft sheet is expected to do well is in terms of total seam length, shown for each Case below:


As can be seen, Case 1 with its perimeter half sheets has a far longer total seam length than the other systems. Comparing Cases 2 and 4 versus Cases 3 and 5 shows the advantage of a 12 ft sheet. In this case study, it reduced seam length by about 20% compared to a 10 ft. sheet for adhered or RhinoBond attachment.

Overall Material Cost


Super Crew Benefits — the 12 ft. sheets reduce welded seam lengths by about 20%, which improves installation efficiency. However, in combination with RhinoBond a super crew can increase efficiency even more:

  • GAF Architectural Detail 307B allows the crew to use the deck sheet as flashing on walls and curbs when using RhinoBond attachment. This eliminates having to weld all of the wall flashings and some curb flashings, depending on the roof size. The crew no longer has to cut down a roll to be added later to flash in the walls and curbs. This reduces the risk of having to clean the deck membrane and makes it easier to close-in a section of the roof by day's end.


  • An experienced crew of the right size can lay out the membrane to eliminate many of the field hand welding, stop/starts, and angle change welds.
  • When roofing a section of a large roof, the main concern at the end of the day is making sure that section is watertight. Detail 307B enables more squares to be laid and secured by the end of the day. Every component of that section might not be completed but at least it can provide protection against moisture infiltration.

Conclusions

A 12 ft. TPO membrane combined with RhinoBond attachment offers several advantages:

  • Increased installation efficiency due to reduced seam length, reduced overall TPO use, and fewer fasteners compared to a traditional mechanically attached system.
  • Wind uplift performance is improved as compared with a traditional mechanically attached system, approaching that of adhered systems.
  • Warranty or guarantee length of a RhinoBond system may be longer than a traditional mechanically attached system.
  • Billowing caused by high wind events is significantly reduced or eliminated.


Note — this analysis was for a generalized roof and didn't include considerations of parapet wall areas or penetrations etc. Always do your own calculations and material usage estimates before making decisions about system design.

The author wishes to thank Mark Lienemann of the GAF CARE team for insights provided to this analysis.

About the Author

Thomas J Taylor, PhD is the Building & Roofing Science Advisor for GAF. Tom has over 20 year’s experience in the building products industry, all working for manufacturing organizations. He received his PhD in chemistry from the University of Salford, England, and holds approximately 35 patents. Tom’s main focus at GAF is roofing system design and building energy use reduction. Under Tom’s guidance GAF has developed TPO with unmatched weathering resistance.

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They have much longer service lives than, for example, finish materials, leading to sustained returns.Specifically, we looked here at how our example building's roof insulation impacted both embodied and operational carbon and energy use. To do this, we calculated the cumulative carbon savings over the 75-year life of our model building. In our example, we assumed R-30 insulation installed at the outset, increased every 20 years by R-10, when the roof membrane is periodically replaced.In our analysis, the embodied CO2e associated with installing R-30 (shown by the brown curve in years -1 to 1), the embodied carbon of the additional R-10 of insulation added every 20 years (too small to show up in the graph), and the embodied carbon represented by end-of-life disposal (also too small to show up) are all taken into account. About five months after the building becomes operational, the embodied carbon investment of the roof insulation is dwarfed by the operational savings it provides. The initial and supplemental roof insulation ultimately saves a net of 705 metric tons of carbon over the life of the building.If you want to see more examples like the one above, check out PIMA's study, conducted by the consulting firm ICF. The research group looked at several DOE building prototypes across a range of climate zones, calculating how much carbon, energy, and money can be saved when roof insulation is upgraded from an existing baseline to current code compliance. Their results can be found here. Justin Koscher of PIMA also highlighted these savings, conveniently sorted by climate zone and building type, here.Support for Carbon Investment DecisionsSo how can you make sure you address both operational and embodied carbon when making "carbon investment" decisions? We've prepared a handy chart to help.First, when looking at lower-embodied-carbon substitutions for higher-embodied-carbon building materials or systems (moving from the upper-left red quadrant to the lower-left yellow quadrant in the chart), ensure that the alternatives you are considering have equivalent performance attributes in terms of resilience and longevity. If an alternative material or system has lower initial embodied carbon, but doesn't perform as well or last as long as the specified product, then it may not be a good carbon investment. Another consideration here is whether or not the embodied carbon of the alternative is released as emissions (i.e. as part of its raw material supply or manufacturing, or "cradle to gate" stages), or if it remains in the product throughout its useful life. In other words, can the alternative item be considered a carbon sink? If so, using it may be a good strategy.Next, determine if the alternative product or system can provide operational carbon savings, even if it has high embodied energy (upper-right yellow quadrant). If the alternative has positive operational carbon impacts over a long period, don't sacrifice operational carbon savings for the sake of avoiding an initial embodied product carbon investment when justified for strategic reasons.Last, if a product has high operational carbon savings and relatively low embodied carbon (lower-right green quadrant), include more of this product in your designs. The polyiso roof insulation in our example above fits into this category. You can utilize these carbon savings to offset the carbon use in other areas of the design, like aesthetic finishes, where the decision to use the product may be discretionary but desired.When designing buildings, we need to consider the whole picture, looking at building products' embodied carbon as a potential investment yielding improved operational and performance outcomes. Our design choices and product selection can have a significant impact on total carbon targets for the buildings we envision, build, and operate.Click these links to learn more about GAF's and Siplast's insulation solutions. Please also visit our design professional and architect resources page for guide specifications, details, innovative green building materials, continuing education, and expert guidance.We presented the findings in this blog in a presentation called "Carbon and Energy Impacts of Roof Insulation: The Whole[-Life] Story" given at the BEST6 Conference on March 19, 2024 in Austin, Texas.References:Architecture 2030. (2019). New Buildings: Embodied Carbon. https://web.archive.org/web/20190801031738/https://architecture2030.org/new-buildings-embodied/ Carbon Leadership Forum. (2023, April 2). 1 - Embodied Carbon 101. https://carbonleadershipforum.org/embodied-carbon-101/

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September 18, 2024

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