RoofViews

Building Science

Wet Concrete Can Ruin a Good Design: Insights to Cement your Success

By Jennifer Keegan

March 25, 2021

Concrete roofing

With the misinformation swirling around the topic of moisture in concrete roof decks, it can be difficult to know the right approach to take to mitigate risk.

  • Are roof failures due to moisture in concrete primarily found in lightweight concrete decks?
  • Do vented decks alleviate moisture in concrete by facilitating downward drying?
  • Is 28 days the right amount of time to let a new concrete deck cure?
  • Are admixtures and MVRA's (Moisture Vapor Reduction Additives) effective in mitigating concerns around moisture in concrete roof decks?
  • Are vapor retarders the answer? What about vented base sheets?
  • What adhesives and insulation and cover board facers are appropriate to use in these roof assemblies?

It's well known in the industry that excessive moisture in concrete roof decks can lead to poor performance of adhesively installed roof assemblies. These performance issues can range from biological growth to roof failures. The industry is still learning about the causes of these failures, and what doesn't work to mitigate the issue. However, there are some basic facts that are often confused, and compounded by out of date information that is still in circulation. This article will attempt to debunk some of the myths.

How this started

Traditionally, when adhered systems were installed, insulation boards were adhered direct to the concrete deck with hot applied asphalt. The continuous layer of asphalt that bonded to the concrete deck functioned as a vapor retarder, mitigating the impact of latent moisture in the concrete from negatively impacting the roof system.

In the late 1990's, newer technologies became popular in the roofing industry, displacing the traditional asphaltic based application methods. Insulation boards were now able to be installed with solvent based and water based adhesives or low-rise foam adhesives.

Beginning in 2000, there was an increase in reported moisture related issues with roofs installed over new concrete roof decks, including moisture accumulation, adhesion loss, adhesive issues with water-based and low-volatile organic compound adhesives, metal and fastener corrosion, insulation R-value loss and microbial growth1. Compounding the problem, restrictions on VOC content in materials resulted in the use of more water based adhesives, which can be "much more susceptible to re-emulsification when exposed to moisture, depending on the adhesive formulation."2

Fact or Fiction

"Moisture in concrete is only an issue with lightweight structural concrete."

As the industry began to pay attention to increased reports of moisture related issues with roofs installed over new concrete roof decks in the 2000's, it was noted that many of these roof failures were primarily over lightweight structural concrete roof decks.

Lightweight aggregates, which are typically an expanded shale, can hold more initial water than traditional 'hard rock' aggregate found in normal weight concrete. According to the NRCA, lightweight aggregates absorb 5 to 25 percent water by weight, where normal weight concrete aggregates typically absorb less than 2 percent water by weight. NRCA's calculations3 indicate that after the 28 day cure time, nearly 3 times the amount of free water can be present in a 6-inch lightweight structural concrete deck than a normal weight concrete deck.

Figure 1: Water added to and remaining in example concrete mixes

FM Global's Loss Prevention Data Sheet (LPDS) 1-29 provides additional requirements for roof systems over lightweight structural concrete, stating that "a great deal of moisture will be released for several months after the concrete has hardened and will be absorbed by above-deck components. This will damage and weaken those components, resulting in damage from winds below design speeds, or premature deterioration requiring replacement."4

While the increase of moisture related issues were initially associated with lightweight structural concrete, by 2015 the number of lightweight structural concrete cases were proportional to normal weight structural concrete cases, indicating that the extra free evaporative moisture in the lightweight aggregate was not the sole cause of the moisture related roof failures.5 Therefore, both normal-weight and lightweight concrete types are now known to be a risk factor for new roof systems. It should be noted, however, that the NRCA and some manufacturers still recommend avoiding lightweight concrete in roofing applications.

"Vented composite decks significantly enhance downward drying."

The use of non-removable forms or composite steel and concrete decks has significantly increased over the past 30 years. These decks allow for much shorter construction timelines, eliminating much of the expense of building and stripping temporary form structures, and are structurally efficient. However, by leaving the steel formwork in place at the bottom of the concrete slab, the concrete can not dry to the underside. Therefore, the roof deck retains a significant amount of water as there are very limited pathways to allow for drying out.

There is an option to vent the steel form deck, which may facilitate some amount of downward drying. However, according to the IIBEC Technical Advisory Bulletin on Roof Covering Systems and New Concrete Roof Decks (IIBEC TA-020-2021), "steel pan decks, whether vented or non-vented, function essentially as a vapor retarder located beneath the concrete, which significantly reduces the amount of drying that can occur from the bottom side of the concrete."

Figure 2: The actual venting achieved by vented metal decks is not yet quantified.

Vented metal decks may allow for some drying of the concrete deck. However, there is no published data to quantify the actual venting achieved. It is presumed to be of "minimal value with respect to downward drying."2 In fact, FM Global's LPDS 1-29 requires additional design considerations beyond a vented metal deck stating that "it will have limited impact on moisture reduction." And, the Steel Deck Institute's Position Statement titled, Venting of Composite Steel Floor Deck" states "The steel deck acts as a vapor barrier...a hypothetical 1.5% open area will increase the diffusion of water by 1.5%, an inconsequential amount."

"Concrete Decks are ready to roof after a 28 day cure"

Water takes a long time to diffuse out of what is typically a four to six inch-thick composite concrete deck. The inherent moisture migration in the concrete structure is exacerbated by new concrete mix designs and the cascading adhesion problems that can occur in roof systems. Significant amounts of water remain after curing is completed, even moreso when lightweight aggregates are incorporated into the mix design.

The historical 28-day cure period for concrete is to achieve appropriate concrete strength and has little significance or correlation to the moisture contained within the concrete. In concrete roof decks, there is very little correlation between cure time and the amount of water remaining. Guideline "rules of thumb," such as not installing the roof system until a minimum of 30 days after pouring and forming, are not particularly effective at reducing or eliminating issues. Assembly design such that moisture doesn't enter the roof assembly is key versus rules of thumb.

"The plastic mat test is the most effective way to evaluate dryness"

Currently, there is no consensus within the roofing industry on an acceptable standard for evaluating the moisture content nor acceptable moisture content levels in concrete roof decks.

Historically, three tests were generally used by the roofing industry to evaluate concrete roof decks. These qualitative test methods were thought to provide visual evidence of unacceptable moisture levels.

  1. The Plastic Mat Test (ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method)
  2. Hot Asphalt Pour and Peel Test (NRCA Test Method)
  3. Calcium Chloride Dome Test (ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride)

The plastic mat test involves securing a plastic sheet to the roof deck and observing condensation accumulation. The Pour and Peel test method involves pouring hot asphalt onto the roof deck, looking for bubbles or frothing, and examining the quality of the adhesion. And the Calcium Chloride test involves placing a canister of the powder under a plastic dome and measuring the amount of water evaporation collected over a 24 hour period.

plastic mat test

Figure 3: The plastic mat test, the hot asphalt pour and peel test, and the calcium chloride dome test are no longer considered reliable to evaluate the dryness of concrete decks for roofing purposes.

These evaluation methods are only reflective of surface moisture and are not effective in detecting moisture levels in lower portions of the concrete slab.2 The industry no longer considers these tests reliable to evaluate concrete roof decks.

The standards and acceptable threshold levels used by the flooring industry do not directly translate to roofing as the concrete roof slab does not exist within the conditioned space and is exposed to the elements. The flooring standard requires conditioning of both the concrete slab and the air above it to a constant service temperature and relative humidity for at least 48 hours. However, this is not feasible for a roof deck. Additionally, the moisture content of the concrete currently exceeds the measurement capability of the in-situ probes used in ASTM F2170.3

"Admixtures / MVRAs will prevent moisture related issues"

Moisture Vapor Reducing Admixtures (MVRAs) are concrete admixtures intended to address moisture in concrete by effectively shutting down moisture vapor movement through the concrete, and MVRAs purportedly deliver a slab that requires no further moisture tests, and no additional topical moisture mitigation systems.6

The concept is to turn the concrete slab into a vapor retarder, slowing the release of its own moisture into the roof system. However, there currently is little to no technical data to substantiate marketing claims made by the vendors of MVRAs nor their ability to lower the water vapor transmission of the concrete by a significant amount.

NRCA partnered with RDH Building Science Laboratories7 to conduct some roofing industry specific research on MVRAs. This study shows permeability values of the concrete roof deck cores with MVRA to be greater than those without MVRA. These test results contradict claims that MVRAs minimize the concrete's ability to release moisture vapor.

To date, MVRAs have been shown in the laboratory and field to have no effect on moisture issues in roof systems. Therefore, their use is not recommended to address the moisture in concrete concerns for roof decks.3,7

Are vapor barriers the answer?

Vapor retarders are designed to reduce vapor diffusion. A class 1 vapor retarder has a perm rating of 0.1 perms or less, and is often referred to as a vapor barrier. A vapor retarder, installed against the concrete, is necessary for expected roof system performance in all but ASHRAE Zone 1.3

Do you specify a class 1 self-adhered vapor retarder over your concrete deck, and believe this has mitigated the risk of moisture in concrete?

While the initial bond of the self-adhered vapor barrier to a "wet deck" may result in acceptable adhesion, what happens when it's exposed to upward vapor drive from curing concrete for a year, or two, or five? It is possible that the bond would be insufficient over time as the moisture migrates out of the roof concrete slab over a period of years, as demonstrated by the SRI Research Report.

To make matters worse, often subsequent layers of roof insulation and roof membrane are also adhered together, relying on the initial self-adhered vapor barrier's bond to the "wet deck" to attach the entire roof system. Insulation facers can delaminate from the substrate or the insulation core and membranes that appear to be initially adhered can lose adhesion due to moisture migration.

While vapor barriers can be a necessary part of roof systems installed over concrete decks, the traditional "rules of thumb" may not be enough. The concerns around long-term adhesion to the concrete deck are leading contractors and designers to consider additional steps to keep the roof system attached to the building. In a recent article,8 the authors provide six alternate design configurations and attachment methods to navigate various scenarios and the attachments, assembly layers, and the fundamental physics that are at work across the interconnected structure and roof systems. Below in Figure 4 is an example of one of the scenarios provided, with the non-traditional design elements highlighted in red for clarity.

figure 4

Figure 4. Example of added roof complexity from evolving structural design

Today's Best Practice

There is certainly a lot of complexity and confusion swirling around moisture in concrete as it relates to roofing. However, there are a few useful industry guidelines to leverage, including the NRCA Roofing Manual, the IIBEC Technical Bulletin, and the Moisture in New Concrete Roof Decks research report by SRI Consultants.

  • Determine in the design phase if the roof deck needs to incorporate concrete for structural, fire, or other reasons. If not, consider a metal deck with a compact roof system above
  • Limit the water-cement ratio for both normal weight and lightweight concrete
  • Pour concrete onto a strippable form when possible; or at least a vented metal deck
  • Allow adequate time in the construction schedule for initial slab drying and protect the slab from re-wetting where possible
  • Utilize a vented base sheet above the concrete slab where possible
  • Install a class 1 vapor retarder (less than 0.01 perms) direct to the concrete deck
  • Eliminate the use of water-based primers or adhesives for the insulation and roof membrane attachment, such as using a low-rise foam adhesive
  • Consider mechanical fastening the roof assembly into the concrete deck
  • Use of coated glass-faced insulation boards and coverboards in lieu of paper faced boards

Specify a roof manufacturer's specific assembly that is designed and tested for wind uplift resistance over concrete decks. Additionally, consider the long term adhesion performance of the vapor barrier when the assembly relies on the adhesion for wind uplift resistance.

Sources Used

1Professional Roofing Magazine – Moisture in Concrete Roof Decks by Mark Graham; February 2017

2IIBEC Technical Advisory Bulletin Roof Covering Systems and New Concrete Roof Decks 02-2021

3Professional Roofing Magazine – What You Can't See Can Hurt You by Mark Graham; August 2012

4https://www.fmglobal.com/-/media/Documentum/Data-Sheet-Individual/01-Construction/FMDS0129.pdf?isGated=true&itemId={A6F00C1A-1B78-40C0-A72E-CEB0702FD38F}

5Moisture in New Concrete Roof Decks research report by SRI Consultants

6ISE Logik Industries MVRA 900 marketing literature

7Professional Roofing Magazine – Putting it to the Test by Mark Graham; February 2020

8Interface Magazine – Structural Concrete Decks, Vapor Retarders, and Moisture – Rethinking What We Know by Helene Hardy Pierce and Joan Crowe; February 2020

About the Author

Jennifer Keegan is the Director of Building & Roofing Science for GAF, focusing on overall roof system design and performance. Jennifer has over 20 years of experience as a building enclosure consultant specializing in assessment, design and remediation of building enclosure systems. Jennifer provides technical leadership within the industry as the Chair of the ASTM D08.22 Roofing and Waterproofing Subcommittee; and as an advocate for women within the industry as the educational chair for National Women in Roofing and a board member of Women in Construction.

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By making our buildings more robust against wind uplift to meet updated standards, we are in effect making them less robust against the negative effects of hot and cold weather conditions.So, how bad is this problem, and what's a roof designer to do about it? A team of researchers at SGH, Virginia Tech, and GAF set out to determine the answer, first by simplifying the problem. Our plan was to develop computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. In other words, we broke the problem down into parts, so we could know how each part affects the problem as a whole. We also wanted to carefully check the assumptions underlying our computer simulation and ensure that our results matched up with what we were finding in the lab. The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these: Roof fasteners have a measurable impact on the R-value of roof insulation. High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners. Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches: Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes. Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study. Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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