Better Understand Why the Combination of Moisture and Concrete Roof Decks Is Troublesome

The primary function of a well-built and well-designed roofing system is to prevent water from moving through into the building below it. Yet, as the Rosemont, Ill.-based National Roofing Contractors Association has observed, an increasing number of “good roofs” installed on concrete roof decks have failed in recent years. Blistering, de-bonding and substrate buckling have occurred with no reports of water leakage. Upon investigation, the roofing materials and substrates are found to be wet and deteriorated.

Wagner Meters offers moisture-detection meters for concrete. The meters are designed to save time and money on a project or job site.

Wagner Meters offers moisture-detection meters for concrete. The meters are designed to save time and money on a project or job site.

Why is this? One potential cause is trapped moisture; there are numerous potential sources of trapped moisture in a structure. Let’s examine the moisture source embedded within the concrete roof deck.

WHY DOES THIS MOISTURE BECOME TRAPPED?

It often starts with the schedule. In construction, time is money, and faster completion means lower cost to the general contractor and owner. Many construction schedules include the installation of the roof on the critical path because the interior building components and finishes cannot be completed until the roof has been installed. Therefore, to keep the project on schedule, roofers are pressured to install the roof soon after the roof deck has been poured. Adding to the pressure are contracts written so the general contractor receives a mile- stone payment once the roof has been installed and the building has been topped out.

Historically, roofers wait a minimum of 28 days after the roof deck is poured before starting to install a new roof. This is the concrete industry’s standard time for curing the concrete before testing and evaluating the concrete’s compressive strength. Twenty-eight days has no relation to the dryness of a concrete slab. Regardless, after 28 days the roofer may come under pres- sure from the general contractor to install the roof membrane. The concrete slab’s surface may pass the historic “hot asphalt” or the ASTM D4263 Standard “plastic sheet” test, but the apparently dry surface can be deceptive. Curing is not the same as drying, and significant amounts of water remain within a 28-day-old concrete deck. Depending on the ambient conditions, slab thickness and mixture proportions, the interior of the slab will likely have a relative humidity (RH) well over 90 percent at 28 days.

FROM WHERE DOES THE WATER COME?

Upon placing the concrete slab, the batch water goes to several uses. Portland cement reacts with water through the hydration process, creating the glue that holds concrete together. The remaining water held in capillary pores can be lost through evaporation, but evaporation is a slow, diffusion-based process. The diffusion rate of concrete is governed by the size and volume of capillary pores which, in turn, are controlled by the water/cement (w/cm) ratio. The total volume of water that will be lost is controlled by the degree of hydration, which is primarily related to curing and w/cm.

A 4-inch-thick concrete slab releases about 1 quart of water for each square foot of surface area. If a roof membrane is installed before this water escapes the slab, it can become trapped and collect beneath the roof system. The water does not damage the concrete, but it can migrate into the roofing system—and that’s when problems begin to occur. For instance, moisture that moves into the roofing system can:

  • Reduce thermal performance of the insulation.
  • Cause the insulation, cover board, adhesive or fasteners to lose strength, making the roofing system susceptible to uplift or damage from wind, hail or even foot traffic.
  • Lead to dimensional changes in the substrate, causing buckling and eventually damaging the roof membrane.
  • Allow mold growth.

A number of factors compound the problem. In buildings where a metal deck is installed, moisture cannot exit the slab through its bottom surface. Instead, the moisture is forced to exit the slab by moving upward. Eliminating one drying surface almost doubles the length of drying time of a concrete slab. The small slots cut in ventilated metal decking have little effect on reducing this drying time.

Ambient conditions also affect the drying rate of a concrete slab since it readily absorbs and retains moisture. Additional moisture may enter an unprotected roof slab from snow cover, rain or dew. Even overcast days will slow the rate of drying.

A MODERN-DAY PROBLEM

Before the introduction of today’s low-VOC roofing materials, historic roof systems didn’t experience as many of these moisture issues. Typically, they were in- stalled onto concrete decks on a continuous layer of hot asphalt adhesive that bonded the insulation to the deck. This low-permeable adhesive acted as a vapor retarder and limited the rate of moisture migrating from the concrete into the roofing assembly. As a result, historic roof systems were somewhat isolated from moisture coming from the concrete slab.

Many of today’s single-ply roof membranes are not installed with a vapor retarder. Moisture is able to migrate from the concrete slab into the roof materials. Modern insulation boards are often faced with moisture-sensitive paper facers and adhered to substrates with moisture-sensitive adhesives. These moisture-sensitive paper facers and adhesives are causing many of the problems.

Rene Dupuis of Middleton, Wis.- based Structural Research Inc. recently presented a paper to the Chicago Roofing Contractors Association on the subject. Some of his findings include the following:

  • Due to air-quality requirements, government regulations curtailed the use of solvent-based adhesives because they are high in VOCs. Consequently, manufacturers changed to water-based adhesives because they are lower in VOCs, have low odor, are easy to apply and pro- vide more coverage.
  • There can be several drawbacks to water-based bonding adhesives. One is that they may be moisture sensitive. Moisture and alkaline salts migrating into roof systems from concrete decks can trigger a negative reaction with some water-based adhesives. This reaction can cause the adhesives to revert to a liquid, or it may alter or delay the curing of some foam-based adhesives. Some adhesive manufacturers have recognized these problems and have be- gun reformulating their adhesives to address these drawbacks.
  • Negative reactions also occur when moisture-sensitive paper facers come into contact with moisture. This reaction typically results in decay, mold growth and loss of cohesive strength. Moisture in the roof system may also cause gypsum and wood-fiber-based cover boards to lose cohesive strength.

Dupuis noted moisture from any source can compromise adhered roof systems with wind uplift when attached to paper insulation or gypsum board. He also said facer research clearly shows paper facers suffer loss of strength as moisture content increases.

PHOTOS: Wagner Meters

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Using Engineered Geofoam for Garden Roofs

For most of the past century, the rooftops of commercial and institutional buildings have largely been places to locate unsightly mechanical systems. Architectural treatments, such as parapets and screens, provide visual relief from such equipment. Now, roofing professionals and building owners increasingly look at the roof as “found space”—a place to be planted and used, instead of hidden.

Facebook’s Frank Gehry-designed MPK 20 building sports a 9-acre green roof using EPS geofoam from Insulfoam.

Facebook’s Frank Gehry-designed MPK 20 building sports a 9-acre green roof using EPS geofoam from Insulfoam.

Throughout the U.S., garden roofs (or living roofs) are growing in popularity with more than 5.5 million square feet installed in 2014, according to Green Roofs for Healthy Cities. Most of that total was for private rather than public projects, indicating this is not just a government trend. In addition to providing attractive and usable open space, garden roofs offer environmental benefits, such as helping to slow and filter urban run-off.

Some of America’s largest companies have installed green roofs. Ford’s Dearborn, Mich., truck plant final assembly building sports one of the world’s largest living roofs at 454,000 square feet. In 2015, Facebook opened its MPK 20 office building in Menlo Park, Calif., with a 9-acre living roof featuring a 1/2-mile walking trail and more than 400 trees.

If you haven’t worked on a garden roof yet, it is likely only a matter of time until you do.

Addressing the Challenges of Garden Roofs

Weighing a fraction of soil, EPS geofoam fill creates ultra-lightweight landscaped features on Facebook’s garden roof.

Weighing a fraction of soil, EPS geofoam fill creates ultra-lightweight landscaped features on Facebook’s garden roof.


Adding plants and park-like amenities to a roof increases the complexity of the roofing assembly. Garden roofs present two primary challenges for roofing professionals to solve: minimizing the dead load and preventing moisture intrusion.

The project team for the Facebook MPK 20 building’s green roof met this two-fold need—and more—with expanded polystyrene (EPS) geofoam.

Weighing considerably less than soil, EPS geofoam is an ultra-lightweight engineered fill that can be used to create contoured landscape features, such as hills and valleys. The material weighs from 0.7 to 2.85 pounds per cubic foot, depending on the product type specified, compared to 110 to 120 pounds per cubic foot for soil.

Despite its low weight, EPS geofoam is designed for strength and has better load bearing capacity than most foundation soils. Geofoam’s compressive resistance ranges from approximately 2.2 psi to 18.6 psi (317 to 2,678 pounds per square foot) at a 1 percent deformation, depending on the product.

The garden roof on Facebook’s MPK 20 building provides ample open space and a half-mile walking trail for employees.

The garden roof on Facebook’s MPK 20 building provides ample open space and a 1/2-mile walking trail for employees.

EPS geofoam is also effective at addressing the second challenge of garden roofs: managing moisture absorption. The moisture performance of the various components in a green roof assembly is critical; retained water imposes additional loads on the roof and increases the risk of water damage to the roof assembly. EPS geofoam meeting ASTM D6817 standards works well here as it only absorbs 2 to 4 percent moisture by volume, even over long-term exposure, and it dries quickly. The moisture performance of EPS has been demonstrated in extensive in-situ applications and real-world testing, including research conducted by the U.S. Army Corps of Engineers’ Cold Regions Research and Engineering Laboratory. After burying EPS in wetted soil for nearly three years, the lab found that the material absorbed only 1.7 percent moisture by volume.

In addition to enabling lightweight, durable landscape features and helping to defend against water, EPS geofoam provides thermal insulation in garden roofs. Roofing professionals have used EPS insulation in roof assemblies for decades because it offers the highest R-value per dollar among rigid foam insulations.

Expect More Demand

Although green roofs currently account for a small portion of the billions of square feet of roofs in the U.S., expect to see more demand for them given their aesthetic and environmental benefits. High-performance materials, like EPS geofoam, can help provide a long-lasting, durable green roof assembly.

PHOTOS: Insulfoam

Spray Polyurethane Foam: A Key Component to Any Net Zero Solution

SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

In July 2014, California initiated the revision process to the 2016 version of Title 24, California’s building energy efficiency codes, which are designed to move the state’s residential and commercial buildings toward zero net energy (ZNE). All new residential construction is to be ZNE by 2020, and all new commercial buildings are to achieve ZNE by 2030. While aggressive, these goals are achievable with the right design implementation and accessibility to proper building materials.

As one of the world’s most influential economies, the state of California has demonstrated its power in leading the other 49 states in the implementation of progressive initiatives. California traditionally takes an environmental stance with a history of enforcing regulations designed to protect the physical environment and health of the state’s residents. These efforts often result in national trending with other states and municipalities following suit with similar regulations. It is widely anticipated a similar phenomenon will occur with ZNE goals.

The design of a ZNE building focuses on the reduction of energy consumption and on the generation of the structure’s own renewable energy (such as via solar panel solutions). Long-term ZNE begins with a quality building enclosure. High-performance attics and wall systems are a key focus of 2016 Title 24 as they make a significant impact in the reduction of peak cooling demand in structures.

SPF may be installed in a continuous layer, eliminating thermal bypasses, and boasts one of the highest R-values of all insulation options.

SPF may be installed in a continuous layer, eliminating thermal bypasses, and boasts one of the highest R-values of all insulation options.

Because of spray polyurethane foam’s unique attributes, the material is widely recognized as an optimal solution for unvented attics, as well as for roofing, walls and ceilings. SPF has the ability to insulate, air and water seal, as well as control moisture throughout the structure, acting as a single-source solution, reducing the need for multiple products.

Energy loss may occur at various points throughout the roof, walls and ceiling via air leakage. Thus the air-sealing ability of SPF is extremely beneficial when trying to improve energy efficiency.

In roofing, SPF acts as a protective roofing solution and as an insulator.

In roofing, SPF acts as a protective roofing solution and as an insulator.

As a thermal insulator, SPF forms in place and fully adheres, almost completely eliminating the cracks and gaps that allow escape of conditioned air. It may be installed in a continuous layer, eliminating thermal bypasses typically found with cavity insulations and boasts one of the highest R-values of all insulation options.

In roofing, SPF acts as a protective roofing solution and as an insulator. The effectiveness of insulation is measured through moisture control, air leakage, health, safety, durability, comfort and energy efficiency factors, and SPF scores exceptional marks in all.

These combined characteristics are integral to SPF’s ability to contribute to total ZNE solutions—solutions, which will become increasingly necessary as the net zero revolution takes hold across the U.S.

Scan for Moisture in a Variety of Membranes and Insulation Thicknesses

Tramex Dec Scanner

Tramex Dec Scanner

The Tramex Dec Scanner is a mobile non-destructive impedance moisture scanner designed for the instant surveying of moisture conditions in roofing and waterproofing systems. Three ranges of sensitivity enable the inspection of a variety of roof membranes and insulation thicknesses. By moving the Dec Scanner across a roof surface in a regular pattern, a continuous reading is obtained and areas that contain moisture can be readily identified. It includes an ergonomic, easy-to-reach and easy-to-operate control panel.

Protect Roof Sheathing from Moisture

Proof Dura-Felt from Insulation Solutions

Proof Dura-Felt from Insulation Solutions

Proof Dura-Felt from Insulation Solutions Inc. is designed to protect roof sheathing from moisture penetration. It provides an essential layer of protection beneath the roofing material, acting as a secondary barrier to moisture that might otherwise be absorbed into the sheathing.

The 10-mil product is composed of 8×8 HDPE laminated to point-bonded PP-coated with high COF co-polymers for traction. At 4.75 ounces per square yard, it offers a tensile strength that meets ASTM D751.