Spray Polyurethane Foam Has Structure-strengthening and Energy-efficiency Capabilities

A high-performance building material, spray polyurethane foam (SPF) is widely used as an effective, lasting roofing solution. With positive benefits, including versatility, thermal insulation, resistance to inclement weather cycling and storms, strengthening of the building envelope, long life span and durability, spray foam has enjoyed increased use among builders and roofing contractors alike.

A roof’s primary purpose is to protect the structure underneath it. As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator. The lightweight material is ideal as a roofing solution when:

 As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator.

As a roofing material, closed-cell SPF acts as a protective roofing mechanism and a thermal insulator.

  • the roof substrate has many penetrations.
  • the roof deck is an unusual shape or configuration.
  • the roof is being applied to a structure located in a severe-weather environment.
  • a lightweight option is needed.
  • a slope application is preferred to provide extra drainage capabilities.
  • keeping the existing roof cover is desired.

STRENGTH AND DURABILITY

SPF is considered a highly durable building material. The physical properties of the foam change little with time, accounting for a life span up to 30 years with regular care and maintenance. SPF roofing systems also strengthen the roof in multiple ways. Roofing spray foams possess a compressive strength of 40 to more than 60 pounds per inch. Spray foam’s adhesion strengthening capabilities are key, especially in locations where severe weather cycling, storms, wind, hail and other conditions are prevalent and commonly cause structure damage. Coastal and hurricane-prone regions are prime examples.

When applied to the interior side of a roof, closed-cell SPF can increase a building’s resistance to wind uplift during severe storms. When SPF is applied to built-up roofing and metal substrates, it increases resistance to wind uplift even further. A study conducted by the University of Florida, Gainesville, in 2007 found that applying closed-cell spray foam under a roof deck provides up to three times the resistance to wind uplift for wood roof sheathing panels when compared to a conventionally fastened roof.

Spray foam is a good solution for unusual configurations and areas with many penetrations.

Spray foam is a good solution for unusual configurations and areas with many penetrations.

Spray foam also is resistant to progressive peeling failure. Caused by wind, peeling happens at the roof’s edges when wind pulls flashings and copings away from their installed positions. Peeling looks like a tin can after it has been cut around the perimeter. When this happens, a chain reaction may occur and lead to catastrophic building failure. After the roof membrane, panels or tiles pull away, the board-stock insulation is exposed, often with less resistance to the lateral and uplift wind forces. Then the sheathing below and the substructure are subject to movement and wind or water damage, potentially leaving the entire building interior underneath open and vulnerable. SPF roofing is continuous, so it provides a water-resistant layer that is well adhered to the substrate.

When the Gaithersburg, Md.-based National Institute of Standards and Technology examined roofs following Hurricane Katrina, it found buildings with spray-foam roofs performed rather well without blow-off of the SPF or damage to flashings. The 2006 “Performance of Physical Structures in Hurricane Katrina and Hurricane Rita: A Reconnaissance Report” found that only one of the examined SPF roofs incurred notable damage, and that damage was confined to only 1 percent of the total roof system. The report concluded spray foam kept the roofs intact, prevented moisture from entering the buildings, and protected the structures from hail and debris.

Hurricane Katrina played a significant role in one of the largest reroofing projects ever on one of the largest metal-framed domed structures in the world: the Superdome in New Orleans. Katrina destroyed the dome’s second roof; the structure’s original roof was constructed with polyisocyanurate foam covered with a fluid-applied elastomeric coating but was replaced in 1989 with a single-ply EPDM roofing system. After the damages suffered during Katrina, the EPDM roof system was replaced with a spray foam roof system.

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Rooftop Alterations, Like Skylights and Roof Monitors, Can Drive Building Value and Performance

Rooftops are an immensely underutilized resource for optimizing building performance. Rooftop strategies can include painting the roof white or installing a solar reflective “cool roof” to reduce summer cooling loads; covering the roof with vegetation to improve insulation, reduce storm-water runoff and provide community spaces; and mounting solar photovoltaic or solar hot-water panels to reduce utility bills.

The multiple functions of rooftop monitors. RENDERING: FCGA Architects

The multiple functions of rooftop monitors. RENDERING: FCGA Architects

Adding daylighting and ventilation through skylights and roof monitors is a strategy with growing popularity and potential. Common sense might lead us to believe that penetrating the roof with skylights and monitors could compromise a building’s insulation and thermal performance. However, with the availability of advanced products, such as glazing, suspended film and high-performance sealants, well-designed and constructed rooftop penetrations can successfully lower energy costs and improve occupant comfort and health.

Rooftop prescriptions vary for every individual project, and a variety of factors must be considered before proceeding with construction. For example, rooftop penetrations will primarily only affect the floor directly beneath the rooftop, so single-story buildings or multistory buildings with a central atrium are ideal. When further determining which types of projects would benefit from roof penetrations, the design team must perform thorough climatic analysis, examine the existing infrastructure and occupancy conditions, and weigh all variables through cost balancing. Before diving deep into analysis, it’s important to understand different types of rooftop penetrations in this capacity and how their design and operational synergies can enhance the value and performance of a building.

Design Synergies

Traditional skylights, tubular skylights and roof monitors are the main types of rooftop daylighting/ventilation penetrations and should be considered individually because of their varying benefits. Traditional skylights offer natural daylight, which can improve the health and productivity of building occupants. Tubular skylights capture sunlight from a small, clear dome on the roof; pass the light through a highly reflective tube; and diffuse the light through a lens into the building. Because of their high efficacy and smaller penetration area, tubular skylights have better thermal performance and are more suitable for harsher climates than traditional skylights.

Roof monitors are vertical fenestrations built into raised structures atop the roof. If the monitors are operational, they contribute exponential building-performance enhancements beyond the other penetration types, including stack-effect ventilation. The figure above depicts the many functions of roof monitors: natural daylighting, ventilation, passive heating and cooling, glare reduction and structural support for rooftop solar-power systems.

As with skylights, roof monitors help disperse natural daylight more evenly and completely throughout a room than windows on the side of a building. When paired with thermal mass, such as concrete or water, vertical glazing on the roof helps capture heat from the sun to offset the building’s heating load.

Glare presents a big problem for worker productivity in buildings; careful design of roof monitors and ceiling systems can help distribute the light and reduce contrast glare. Finally, monitors can be topped with angled roofing that matches the optimal sun exposure angle for solar panels mounted atop.

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Rooftop Equipment Mounting and Penetrations for Low-slope Standing-seam Metal Roofs

Standing-seam metal roofing offers a durable, sustainable alternative to other roof types and can provide maintenance-free service for five to 10 decades. Sadly, this exceptional lifespan often is sabotaged with the mounting of essential rooftop equipment and ancillary mechanicals.

Metal roofing can make use of special seam-clamping hardware that grips the standing seam without puncturing the membrane. Seam clamps have made metal roofing a preferred roof type for mounting photovoltaic solar arrays. PHOTO: Metal Roof Advisory Group Ltd.

Metal roofing can make use of special seam-clamping hardware that grips the standing seam without puncturing the membrane. Seam clamps have made metal roofing a preferred roof type for mounting photovoltaic solar arrays.

Regardless of the roof type involved, consultants generally agree that the best way to prevent roof-related problems is to clear the rooftop of everything possible and just let it function as a roof—not a mechanical equipment platform. However, such a perfect roof continues to elude us, as it becomes necessary or convenient to mount HVAC equipment, screens to hide it, piping to fuel it, scuttles to access it and walkways to service it. The list of rooftop mountings also may include plumbing vents, satellite dishes, lightning protection, snow retention systems, solar collectors, advertising signage and fall-protection systems to maintain all the foregoing. To help achieve relatively trouble-free roofs, this segment provides some basic understanding of the dos and don’ts in situations where rooftop equipment mounting is requisite.

Penetration-free Attachment

A good “first rule” about any rooftop mounting is to avoid penetrating the membrane whenever possible. While this may seem obvious, the tenet is often violated with standing-seam metal. The norm for attaching things seems to involve anchoring the item to the structure through the roof. When this happens, it not only threatens weather integrity, but can also violate the membrane’s thermal-cycling behavior by inadvertently pinning the panel to the structure. Such a point of attachment will fatigue and fail from forces of thermal expansion within a short time. Fortunately, scores of items and equipment can be securely mounted to metal rooftops without any penetration whatsoever, actually making metal roofing more user-friendly than other roof types.

In terms of mounting ancillaries, metal roofing can use special seam-clamping hardware that grips the standing seam without puncturing the membrane. Unlike many other types of roofing, metal is a rigid, high-tensile material. The seam area creates a beam-like structure that can provide convenient anchorage for walkways, solar arrays, condensing units and gas piping without harming the roof’s weathering characteristics. Mechanicals can be safely and cost-effectively secured to these seam clamps, leaving the roof membrane penetration free. Seam clamps can provide holding strength of up to several thousand pounds on some profiles and gauges, last the life of the roof and preserve thermal-cycling characteristics. Using seam clamps when possible for ancillary mounting will eliminate unwanted holes and other potential problems.

Seam clamps allow even cumbersome ancillary items to be attached to metal roofs without penetrating the rooftop. PHOTO: Metal Roof Advisory Group Ltd.

Seam clamps allow even cumbersome ancillary items to be attached to metal roofs without penetrating the rooftop.

Clamps should be made only of noncorrosive metals—typically, aluminum with stainless-steel mounting hardware. These metals are compatible with virtually anything found on a metal roof, except copper (with which there are dissimilar metallurgy issues). Dissimilar metals in electrolytic contact will induce galvanic corrosion of the less noble metal. In cases involving copper roofing, brass clamps should be used with stainless-steel hardware.

Seam clamps generally integrate with the profile and seam folding, and in some way “pinch” the seam material to anchor them in place. Preferred methods of doing this involve setscrews tightened against the seam causing a detent in the seam material that in turn creates a mechanical interlock of the setscrew, seam and clamp, providing the greatest holding strength and durability. Setscrews should have round, polished points to prevent galling metallic coatings, which can lead to corrosion. In like fashion, and regardless of the method of engagement, any clamp device should avoid any sharp points or nodes that could potentially pierce or gall metallic coatings of steel or cause fatigue and fracture points of other metals.

It also is important to remember that any loads introduced into the clamp will be transferred to the panels and their anchorage to the structure. Consequently, anchorage must be capable of withstanding the added load. The best practice is to utilize clamps that have been appropriately tested for material and seam-specific holding strength; be sure in-service load does not exceed that of the published holding strength, including factors of safety. The roof manufacturer should also be consulted with respect to approval of devices used.

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