The Success of Your New (Replacement) Roof Depends on Adjacent and Connected Elements, including Masonry

Although the name of this publication is Roofing, the roofing/waterproofing/construction industry recognizes more and more that the building envelope is a fully integrated and interrelated assembly of systems.

masonry cracks due to freeze thaw

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As such, I feel the need to discuss the importance of water resistance and structural integrity in existing wall surfaces, which are adjacent and connected to your project’s new (replacement) roof system. The focus of this article is not how to design a replacement roof system but how to address adjacent masonry to ensure it doesn’t work against the success of the new roof.

These principles actually apply to any wall system that connects, generally above and adjacent, to your roof, but masonry poses some distinct concerns. Water intrusion, thermal movement and structural integrity of this masonry, along with locations of embedded flashing, all come into play as the new roof system is properly integrated into the adjacent rising wall, parapet wall or even perimeter edge wall beneath the roof.

COMMON MASONRY ISSUES

Thomas W. Hutchinson, AIA, FRCI, RRC, a regular Roofing contributor, has said, “long-term service life is the true essence of sustainability”. Moreover, designers specify (for owners to buy) warranties of 20, 25 years or more with new roof systems. It’s just good common sense that you can’t allow a new roof to be jeopardized by water intrusion from an adjacent system because of an oversight in the original analysis of the situation.

Many of us have been called by an owner who says his or her new roof is leaking, only to find roof-mounted equipment or an unrelated system is actually leaking. However, if the leak is stemming from another aspect of the building envelope, such as an adjacent parapet or rising wall, which is now jeopardizing the investment made on a new roof, that you (the designer) should have foreseen, it makes for a very difficult position. The roofing system manufacturer, who holds the warranty, and the owner are going to look at you as being responsible.

masonry

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Let’s examine three common occurrences using actual case studies. All three situations, which occurred on schools in the Northeast, exemplify the condition of adjacent masonry was deficient and had to be corrected, adding a significant degree of scope and cost to the project to guarantee a roof design that would perform over the long haul. These three cases cover:
1. Repairing the masonry and covering it.
2. Altering the masonry to change the location of embedded flashings.
3. Replacing structurally unsound/failed masonry with another material.

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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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Several ‘Sandwich’ Roof Assemblies Mitigate Sound Transfer

We all want a roof over our heads to protect us from the cold winter months, hot summer months and precipitation year round. How much thought goes into the sound-control construction of a roof, though? Have you considered the acoustic properties of your roofing system? Admittedly, acoustics is not a topic that many roofing contractors think about. The construction of a roof, however, can have a significant impact on the sound quality of the building interior. While this may not seem important in every project, it can be a critical element of the design for concert halls, theaters, auditoria and even school classrooms.

Sound Isolation

The acoustics of a space depend on many criteria, including sound isolation, sound reflection, impact noise and sound transfer. In many cases, particularly in noisy, urban environments, there is a need to prevent loud outside noises, such as traffic, sirens and airplane noise, from entering quiet spaces. Sound isolation depends on the entire envelope of a space, including external walls, windows and roofs.

Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective sound control.

Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective
sound control.

Historically, roofs over sound-sensitive spaces have been built with fairly dense materials, such as concrete, which by themselves are relatively effective in blocking sound transfer. As construction methods have developed, however, more lightweight construction is being used. If thought and care are not given to the assembly, these lightweight construction methods can cause serious issues with acoustics. Rain noise, mechanical noise and other exterior sounds can all transfer readily through a thin, lightweight roofing system.

In an effort to use lighter-weight construction, a “sandwich” assembly may be used to mitigate sound transfer. Similar to an Oreo cookie, a sandwich assembly’s outer layers are comprised of a heavy, dense material, and the inner filling consists of insulation and/or airspace. The materials of this assembly can differ from concrete to roofing board, rigid insulation to fibrous insulation, gypsum board to acoustic ceiling tiles. The components can be combined in a variety of ways, each with varying levels of sound isolation.

One of the principle phrases often heard when discussing sound isolation is “mass air mass”, which refers to the separation of two bodies of mass by an air space. The greater the mass and the deeper the air space, the more sound isolation will result. For this reason, a heavy mass, such as 5-inch concrete, followed by a deep air space, such as an 18- to 24-inch ceiling cavity in which ducts are run, followed by a continuous layer of drywall ceiling will provide a high level of sound isolation. Additional steps, like adding sound-absorptive material to the air space and/or using resilient connections when supporting drywall, further improves the sound isolation of the assembly.

Sandwich Roof Assemblies

Several sandwich roof assembly approaches are possible, including:

Good: Multiple layers of dense roofing board (at 2.5 psf per board, a final density of 10 psf or four-ply is often recommended) on either side of insulation, which ideally would be a sound-absorptive fibrous fill, like mineral wool, can reduce sound transmission. This approach is similar to a “floating floor”, often used in interior spaces to isolate sound transfer from one room to another. (Equivalent Sound Transmission Class, or STC, ratings can range from low 50s to low 60s, depending on whether a ceiling is included below the deck.)

Drywall ceilings hung on resilient hangers in conjunction with a lightweight roofing system provide even greater sound isolation by virtue of the resilient connection or “decoupling” of the drywall layer from the rest of the building structure.

Drywall ceilings hung on resilient hangers in conjunction with a lightweight roofing system provide even greater sound isolation by virtue of the resilient connection or “decoupling” of the drywall layer from the rest of the building structure.

Good: Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective sound control. These can be part of a sandwich approach with airspace or rigid insulation between soil and a more-dense roofing material, similar to the roofing board described in the previous example. The mass-air-mass combination is similar to the approach just mentioned, and the benefits of green roofs appeal to many building owners for a multitude of reasons, including minimizing urban heat islands and storm-water management.

Good: A 5-inch slab of normal-weight concrete (150 pcf) has a density of 62 psf. This tried-and-true method is still used regularly and often proves to be the most cost-effective method of enclosing a space. The best sound isolation will occur if this is used in conjunction with a ceiling below, but on its own it still provides a reasonable level of isolation in many environments. This isn’t technically a sandwich system unless paired with a ceiling below or a green roof above. (Equivalent STC ratings can range from low 50s to low 80s. The highest ratings require pairing a resiliently hung ceiling with the concrete, as described under “Multi-function Roof Assemblies”.) IMAGES: Threshold Acoustics LLC [Read more…]

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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Are You ‘PV Ready’?

Commercial rooftops are an attractive platform for the installation of solar photovoltaic (PV) electricity-producing systems. These low-slope roofs offer an economical and sustainable structural foundation for renewable solar energy. As an example, one of the largest roof-mounted PV systems in North Carolina has been online for several months at the Old Dominion Freight Line Inc. vault logistics facility in Thomasville. Almost 7,700 solar panels completely cover the warehouse’s 160,000-square-foot roof and produce enough power (1.8 megawatts) to offset more than 90 percent of the building’s annual energy costs.

Success stories like Old Dominion’s are becoming increasingly common in the sunny Carolinas. However, it is important to remember a roof’s function is, first and foremost, to protect the building’s contents and people from the elements. In this regard, roofing professionals need to anticipate the potential risks associated with the installation of a roof-mounted PV system (array). This sort of due diligence is particularly important when installing PV systems on existing warranted roofs.

A broad selection of membranes and thicknesses are available for consideration when a PV installation is planned. Photo courtesy of GAF, Wayne, N.J., and Protech Roofing Service, San Diego

A broad selection of membranes and thicknesses are available for consideration when a PV installation is planned. Photo courtesy of GAF, Wayne, N.J., and Protech Roofing Service, San Diego

To help in these industry efforts, members of Waltham, Mass.-based SPRI—the trade association that represents sheet membrane and component suppliers to the commercial roofing industry—have developed “PV Ready” roof assemblies and guidelines designed to provide maximum protection for the roof (and maintain its warranty coverage).

In September, SPRI’s technical committee and board of directors also approved and distributed to its members Technical Bulletin 1-13A, “Summary of SPRI Membrane Manufacturer Photovoltaic (PV) Ready Roof Systems and Services”. The bulletin contains general guidelines from SPRI related to “PV Ready” roof assemblies. This article goes into more depth about issues related to PV installations, particularly on existing warranted roofs.

Ask the Right Questions

The installation of a PV system on an existing warranted roof raises many important questions for the roofing professional and building owner. For example, will the roof accommodate the added weight of the PV array? Logistically speaking, before property owners decide on a solar-power system, they will need to determine whether their roofs are sturdy enough to support
the additional loads put on the existing roof structure by the solar array.

An average solar panel and support system typically add a minimum of 3 to 4 pounds per square foot to the existing roof. It is the responsibility of the roofing professional to ensure this additional weight does not exceed the load limits determined by the building’s designer.

From an economic (life-cycle-cost) point of view, it makes sense the service life of the existing roof membrane will come close to matching the projected service life of the PV system. If not, a complex and costly reroofing project may be required long before the solar panels need to be replaced. In general, the underlying roofing system must provide the same minimum investment horizon—generally at least 25 years—to realize the full potential of the rooftop PV system.

Most PV arrays require penetrating the roof membrane. Even non-rack-type systems may include electrical conduits, wiring and other components that may need to be flashed in a professional manner. It is essential the responsibility for this flashing work rests with the roofing contractor.

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