From Screw-down to Standing-seam Metal Roofing

Time to reroof an old screw-down metal roof? Are you thinking about upgrading to a new standing-seam roof? Great idea! Today’s new standing-seam roofs are truly state-of-the-art; available in many profiles and finishes; and, more importantly, address many of the issues encountered in older generation screw-down metal roofs.

Caulk, roof coating and tar patches were used to cover leaking fasteners and panel end laps.

Caulk, roof coating and tar patches were used to cover leaking fasteners and panel end laps.

The screw-down metal roof and wall panel has been the backbone of the metal building industry since its inception and still represents a significant part of the total market. Screw-down panels are lightweight, durable, inexpensive and strong enough to span up to 5 feet between structural supports. Screw-down roofs and walls also have a wonderful physical property: The panels can and frequently are used as “diaphragm bracing,” securely holding the building’s roof purlins and wall girts in position, adding rigidity to the structure in much the same way drywall strengthens stud walls. This is a huge material—and labor—cost saver!

The early systems were not without problems, however; much of the technology we take for granted today did not exist in the early years of pre-engineered buildings. Many roofs during the late ’60s thru early ’80s were installed using 10-year life fasteners to secure a 30-plus-year life roof.

The fastener issue seems crazy today given the numerous inexpensive, long-life, weathertight, self-drilling screws available. Back when I started in the metal building industry, you could have the newly developed “self-drilling” cadmium fasteners or “self-tapping” stainless. Self-tapping meant you had to pre-drill a hole in the panel and purlin to install it—a much slower and more expensive process. Most of us used the less expensive but (unknown to us at the time) fairly short-life cadmium-coated fasteners and often never provided the option of a stainless upgrade to our customers.

Another shortcoming with screw-down roof panels is that, generally speaking, screw-down panels on metal buildings should be a maximum length of about 80 feet. Longer roof-panel runs frequently suffered rips or slots in the metal caused by expansion and contraction. Metal panels expand and contract at a rate of about 1 inch per 100 feet of panel run. This is normally absorbed by the back and forth rolling of the roof purlin and some panel bowing, but after 80 feet or so they can no longer absorb the movement resulting in trauma to the panels and trim. I have frequently seen this 80-foot limit exceeded.

a rusted fastener has caused the surrounding metal to corrode and fail.

A rusted fastener has caused the surrounding
metal to corrode and fail.

Standing-seam panels eliminate both of these shortcomings. The panels are attached to “sliding clips”. These clips are screwed to the purlins and seamed into the side laps of the panels securing them and thus the panels have very few, if any, exposed fasteners. The clips maintain a solid connection with the structure of the building while still allowing the panels, which can be 150 feet or longer, to move with expansion and contraction forces without damage.

This is great news for the building owner: You’re providing a more watertight roof, few if any penetrations, and expansion and contraction ability. It does come with a catch, however; standing-seam panels, because they move, do not provide diaphragm strength. The building’s roof purlins must have significantly more bridging and bracing to keep them in their correct and upright position. This is automatically taken care of in new building design but when it comes time to reroof an older building, removing the existing screw-down roof could remove the diaphragm bracing it once provided and make the building structurally unsound. Yes, that’s bad!

PHOTOS: ROOF HUGGER INC.

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Wind Loading on Rooftop Equipment

I recently attended a continuing-education conference for civil/structural engineers that discussed changes in the 2012 International Building Code (IBC) and the referenced ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures”. During the seminar, the question was asked: “Who is responsible for the design of wind loading to rooftop equipment as defined in the IBC and Chapter 29 of ASCE 7-10?” The most accepted response was to add a section in the structural general notes that wind design on rooftop equipment is to be designed “by others”.

A structural engineer designed the metal support system and load transfer from the new HVAC unit down through the structure.

A structural engineer designed the metal support system and load transfer from the new
HVAC unit down through the structure.

The design requirements for wind loading on rooftop equipment have been included in previous editions of the IBC and ASCE 7, but significant changes have been included in ASCE 7-10. The increased attention is in part because of more severe wind events in recent years. While it is not the primary responsibility of the roofing consultant or contractor to evaluate the systems being placed on the roof, it is good to understand the code’s requirements for loading to rooftop equipment, how the load is determined and applied, and how the load is transferred to the building structure.

CODE REQUIREMENTS

The primary focus of the roofing professional in the IBC is concentrated on Chapter 15 (Roof Assemblies). While there are requirements in Chapter 15 addressing rooftop structures, these requirements, particularly in relation to wind loading, extend beyond Chapter 15. It is therefore imperative to be familiar with other sections of the code.

For instance, Section 1504 (Performance Requirements) refers the user multiple times to Chapter 16 (Structural Design) for wind-loading-design requirements. While roof manufacturers typically prequalify their systems based on various industry standards (ASTM, FM, ANSI, etc.), rooftop equipment supports are not typically prequalified because of the variability of placement and conditions. Similarly, new to this code cycle, Section 1509.7.1 includes the requirement for wind resistance for rooftop-mounted photovoltaic systems per Chapter 16 of the IBC. Other industries or trades have similar requirements. Section 301.15 of the 2012 International Mechanical Code and Section 301.10 of the 2012 Fuel and Gas Code require “equipment and supports that are exposed to wind shall be designed to resist the wind pressures in accordance with the IBC”.

Section 1609 of Chapter 16 (Wind Loads) applies to wind loading on every building or structure. Section 1609.1.1 provides two design options. The designer can use chapters 26 to 30 of ASCE 7-10 or Section 1609.6 of the IBC. Note however that Section 1609.6 is based on the design procedures used in Chapter 27 of ASCE 7-10, which does not address wind loading on rooftop equipment and thus is not applicable. Chapter 29 of ASCE 7-10 (Wind Loading on Other Structure and Building Appurtenances) contains the procedures used to determine wind loading on rooftop structures and equipment.

DETERMINING AND APPLYING WIND LOADING ON ROOFTOP EQUIPMENT

Properly specified ballasting blocks are designed and formed to better address the freeze/thaw cycle.

Properly specified ballasting blocks are designed and formed to better address the freeze/thaw cycle.


To determine wind loading on rooftop equipment, the first step is to identify the building Risk Category (formerly the Occupancy Category) and the building location. The Risk Category is determined from Section 1604.5 and Table 1604.5 of the IBC or Table 1.5-1 of ASCE 7-10. There are slight variations in the two codes but typically each will produce the same Risk Category.

The Risk Category and the location are then used to determine the design wind speed based on published wind-speed maps, available in Section 1609.3, figures 1609 A to C of the IBC, or Section 26.5.1, figures 26.5-1 A to C of ASCE 7-10. It can be difficult to read these maps to select the appropriate wind contour line, specifically along the East Coast. The Redwood City, Calif.-based Applied Technology Council (ATC), a non-profit that advances engineering applications for hazard mitigation, has digitized the maps providing a valuable resource for determining design wind speeds by GPS coordinates or the building’s address. Visit ATC’s wind-speed website. Note however that it is always advisable to cross check this design wind speed with the maps in the adopted code or with the local building authority.

PHOTOS: MIRO INDUSTRIES INC.

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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

Click to view larger version

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

Click to view larger version

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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