Myths About Metal Roofing: Heat, Wind and Lightning

Properly detailed and installed metal roofing is one of the most resilient, lasting, efficient and attractive kinds of roofing systems for commercial and institutional buildings. Yet there are plenty of questions about metal roofing, and building teams often find time in project meetings to address the most common, recurring topics and myths.

Facts vs Myths About Metal Roofing

I call these “mythbuster meetings,” because many of the questions are fabrications – concerns arising from less savvy professionals or from competitive “selling points.” Among the most prevalent untruths:

Myth: Wind uplift affects metal roofing more than other roofing types.

Reality: While the noncontinuous nature of metal roof attachments makes them susceptible to wind uplift concerns, most roofing types are prone to similar effects. ASCE/SEI calculations for wind loading and FEMA studies of storm areas have shown that properly applied metal roofing outlasts other roof assemblies during hurricanes and tornados.

Building geometry affects how well the roof survives, regardless of roof type. Engineering determines how many insulation board fasteners are needed, and the optimal and safest distances between clips for standing seam systems at corners and perimeters, where the forces are greatest. The interlocking or “active fastening” helps metal roofing pass severe wind and uplift tests including ASTM E1592, UL 580 and UL 1897, and the Miami/Dade County codes, according to a report from Stanford University.

Myth: Metal panels get hotter and have more thermal bridging because metal conducts heat so well.

Reality: Depending upon the surface finish, metal roofing can “provide enhanced energy efficiency with its solar reflectance and infrared emittance properties […] to meet the climate requirements of the building,” according to the Stanford University paper and research highlighted by the Cool Metal Roofing Coalition.

As compared to other roofing types, metal roofing tends to be highly reflective and is available with high emissivity. Insulated metal roofing panels have foam insulation that delivers R-values up to R-8.515 per inch thickness and total roof U-factors that exceed those of many other roofing types, helping projects meet strict energy code rules.

Myth: Metal roofs are more likely to get hit by lighting than any other roof types.

Reality: That is bunk; simply untrue. You can read my detailed blog on the subject, or for serious mythbusters refer to the Metal Construction Association’s Technical Bulletin MCA13a, which gives a full and authoritative overview.

As the MCA summarizes, “Because metal roofing is an electrical conductor and a noncombustible material, the risks associated with its use and behavior during a lightning event make it the most desirable construction available.” That’s right: The best option for lightning risks.

I hope some of the above information provided insight and assurance about building with metal roofs. If you have any additional questions or concerns, submit them here to our technical experts.

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Wind Designs for Metal Roofs

One of the most important requirements for roof installation is ensuring that a roof stays in place when the wind blows.  The core concept is that the roof’s wind resistance needs to be greater than the wind loads acting on a building’s roof.  Wind resistance is most commonly determined by a physical test; wind loads are calculated.

Calculating Wind Loads

Wind loads are based on the design wind speed (which is based on the geographic location of the building), height of the roof, exposure category, roof type, enclosure classification and risk category.  The height of the roof, and exposure and risk categories are factors that are used to convert design wind speed to an uplift pressure.  Wind speed maps and the rules to calculate wind pressures are found in Section 1609, Wind Loads, in the 2012 or 2015 IBC.  The information is based on an engineering standard written by The American Society of Civil Engineers, “ASCE 7-10, Minimum Design Loads for Buildings and Other Structures.”Wind Uplift Testing_2

Defining Exposure Risk Category

Exposure categories relate to the characteristics of the ground, such as urban and suburban areas or open terrain with some obstructions or flat areas like open water.  There are 4 risk categories.  Category I is low risk to humans, such as agricultural facilities. Category III includes, for example, buildings for public assembly, colleges and universities, and water treatment facilities.  Category IV includes essential facilities like hospitals and police stations.  Category II is everything else—most roofs are Category II. A building shall be classified as enclosed, open or partially enclosed. The enclosure classification is used to determine the internal pressure coefficients used to calculate design roof pressures.

Determining Wind Pressures

Contractors should work with a structural engineer or the metal panel manufacturer to determine the wind pressures for each roofing project.  Wind pressures are determined for the field of the roof, the perimeters and the corners, where loads are largest.  Only after determining the design pressures can the appropriate metal panel roof system and attachment requirements be designed.

Testing Uplift Resistance

Physical tests are the most common method to determine uplift resistance.  Panel width and profile, metal type and thickness, clip type and frequency, type and number of fasteners, and the roof deck contribute to the uplift resistance of every metal panel roof system.  Metal panel roof systems installed over solid substrates (with concealed clips or through-fastened) can be designed using the following test standards: FM 4471, ASTM E 1592, UL 580, or UL 1897.  Metal panels installed over open framing can be designed using either ASTM E 1592 or FM 4471.  Manufacturers run these tests; uplift resistance data is available for most metal panel roof systems.  Installers can get this data directly from manufacturers or from web-based listing services provided by FM and UL.

Designing a Legal Metal Roof System

Wind loads and wind resistance information is necessary to verify code compliance.  Get it for every project you install!  Using systems that not only have been tested to the correct tests, but using systems that have uplift resistance greater than the design loads is key to a successful installation, and quite frankly, key to installing legal roof systems.

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Roofing Underlayment and its Attachment Requirements

The International Residential Code (IRC) is commonly considered to be a prescriptive code, which means there are many requirements included that provide specific directions. Prescriptive-based code language provides a simpler method of enforcement for inspectors. And shouldn’t that be the case for one- and two-family dwellings, where well built and affordable is the goal?

In Chapter 9 of the 2015 IRC, the underlayment requirements for steep-slope roof coverings are included in three tables—material types, application and attachment requirements. Each table includes specific information for metal panels and separates out high-wind areas (defined as greater than 140 mph, and is only in the southernmost portion of Florida).

Material Type Requirements

Underlayment types for metal panels needs to only comply with manufacturer instructions. D226 and D4869 underlayments are viable options, as long as metal panel manufacturers allow them. And very importantly for metal panels, synthetic- / polymer-based underlayments are a viable option, again, as long as the panel manufacturer allows them to be used.

For metal panels in high wind areas, only D226 Type II and D4869 Type IV are allowed. In other words, only the heaviest materials are allowed in the highest wind zones.

Application Requirements

Simply put, underlayment should be applied according to the manufacturer’s installation instructions. For high-wind areas, specific application requirements are provided:

“For roof slopes from two units vertical in 12 units horizontal (2:12), up to four units vertical in 12 units horizontal (4:12), underlayment shall be two layers applied in the following manner: apply a 19-inch strip of underlayment felt parallel to and starting at the eaves. Starting at the eave, apply 36-inch-wide sheets of underlayment, overlapping successive sheets 19 inches, and fastened sufficiently to hold in place. For roof slopes of four units vertical in 12 units horizontal (4:12) or greater, underlayment shall be one layer applied in the following manner: underlayment shall be applied shingle fashion, parallel to and starting from the eave and lapped 4 inches. End laps shall be 4 inches and shall be offset by 6 feet.”

Underlayment Attachment Requirements

Underlayment should be attached according to the manufacturer’s installation instructions. For high-wind areas, specific attachment requirements are provided:

“The underlayment shall be attached with corrosion-resistant fasteners in a grid pattern of 12 inches between side laps with a 6-inch spacing at the side laps. Underlayment shall be attached using metal or plastic cap nails or cap staples with a nominal cap diameter of not less than 1 inch. Metal caps shall have a thickness of at least 32-gage sheet metal. Power-driven metal caps shall have a minimum thickness of 0.010 inch. Minimum thickness of the outside edge of plastic caps shall be 0.035 inch. The cap nail shank shall be not less than 0.083 inch for ring shank cap nails and 0.091 inch for smooth shank cap nails. Staples shall be not less than 21 gage. Cap nail shank and cap staple legs shall have a length sufficient to penetrate through the roof sheathing or not less than 3/4 inch into the roof sheathing.”

Self-Adhesive Underlayment Options

Of course, there are exceptions to these requirements. The first is to use a self-adhesive underlayment (i.e., ice dam protection) over the entire roof. The material needs to comply with ASTM D1970, “Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection” and be installed per the metal panel manufacturer’s requirements. The code also points out that roof ventilation must be considered because a self-adhesive sheet is most often an air barrier and a vapor retarder. Concerns with moisture are quite relevant when these types of materials are installed over the entire roof deck. The second exception is to tape the seams of the roof deck with 4-inch wide strips of D1970 material, and then cover the deck with underlayment. The second exception is not widely used, except when trying to reduce, or eliminate, air flow through the deck while allowing moisture to escape.

IRC Requirements for Attaching Metal Panels

The IRC also includes some, but not many, requirements for the attachment of metal panels. The IRC requires metal panels be attached per manufacturer’s installation instruction and “be secured to the supports.” This implies fasteners should be attached to purlins or rafters, but one could easily argue the roof deck is the support for the metal panels. However, the IRC does provide specifics for fasteners used to attach metal panels, but the following is only applicable if manufacturer’s instructions don’t include fastener requirements. The IRC states:

“In the absence of manufacturer’s installation instructions, the following fasteners shall be used:

  1. Galvanized fasteners shall be used for steel roofs.

  2. Copper, brass, bronze, copper alloy and 300-series stainless steel fasteners shall be used for copper roofs.

  3. Stainless steel fasteners are acceptable for metal roofs.”

The Importance of Following IRC and Manufacturer Instructions

The IRC is a prescriptive code and there are many specific requirements for underlayment and metal panels. But because of the wide variety of styles, the IRC appropriately requires installation according to manufacturer’s instructions. It’s important to specify a new roof using both manufacturers’ instructions and IRC’s specific requirements. And, remember, a metal roof will have a long service life, so the underlayment’s service life should equal that of the metal roof. Don’t be shortsighted when designing for longevity.

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3 Energy-Saving Technologies to Consider with Metal Roofs

A roof’s primary function is to keep a building weatherproof. A roof’s secondary function—and approaching nearly equal importance—is to be an energy-efficient element of the building envelope. From an energy efficiency standpoint, we’re accustomed to the inclusion of insulation. Are we as accustomed to the ideas that roof color and air leakage matter for energy efficiency? The building industry is embracing all of these technologies in an effort to save energy.  So how does an installer make it all work?

Insulation

NAIMA.org

Photo Courtesy of NAIMA

Insulation requirements for roofs on metal buildings (according to the 2015 IECC) range from R-19+R-11 LS up to R-30+R-11 LS, depending on climate zone. The first layer is draped over the purlins and requires a thermal spacer block with an R-3.5. A second layer is installed at perpendicular and is required to include a liner system (LS), which is a continuous vapor barrier installed below the purlins and is uninterrupted by framing members. The crisscrossed layers help reduce convective air movement within the insulation layer, making the insulation layer more effective. And, good news!—the vapor barrier can also be an air barrier. So, on to air barriers.

Air Barriers

Even small air leaks in buildings can account for a 30 to 40% heat loss during heating season (winter), regardless of the amount of insulation. It can’t be overstated—air barriers are critical to an energy-efficient roof and overall building envelope. The LS, or vapor barrier, can be an air barrier only if the seams of the LS are sealed to prevent air passage. The junction between the air barrier in the roof and walls is critical; it must be joined to be continuous. Often, a separate material (adhered membranes or spray-applied foams) is used as the transition from wall to roof. Or, the roof and wall air barriers might end on opposite sides of a perimeter beam or purlin, connecting the two air barriers. Also, any penetrations through the roof need to be sealed to the air barrier. Being continuous/having continuity is key to constructing a properly functioning air barrier!

Roof Color

We’ve heard a lot about roof color. Where air conditioning is prevalent (e.g., the Southwest), highly reflective roofs make sense, especially if there is minimal insulation. Where heating is prevalent, roof color becomes less effective for energy efficiency for a couple reasons. One, buildings require significant amounts of insulation, and two, there is much less direct heat gain from the sun over the course of a year. Where heating and cooling are both used regularly (e.g., Nashville, Chicago), it’s not a matter of “black or white.” There are many metal roof colors that are moderately reflective, so they balance reflectivity and heat gain as the seasons change.

Contemplate the interaction of insulation, roof color and air barriers on each metal roofing project.

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Better Barriers: Meeting Thermal Performance, and Controlling Air and Moisture

Panelized metal exteriors have joints. It’s just a rule of best-practice design. Yet these joints are seen by some as interruptions in the façade or roof, when in fact they are connections — the opposite, one can argue, of the word “interruption” that suggests a discontinuity.

Edie's CrossingIn fact, engineered metal panel systems offer arguably the best possible continuous exterior system. Not only are they properly applied exterior to the building structure—outboard of columns, joists and girts—but they are also designed to ensure an unbroken chain of thermal control and barrier protection. Combined with controlled penetration assemblies as well as windows, doors and skylights that are engineered as part of the façade and roof system, the insulated metal panel (IMP) products provide unequaled performance.

That’s the main reason that specialized facilities designed for maximum environmental barrier control are made of IMPs: refrigerated warehouses, R&D laboratories, air traffic control towers and MRI clinics, to name a few.

But any facility should benefit from the best performance possible with metal roofing and wall panels. Consider insulation shorthand for the code-mandated thermal barrier required for opaque wall areas in ASHRAE 90.1 and the International Energy Conservation Code (IECC). For a given climate zone, says Robert A. Zabcik, P.E., director of R&D with NCI Group, the project team can calculate the functional amount of insulation needed by using either the “Minimum Rated R-values” method or the “Maximum U-factor Assembly” calculation. For IMPs, teams use the Maximum U-factor Assembly, which can be tested using ASTM C1363.

With IMPs, the test shows thermal performance values up to R-8.515 and better per inch of panel thickness, meaning that a 2.5-inch-deep panel would easily meet the IECC and ASHRAE minimums.

With metal roofing panels and wall panels, a building team can achieve needed energy performance levels with this single-source enclosure, providing a continuous blanket of protection.

The same is true for air and moisture control. In a July 2015 paper by Building Science Corp., principal John Straube wrote, “Insulated metal panels can provide an exceptionally rigid, strong and air impermeable component of an air barrier system.” He noted that, “Air leakage condensation cannot occur within the body of the insulated metal panel, even if one of the metal skins is breached, because all materials are completely air impermeable and there are no voids to allow air flow.”

In terms of water control, Straube writes that IMPs have a continuous steel face that is a “high-performance, durable water control layer: water simply will not leak through steel, and cracks and holes will not form over time. The exterior location of the water barrier,” he adds, “offers some real advantages.”

Clip-Fastener-AssemblyEnfold_blog

Connecting the panels at transitions, penetrations and panel joints is the key, of course. Straube notes that sealant, sheet metal, and sheet membranes are effective and commonly used to protect joints.

In my experience, these joint details are incredibly effective. They often outlast most other components of the building. Even more important, they help make IMPs better barriers that meet thermal, air and moisture performance needs. They help make metal panels one of the best choices of all.

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Reroofing with Steep-slope Metal Panel Roof System Over an Existing Low-slope Roof: Part 2

Let’s continue the discussion about converting low-slope roofs to steep-slope metal roofs. Part 1 discussed attachment of framing, the new attic space, ventilation and condensation issues, and drainage.

Before After Retrofit

Reroofing Code Requirements 

Converting a rooftop is a specialized type of reroofing.  The codes specifically allow this via an exception that says “complete and separate roof systems, such as standing-seam metal roof panel systems, that are designed to transmit the roof loads directly to the building’s structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings.”

To meet this code requirement—and to not have to remove the existing roof system—the loads must bypass the existing roofing system. This is critical to create a load path from the new structure to the existing structure for dead loads, snow loads, rain loads, and uplift (e.g., wind) loads.

Structural Loads & Wind Resistance

IBC’s Chapter 16, Structural Design, includes all the required information and design methods to determine the dead, snow, wind, and rain loads acting on the building.  The new framing members and their connections, as well as the new metal panels and their attachments to the new framing, must be able to resist the loads acting on the building.  The resistance must exceed the loads.  Most often, wind resistance loads control the design.  Manufacturers and structural engineers should be consulted for material specifics and fastener requirements.

Fire Resistance

Fire resistance for a converted roof needs to meet the requirements of the model codes.  Check with manufacturers for fire classification of the system installed, and ensure it meets the minimum class (A, B, or C) required in the project location.  See the blog “Fire resistance of metal panel roof systems” for more information.

Insulation

For all types of reroofing, the most recent insulation requirements need to be met.  In most cases, additional insulation will be necessary.  Insulation can be placed at the attic floor (i.e., on top of the existing low-slope roof) or directly under the new metal panels.  Where the new roof meets the wall is very important for continuity of the overall building envelop insulation; lack of continuity is energy inefficient and may be a point of condensation.  The location of the new insulation needs to be coordinated with the ventilation plan and condensation potential should be considered. See Part I for more information.

While reroofing with metal can be an aesthetic improvement and solve leak issues, structural loads and wind resistance, fire resistance, and insulation requirements are necessary considerations when converting from a low-slope roof to a steep-slope metal panel roof system.  Don’t overlook the basic code requirements, or the need to deal with heat, air, and moisture issues of the new attic space.

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A Common Misconception About Determining Thermal Resistance

Photo courtesy of ENERGY.GOV

Photo courtesy of the U.S. Department of Energy

As an architect, you’re required to design a building’s wall to meet the code-required R-value (or U-factor) in the International Energy Conservation Code. So you design the wall and add up the manufacturer-stated R-values of the components.  Done, right? That method only makes sense if walls have no joints, seams, windows, or doors! Let’s think about this.

Accounting for Thermal Discontinuities

The manufacturer-stated R-value of an insulated metal panel (IMP) should really be the R-value in the center portion of the panel, if the manufacturer uses terminology consistent with ASHRAE 90.1. However, a wall is made up of many IMPs, and there are joints between the IMPs.  We’ve all seen the infrared photos showing the heat loss at joints between panelized anything—plywood, insulation boards…and IMPs. The joints between each and every IMP are thermal discontinuities, commonly called thermal bridges. These are locations where the R-value is not what you read in the manufacturer’s literature. There are also metal clips and attachments that reduce the R-value of the IMP wall system. If you’re designing a wall system, don’t specify the R-value of the panel and assume it is the R-value of the wall system!

Calculating the R-Value of a Complete IMP System

A building owner deserves a wall that meets or exceeds the code-required minimum R-value or U-factor. The mechanical engineer needs to properly size the building’s mechanical systems based on the ‘real’ characteristics of the building envelope.

Let’s put some numbers behind this idea. Let’s consider a 42 inch-wide panel, 2 inches thick, with a stated R-value of 12. The outer surface of the panel is close to the exterior temperature—say 30 degrees. The metal wraps through the joint, decreasing the temperature of a portion of the metal on the backside of the panel everywhere there is a joint. Clearly this reduces the overall R-value of the IMP as a system.  Let’s estimate that the thermal bridging effect of the joints reduces the R-value 5 inches along the edges of the panels to an R-6. That means 30 inches of the panel has an R-12, and 10 inches of the panel has an R-6. That calculates to an average R-value of 10.5 for the panel overall, which is more than a 12% loss of R-value. This is why blindly using the famous equation of R=1/U is dangerous. That equation is only true if the R-value and U-factor involved are consistent with how thermal bridging is or isn’t represented.

U-Factor Testing for Higher Accuracy

It’s clear that the panel joints are thermal bridges, but the extent of loss is really an educated guess. But there is a solution! The forward-thinking IMP manufacturers are performing U-factor testing and finite element modeling, and that includes joints between panels. The U-factor testing is a more accurate determination of thermal resistance.

As an architect designing the wall system, if you use stated R-values, recognize that you’ll need to account for the loss of R-value because of the joints. Or, simply specify panels whose manufacturers are determining the U-factor for their IMPs!

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Wellness and Envelopes: Four Ways Single Skin & Insulated Metal Panels Keep Us Healthy

SONY DSCIs there a connection between building design and human health?

We know the answer must be yes, but figuring out how the connection works is the job of experts like the team behind the WELL Building Standard®, a new certification that takes on the question. Among the solutions that can help make a building better? Metal roofing and siding, according to many healthy building experts.

First, let’s learn about WELL. According to the International WELL Building Institute, the WELL Building Standard “takes a holistic approach to health in the built environment addressing behavior, operations and design.” Their performance-based system measures and monitors such building features as air, water, nourishment, light, fitness, comfort, and mind. Two ratings have been offered: WELL Certified™ spaces and WELL Core and Shell Compliant™ developments. Done properly, these “improve the nutrition, fitness, mood, sleep patterns, and performance of occupants.”

Pilot programs are currently available for retail, multifamily residential, educational, restaurants and commercial kitchens projects. In many of these projects, the use of metal claddings and insulated metal panels (IMPs) is recommended by many health-focused professionals. Why?

1. Occupant comfort. IMPs tend to have excellent R-values and very good thermal efficiency – including long-term thermal resistance, or LTTR, a key measure of how the building will perform over time. For the wellness factor from pure thermal comfort, IMPs are highly effective over conventional construction.

2. Nourishment of people and earth. IMPs are often made with recycled metals and improve the energy performance of the building. With energy cost savings ranging from 5 percent to 30 percent, they cut the carbon footprint of the facility. Plus the interior and exterior skins include up to 35 percent recycled content – and they are 100 percent recyclable – reducing impact on the global carbon load.

3. Daylight for all. Using metal roofs with skylights or light-transmitting panels in conjunction with integrated dimming lighting is a highly cost-effective strategy, and IMP systems also have integrated window systems that increase available sunlight within building interiors. Light is essential for healthy buildings, and daylight is the best kind of all.

In addition, because rigid insulation per inch offers more R-value than per inch of fiberglass insulation and IMPs have metal liner skins, day-lighting fixtures such as light tubes can be integrated more easily with these roofs.

4. Proper moisture and air control. Issues such as leaky walls and wet, moldy construction materials are anathema to wellness, and must be controlled for healthy building certifications. Mold has a negative impact on indoor air quality and indoor environmental quality, and one of the main culprits is trapped moisture. This can also corrode the metal studs and furring members, even if they are galvanized, leading to structural issues such as reduced fastener pullout resistance and leaks.

How Does a Building Become WELL Certified?

IMPs used as either rainscreens or as sealed barrier walls backing up a rainscreen are shown to protect against moisture issues and mold over time. They also serve as a continuous layer of insulation and air barrier. In this way, the single-component system can eliminate the need “for air barriers, gypsum sheathing, fiberglass insulation, vapor barriers, and other elements of a traditional multicomponent wall system,” says one industry executive. In fact, many masonry buildings are being upgraded with IMP retrofits on the exterior, directly over the old concrete, brick or stone.

All of these traits of IMPs certainly contribute to more healthy buildings, but do they add up to WELL Building certification levels, such as Silver, Gold or Platinum?

To get there, building teams must undergo an on-site WELL Commissioning process with rigorous post-occupancy performance testing of all the features. If it meets the “preconditions” — the WELL features necessary for baseline certification — WELL Certification is given. If the team pursues “optimization features,” the higher levels of achievement are granted.

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Fire resistance of metal panel roof systems

Ditch Witch (12)Metal is inherently fire resistant.  The codes acknowledge that; however, certain limits are placed on metal’s fire resistance when used as part of a metal roof system.

Metal panels transfer heat very well—they get hot quickly and give up heat quickly.  And, in many cases, there is a building component (roof deck, framing) directly under metal panels.  Metal roof systems are required to be fire classified because of the concern about the combustibility of the materials under the metal panels.

Fire Resistance Classifications

The 2012 and 2015 IBC, in Section 1505 of Chapter 15, states that fire classification of roof assemblies is based on two tests—ASTM E108 and UL 790—that are fundamentally identical.  Each requires a spread of flame test and a burning brand test.  Tested roof systems are fire classified Class A, B, or C, where the most fire-resistant roof assemblies are Class A, and Class C is least resistant.

Building Code Fire Resistance Requirements

Building codes establish fire resistance requirements for roofs based on the type of construction (e.g., concrete/steel, wood) for the building.  A common misconception about roofs’ fire ratings is that building codes require Class A.  Not true—the IBC does not require Class A roof assemblies for any type of construction!  Only roofs on buildings located in wildfires zones (e.g., Southern California) will likely be mandated to be Class A.  (It is worth mentioning here that the vast majority of low- and steep-slope roof systems sold and installed in the U.S. are Class A.)

The building code lists a number of roof types deemed to be Class A (in other words, testing is not required).  Appropriately, metal panels are included: ferrous (steel) and copper shingles or sheets, metal sheets, and shingles on noncombustible decks (e.g., steel, concrete—not wood), or on noncombustible framing where a deck is not included (e.g., directly over metal purlins).  The key is that the deck or framing is noncombustible.

If metal panels are installed over combustible decks, the assembly needs to be tested using ASTM E108 or UL 790.   An exception for combustible decks is that 16 oz./sq. ft. copper (or thicker) can be installed over combustible decks and be considered Class A without testing.

Building with Fire Safety in Mind

The code requirements for fire resistance of metal panels are logical and not overly burdensome.  Most metal panel manufacturers have tested their roof assemblies, and most, if not all, metal panels and shingles can be used in Class A fire-rated roof systems.

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Reroofing with steep-slope metal panel roof system over an existing low-slope roof: Part 1

Reroofing w Steep Slope

You’ve probably seen many articles and photos of buildings with low-slope roofs converted to steep-slope metal panel roofs.  Aesthetics and leakage are driving these conversions, but mostly leakage of the low-slope roof.  Owners are frustrated with leaks and believe a steep-slope metal roof is their answer, and it certainly can be!  There are a number of issues to be resolved and decisions to be made to ensure long-term performance of the new metal roof.

Retrofitting a roofing system

A new structure, such as MBCI’s retrofit systems, should be attached directly to the existing structural members.  Removal of small sections of the existing roof is required for direct attachment.  Fastening through the existing roof means attaching through a soft substrate, allowing for compression over time and movement of the fasteners, and their eventual loosening.  If possible, installing new flashing at the attachment points (with spray foam?) provides a second water barrier.

Steps to take after retrofit installation

Conversion to a steep-slope metal roof creates an attic space that now needs to be ventilated to remove heat and moisture, but also to provide adequate airflow for HVAC intake requirements.  Determine intake air requirements—how many air changes per hour are required in the attic to satisfy fresh-air intake requirements?  Significantly more than code-required ventilation amounts may be needed if many HVAC units are enclosed.  Inadequate ventilation could result in an extremely hot and humid attic space and overworked mechanical intakes.  Poor ventilation could increase the need to cool a building, especially single-story buildings (e.g., schools) converted from low-slope to steep-slope roofs.  Conversely, the vent stacks are also enclosed.  Perhaps vent stacks need to be vented through the roof?

To prevent condensation issues in the newly created attic space, it’s prudent to install mechanical ventilation units that are controlled by a humidistat, not a thermostat.  Maximum humidity levels can be determined for summer and winter seasons, and the humidistat can be set to remove humidity and air when the maximum relative humidity levels are reached.

Drainage at roof edges must be well thought out.  If the new roof includes overhangs, gutters and downspouts and eave vents can be installed.  If there are parapets, creating an “internal gutter” where the new metal roof meets the parapet can be difficult to detail properly for a long-term, low-maintenance solution.  Overhangs and eaves, even where parapets exist, are the best solution.

This is blog one of two.  More on this topic next time!

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