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Steel Building Erectors Article: FIELD TECHNIQUES - Retrofit Roof Solutions
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PRE-ENGINEERED METAL buildings (PEMB's) have grown to be more than 60 percent of all new, 1 and 2 story nonresidential construction in the United States. The 26-gauge (0.017 inch normal thickness) coated-steel R-Panels - with 36 inch (914 mm) wide coverage and 1 to 1 inch (25 to 38 mm) tall trapezoidal shaped ribs-spaced 12 inches (305 mm) on-center have been the principal roof type for PEMBs during the past 60 plus years. About 30 to 50 billion square feet (278.7 to 464.5 million m2) have been installed.

But fixes for metal roof leaks are not simple and can be quite expensive. The simplest, least expensive R-Panel repair solutions are applied to the roof surface. Replacing problem fasteners and sealants, as well as surface-applied roof coatings, and overlaying the existing roof with a new roof are straightforward solutions. However, to repair existing roofs, improperly installed panels, flashings, penetrations, etc, must be disassembled and reassembled to provide a long-term weather tight solution. When weighed against the potential for business interruption, repair of an existing roof becomes less attractive than a roof overlay solution.

On PEMB's, R-panels are installed over 16 gauge or heavier cold-formed Z or C shaped steel purlins spaced 5 feet (1.5 m) on-center. In some cases, the purlins supporting the roof panels are bar joists, wood joists or other type of structural member.

Screw-type fasteners are used to connect the panels to the purlins and joint fasteners, commonly referred to as stitch screws, primarily are self-drilling screws. Self tapping screws can be used in predrilled, or prepunched, panels and purlins. Carbon-steel or aluminum fasteners are sometimes utilized. Fasteners are, at a minimum, spaced 6 inches (152 mm) on center at the panel sidelaps and 12 inches (305 mm) on-center for a panel-to-structural member connection.

A gasketed washer is used under the fastener head to weatherproof the fastener hole through the panel. Until the early '90s, the primary gasket material was neoprene, which tends to dry and crack after a few weather cycles, creating a potential source of leaks. Today, more durable ethylene polypropylene diene monomer (EPDM) is widely used for the washer gasket material.

Field applied tape and pumpable butyl rubber-based sealants are used to water-proof panel sidelaps and endlaps. Butyl sealants vary in quality because of the percentage of butyl rubber versus chalk filler material. High-quality sealants will last 20 plus years while lower quality sealants will last as little as a few years before losing elasticity and contributing to leak problems.

Knowing the typical profile, evaluate the suitability of various metal roof solutions. First, existing conditions must be thoroughly investigated and evaluated. Corrosion on top and bottom panel surfaces is a large concern. The integrity of the panel-to-purlin attachment must be evaluated.

Warning: An obvious fall hazard exists at the edge of a roof, and less obvious fall hazard exists at the interior of the roof. The interior roof hazard is the potential to fall through a rusted, deteriorated metal panel or a skylight. Roof coatings conceal roof panel deterioration and translucent fiberglass panels.

The thickness condition and location of any existing insulation and vapor retarder should be investigated. The presence of any moisture in the insulation and/or the bottom side of the R-panel should be noted. A dew-point analysis based on the building usage should be performed to determine the proper amount of insulation required and location and required perm rating of the vapor retarder.

PEMBs are optimized for the original, code-required loads and do not provide for the added dead load of any future roofing materials. Reroofing a PEMB should include a structural analysis of the current system by a qualified engineer familiar with PEMB design assumptions and construction. Changes in American Iron and Steel Institute cold-Formed Steel Design Manual 2002 Edition, have reduced the capacity of cold-formed steel purlins from the 1980, 1986, 1996, and 2001 editions. Existing purlins that worked using earlier editions will not support the same loads when designed with the current edition.

Model building codes include very little about retrofitting metal roofs or metal buildings. The 1997 Uniform Building ode contains a chart in the "Appendix to Chapter 15 - Reproofing" indicates an existing metal roof only can be retrofitted with another metal roof. This restriction can be explained by considering the adequacy of a 26-guage R-panel or 24-gauge standing-seam roof panel as a supporting, structural deck for the application of any conventional roofing materials. Conventional roof warranties contain exclusions for "excessive building or structure movement".

Solutions for leaking or deteriorating metal roofs come in many shapes, sizes and costs. The pros and cons of each solution must be evaluated against the overall cost, expected service life, compatibility with the PEMB and suitability of the materials for the application.

Available through-fastened R-panel metal roof solutions include:

  • Tear off and replace the existing roof with a new metal roof
  • Coat the existing roof with elastomeric (acrylic-and-urethane-based) coating for waterproofing or asphalt-base coatings containing aluminum to inhibit rust
  • Overlay the existing roof with spray-polyurethane foam (SPF), a single-ply membrane roof, or a metal retrofit using a through-fastened R-panel roof or standing-seam roof.

    Tear off and replacement of a 26-guage R-panel roof with another metal roof adds little of any weight. A through-fastened R-panel roof cannot be replaced directly with a standing-seam metal roof. The PEMB purlin design takes advantage of the through-fastened R-panel roof to help brace the purlin top flange. The through-fastened R-panel roof might have been used as a wind diaphragm similar to a structural roof deck. A standing-seam roof will not brace the top flange of the purlins not act as a wind diaphragm.

    Safety is a consideration while working over open purlins during tear-off and replacement. Building contents and/or operations are susceptible to damage and/or interruption because of weather. A new R-panel roof can replace the old R-panel roof without affecting the existing PEMB design. To evaluate a tear-off and replacement solution using a standing-seam roof, qualified engineer should check the PEMB design.

    A roof coating adds minimal weight to an existing roof and can be applied with spray equipment, by brush or by paint roller. Coatings come in a variety of colors with bright white being solar reflective, providing an energy benefit in the summer cooling months. Most coatings have good flexibility and elongation to accommodate movement and thermal stress. There are many different types of coatings to address leaks and rusting of metal panels. Acrylic-based elastomeric coatings are popular for addressing leak problems, and asphalt-based coatings containing aluminum are popular as rust inhibitors. (Coatings applied over skylight panels or deteriorated steel panels can create a fall safety hazard for foot traffic on the roof.)

    The condition and surface preparation of the existing panel can affect the adhesion of the coating. If a coating loses adhesion, the resulting separation of the coating from the panel surface can allow moisture to become trapped, resulting in accelerated rusting of the existing panel. Surface preparation for the application of coating requires all foreign materials (sealing products, previous coatings, metal oxidation, loose paint, etc.) to be fully removed. The applicator must be skilled in the proper application methods for the coating and allowable environmental conditions, including temperature, humidity and wind.

    Most coatings can accommodate uniform panel movement. Movement concentrated at panel lap joints and flashings can exceed a coating's allowable elongation. Excessive vertical deflection from live loads or even horizontal deflection from wind loads can damage the coating. Coatings will not span over holes or gaps in the panel. This can be addressed by embedding a reinforcing fabric in the coating. Reinforcing fabrics have little elongation, reducing the elasticity of the coating system.

    SPF is a two-part chemical compound that is mixed at the point of application on the roof. SPF is lightweight, self-insulating roof that is easily with spray equipment. SPF adheres to most types of substrates and can be sprayed to vertical surfaces (parapet walls and curbs) and sloped surfaces. Many protective coating options are available for application over the cured foam to prevent attack from ultraviolet light. SPF relies on the integrity and stiffness of the supporting "roof deck" to carry any roof loads.

    SPF application is dependent on the conditions of existing panels, as well as environmental conditions, including temperature, humidity, wind, etc. Outside the Southwestern United States, few areas have many days with weather conditions that fall within the published SPF application guidelines.

    Improper SPF application can introduce voids in the foam that becomes the vehicle through which moisture can enter and become entrapped, resulting in accelerated deterioration of the existing R-panel. SPF cannot accommodate thermal or deflection movement without damage. Applying foam over integral skylight panels can create a fall hazard. SPF is susceptible to attack from birds and animals and supports plant growth.

    Single-ply membrane roofs (EPDM, TPO, PVC, etc.) are available as fully adhered and mechanically fastened applications. A reinforced single-ply membrane can be installed over an existing metal roof fastened to the underlying purlins or existing 26-guage R-panel. A qualified engineer should verify the design and attachments if the single-ply system to the PEMB. Using the 26-guage R-panel roof as the only attachment for a single-ply membrane overlay is not recommended and may void the single-ply roof warranty.

    When the single-ply membrane sheet is mechanically fastened to the purlins, the sheet width should be 5 to 6 feet (1.5 to 1.8 m)

    Unreinforced single-ply membranes provide as much as 200 to 300 percent elongation, however, reinforced membranes provide 10 percent or less elongation, which must accommodate the expected thermal and wind movement of the PEMB and R-panel.

    Single-ply membrane systems are relatively easy for contractors to install and require little or no specialized equipment. White membranes are reflective, providing an energy benefit in summer months. However, dirt accumulation can negate this benefit.

    A single-ply membrane overlay uses rigid board insulation. (1- to 1 inch 25 to 38 mm thick based on R-panel rib height) to fill the area between the ribs of the R-panel. A continuous layer of rigid insulation is applied over the panel ribs and fastened to the R-panel or purlin with insulation plates and screws. Using polystyrene insulation can affect the roof fire rating. Wood blocking is installed at roof edges to secure the roof flashings and membrane terminations. The weight if a single-ply overlay, including 1 inches (64 mm) of rigid insulation, is about 1.25 pounds per square foot (psf).

    A metal roof overlay can utilize an R-panel roof or standing-seam roof as the new roof system. The overlay also can incorporate a new purlin directly over and attached to the existing purlins. This new purlin can serve as the support for the new roof and also create the depth necessary to add the desired amount of new insulation. The cost of a metal roof overlay using a 26-guage R-panel roof system will be about 15 to 20 percent less than a 24-guage standing-seam metal roof system overlay.

    The criteria to evaluate for an R-panel or standing-seam roof are:

    Longevity and Serviceability

  • An R-panel roof can have a warranted, weathertight service life up to 10 years. The panel finish can last in excess of 20 years, however, exposed fastener gaskets, panel lap joints, thermal movement, penetrations, etc., can reduce the service life of an R-panel roof.
  • Standing-seam metal roofs can provide warranted, weathertight service for 20 years or longer.
    Purlin Spacing
  • R-panel roofs can be designed to span purlins spaced in excess of 5 feet (1.5 m).
  • Wind-uplift design limits a 24-gauge steel standing-seam metal roof to a nominal 5-foot (1.5-m) span or less. Most standing-seam systems have not been rated in industry wind-uplift tests, such as UL 580 (class 90 rating), FMRC Standard 4471 (FM I-90 rating) or ASTM E-1592 for spans more that 5 feet (1.5 m).

    Insulation Thickness
  • High R-values cause all roofs to experience large temperature differentials and in turn more thermal movement.
  • R-panel roofs have fiberglass-blanket insulation thickness limitations as low as 3 inches (76 mm). Insulation thickness is limited because of the potential for indentations in the panel around the heads of the tightened fasteners. The indentations hold water at low slopes and can cause roof leaks.
  • Most standing-seam metal roofs are designed to accommodate up to 6 inches (152 mm) of insulation without problems.

    Length of Panel Run
  • Steel R-panel roofs have limitations for continuous panel runs of less than 150 feet (46 m) and some roofs as short as 120 feet (37 m). The length limitation is because of the lack of any provision to handle thermal movement inherent with most R-panel roofs.
  • Standing-seam metal roofs designed to accommodate thermal movement can have continuous panel runs as long as 200 to 250 feet (61 to 76 m). This length is determined by allowable panel clip movement and movement allowed by trims and flashings.

    Roof Slope
  • R-panels never should be used at slopes less than :12 (5 degrees). The steeper the slope, the faster the water flows off the roof and less chance for roof leaks.
  • Standing-seams metal roofs eliminate up to 90 percent of the exposed through fasteners and place the panel sidelap joint well above the water flow plane. Standing-seam roofs can be installed on slopes as low as :12 (1.2 degrees).

    A standing-seam metal roof overlay can be installed over an existing R-panel roof utilizing two methods. A new purlin can be installed as the support for the new roof system.

    By using different depth purlins (typically from 1 to 12 inches (25 to 305 mm deep) the desired insulation thickness can be achieved. This type of retrofit system can be installed over R-panel roofs, as well as other types of existing metal roofs, including over standing-seam roofs. This retrofit system easily can be installed over out-of-module existing roofs. The weight of the system typically is less than 2 psf.

    A new standing-seam metal roof system is fully compatible with the existing PEMB system. Another advantage to this type of retrofit system is the ability to change the slope and drainage direction of the new roof to eliminate valley gutters to increase the slope if necessary or change the appearance of the building.

    Extruded polystyrene blocks are used to transfer gravity loads to the existing purlins. The new roof is only 1 inches (38 mm) above the existing R-panel, allowing a small amount of insulation, the primary purpose of which is to control condensation. This retrofit system is designed to be installed over R-panel roofs that are not severely out-of-module. The weight of the retrofit system is about 1 psf.

    Most PEMBs more that 30 years old have 1 inches (38 mm) of fiberglass insulation with a vinyl facing. The fiberglass insulation is compressed and provides little R-value. The old vinyl facing becomes soiled, brittle and torn and does little to prevent vapor migration because of the 1.0+ perm rating. An important consideration for any reroof project is the condition of the existing insulation and vapor retarded. The existing insulation plus any new insulation must be evaluated using a dew-point analysis against the building usage and historical, local weather conditions.

    On retrofit roof projects, insulation is added above the existing R-panel roof. To address the correct vapor retarder and dew point location, a good solution is to remove the existing insulation from the underside of the R-panel roof. By cutting through the facing and insulation next to the purlins with a razor knife, the old insulation is easily removed. The existing R-panel roof acts as the new vapor retarder and provides a like-new reflective ceiling on the building interior.

    When considering an R-panel roof retrofit project, think about the suitability of the various retrofit roof solutions and choose the system that meets code requirements and provides the best overall performance and value for the building interior.

    As in every retrofit application, a qualified engineer should check the existing building and the connections of the new roof to the existing purlins or panel. The 26-guage R-panel is not adequate as a roof deck for most proposed retrofit systems. The condition of existing R-panel can determine the retrofit solution selected.

    A dew-point analysis will determine the optimum amount of insulation to add with the retrofit roof. By selecting the right retrofit solution, the new roof system can provide the owner with many years of worry-free service life.

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