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Retrofitting a Low-Slope Roof

Get your flat roof retrofitted right

According to recent studies, about 25 percent of U.S. commercial, institutional and public buildings are now 55 years old or older. Much of that commercial, industrial and school space is flat-roof stock that has reached the end of its service life. A flat roof can be replaced again or retrofitted with a sloped metal roof system that can be a long-term solution to roofing problems.

"There are a handful of manufacturers who make some very successful and nationally approved systems for low-slope metal retrofit roofs," says Dave Rowe, director of product management at Englert Inc., Perth Amboy, N.J. "A variety of light-gauge steel built-up framing and nested sub-purlin systems allow the architect or owner to retain the building's structural properties while correcting chronic problematic roof geometry that will only repeat the current roof headaches. For example, low-slope retrofit systems can solve problems that may result from existing roof geometry, such as poor drainage, snowdrifting or the addition of a new building adjacent to the existing structure."

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A sloped retrofit metal roof incrementally extends the life expectancy of the building, reducing roof maintenance costs. "The slope could be increased to improve solar panel performance," says David Stermer, director of engineering at Metal Sales Manufacturing Corp., Louisville, Ky. "Also, the slope could be increased to improve the aesthetics of the roof by adding hips, valleys and color. Increasing the slope can improve the weather tightness by making precipitation run-off faster."

There are other reasons a low-slope metal roof should be retrofitted into a conventionally sloped roof, such as when a simple roof-over will not eliminate the risk of leaks, says Tim Lane, president of TopHat Framing Systems in Painesville, Ohio. "This may occur, for example, when there are interior gutters inside the wall line," he adds. And, "when the slope of the roof is just too low. In this case, it's unlikely that a weatherability warranty will get issued. Also, when the owner or designer wants to change the architectural features of the structure, for example, he can completely change the look of a '70s style building into something that looks modern."

Pre-retrofit evaluation

Before retrofitting a low-slope roof there are some evaluations that must be undertaken to ensure its success. The contractor must first do a thorough investigation of the roof including an evaluation of the existing building's structure by a professional engineer. This can determine the products and methods required for it, and the existing structure's structural capability to accept the retrofit.

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According to Don Riggs, assistant general manager of project service at MBCI in Houston, a contractor must first ask the following questions before retrofitting a low-slope metal roof:

• Is it due to poor installation, atmospheric deterioration causing corrosion, or a breakdown of the metal and sealants over time?

• Is the existing roof a through-fastened system or concealed-fastener standing seam?

• Is the end user wanting to install the same type roof, or in the case of a through fastened roof, upgrading to the concealed fastener standing seam?

• Does the end user wish to upgrade the insulation values of the building?

• Does retrofitting the building require meeting current building code requirements for wind design?

• Is the existing building designed to accommodate the additional weight of a retrofit system or will the existing roof need to be removed and replaced? Additionally, according to Lane, the following questions should be asked:

• Can we meet the needs of serviceability?

• What is the function or future purpose of the building?

• How do new building codes compare with original codes? For example, new code often requires that frames are closer together.

• What has changed in or around the original structure? For example, is the owner hanging heavy equipment from the ceiling that needs to be designed around?

• Are the existing building, new frame and new roof compatible?

Rick Dodge, vice president of Paramount Metal Systems, a design-build contractor specializing in pre-engineered steel structures and roof systems in Little Rock, Ark., uses an inspection sheet his company fills out during a physical inspection of the roof being retrofitted. "Depending on the design criteria, we will also do pull-out tests for the fasteners to be used in anchoring the new substructure to the existing roof. Be sure the roof you are going over will carry the new roof load and determine the correct fasteners to secure the new sub-framing to the original structure."

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Once the investigation is completed, the contractor and end user must determine the retrofit cost based on the contractor's recommendations to the end user. "It is always important in retrofit to establish a contingency of funds for items that are impossible to discover during the initial evaluation phase," says Riggs. "The contractor with a completed agreement is then responsible for complying with the requirements of the building code which may include submission of plans and engineering to the proper code official. Mobilize the site by making sure all safety concerns are addressed especially of building occupants as most retrofits are performed on occupied buildings. Provide the owner with a plan for the safe entrance and exit of the building occupants, as well as areas around the building that need to have restricted access to prevent injuries due to falling material or equipment movement."

Square and true

All roofing, low-slope retrofit or otherwise, must be installed square and true. But, most existing buildings, due to settling, will be out of square and the new roof must be installed perpendicular to the existing building's eave. Custom flashings may be required to close the new roof at the rakes of the building due to the out of square conditions. "If custom framing is required, the crews must make sure the new framing is installed over the existing framing of the building," says Riggs. "Always make sure when installing new framing to the structure that the penetration of the fasteners to the structure are waterproofed to prevent leaks prior to the installation of the roof."

From flat to fixed

Once these and other pre-installation steps are successfully completed, the low-slope roof retrofit can begin. "This system would consist of a base clip, variable height post, and a purlin system that would create the required positive pitch for a new metal roof panel system," says Chuck Howard, PE, president of Metal Roof Consultants Inc., Cary, N.C. "This sub‐framing system would need to be braced to make it structurally sound prior to the installation of the roof system. Then a standing seam metal roof system would be installed to the sub‐framing system. Insulation could be installed in the resultant roof cavity to increase thermal resistance of the new roof assembly. Positive ventilation between the existing roof and the new metal roof is required to remove any remaining moisture in the existing roof, as well as eliminate the possibility of continual condensation within the cavity." Stermer breaks down low-slope roof retrofit into four primary components:

• Attach the base supports to the existing structure

• Install the posts and framing members, including bracing

• Install the gable wall girts and panels

• Finally, install the new metal roof

 "A low-slope metal roof will need framing to develop a steeper slope, while a conventionally sloped roof does not need the framing associated with a change in slope," he adds. "In locations with high snow loads, the change in slope may create step conditions that must be designed for greater snow loads."

Lane agrees that installing a direct-connect retrofit system is just as easy as it sounds. "Directly connect the pre-punched retrofit member to the frame with pre-approved screws, one screw per hole," he says. "Once the frame is set, load roof panels right on top of frames and install." What makes low-slope roof retrofit unique? With a conventionally sloped metal roof, there is no need for the variable height post to create positive fall.

"An engineered system, such as a Roof Hugger system [from Lutz, Fla.-based Roof Hugger] can be utilized to separate the new roof from the existing roof in an existing metal roof condition, or a clip and purlin (minus the variable height post) can be installed over an existing shingle roof, providing a structural plane onto which to place a new metal roof system," says Howard.

When dealing with a flat roof as opposed to one with a higher pitch, gravity becomes a consideration. "Material movement and manpower movement on a low-slope structure is a lot easier," Lane says. "Fatigue is much less on ankles and body. For work on conventional slope roofs, safety becomes an even more important consideration. For example, when you are putting up panels you need to keep them secure until they are completely fastened. Unfortunately it can be easier to damage material, injure workers, damage the building, or even injure employees inside the building. On a pitched roof, you are very visible to the public, which means unsafe practices are even more likely to be noticed. Work on a traditionally sloped worked could have more scrutiny and will be a chance for you to demonstrate the quality of your company's work."

All retrofits must be installed according to the manufacturer's requirements. Following the manufacturer's details will ensure compliance with warranty. "If manufacturer inspections are required, make sure the inspector is available in order to not delay the project," says Riggs.

Additionally, there are consistent standards and guidelines with respect to all retrofit metal roofing. "The International Building Code does allow this type of system to be installed over two existing roofs, without the removal of the existing roofs, since the new system will be loading the building structural members and not the existing deck," says Howard. "Other than that, sound engineering principals must be used to design the new sub‐framing and the attached metal panel system, both designed to satisfy wind loads established by ASCE 7 (current edition). Finally, only panels that have undergone sufficient ASTM E 1592 structural uplift testing should be used for these systems. This testing is necessary to be able to structurally determine how the total framing and panel system will withstand the required wind uplift values."


Retrofit Tools

For a low-slope retrofit, most of the tools of the trade are the same as for any metal roof system, such as snips, screw guns and caulk guns. "For most cases, it's just your normal metal roof/panel installation tools, screw guns, nibblers, snips/ shears, roof seamer, if applicable," says Rick Dodge, vice president of Paramount Metal Systems, Little Rock, Ark. "Equipment can vary, but it can include a forklift for off-loading, a crane sometimes to hoist bundles of roofing, or boom lift."

"A chop saw is used to cut members to length," says David Stermer, director of engineering at Metal Sales Manufacturing Corp., Louisville, Ky. "A screw gun is used to attach framing members and attach the roof to the supports with selfdrilling screws. A mechanical seamer and hand seamer are used to seam standing seam roof systems. A rivet tool and drill are needed to fasten trim together with pop rivets. Other tools such as level, chalk line and tape measure are needed to properly orient the roof. Tools such a snips, shears and hemming tools are needed to customize panels and trims at hips and valleys. A caulk gun is needed to apply tube sealant."