Five Retrofit Choices Using Metal Roofing Systems

Buildings usually need new or replacement roofing installed at some point during their service lives. New metal roofing is an ideal roofing retrofit option due to its longevity and durability, which can extend the life of the roofing and require fewer replacements over time. The aesthetic appearance of a new metal roof can also make a notable visual impact to upgrade the building itself.

When considering such a metal roof retrofit on an existing building, the first thing to realize is that there are a number of choices available. Here is a quick look at five of the most common ones that can be selected from and tailored to suit a particular building. Note that since all of them will add some structural loading to the roof, all the retrofit solutions should be reviewed by a professional structural engineer to be sure the existing building can accommodate the changes.

1. Double-Lok® Clip System

When an existing building has an existing PBR roof panel or similar, a Double-Lok® clip and panel system can be installed directly over the existing roof with minimal alterations. In this case, 2″ standoff clips are run from eave to ridge and fastened through the existing roofing panels down into the existing purlins. Then new standing seam metal roofing panels are installed along these clips so they are held 2″ above the purlins and 3/4″ above the major ribs of the existing panel roof. Because of these clearances, the cavity between the old and new roofs can either be vented or additional insulation can be added. Venting the cavity can be a good option when the existing roof slope is 3:12 or greater, as this allows for convective air flow during the summer and can help reduce the chance of ice damming in the winter. For any slopes of 1/2: 12 or greater, adding insulation can improve the overall thermal performance of the building. Generally, this system allows for considerable versatility, even if it takes more parts and labor than some other solutions.

2. Retrofit Roof Panels Over Existing

For cases where a standoff isn’t sought, new, non-structural metal roof panels with a specifically retrofit profile can be installed directly over existing roof panels on roof slopes of 1/2: 12 and greater. That means there is no need to tear out the old roof, since the new panels are through-fastened into the old ones and the purlins below, all in a pattern that avoids the original fasteners. Such retrofit panels are available in 29 and 26 gauge in full range of colors and come with a factory-applied vapor barrier on the underside. Even light-transmitting panels are available for this retrofit panel system.

Retrofit
5 most common choices for retrofit metal roofing systems

3. Notched Sub-Purlins

A variation on the metal-over-metal roofing approach is to use specifically designed z-purlins with notch-outs that are screwed down to the existing purlins. Unlike other systems that run retrofit support members from the eave to the ridge, the notched purlins run parallel to both (in line with existing purlins) with the notches spaced to clear the ribs or corrugations in the existing metal roofing. Profiles are available to match virtually any metal roof produced in the last 50 years. That means they can be installed over existing roofing and, in the process, provide enhanced structural capabilities because of the added sub-purlins.

4. Grid System

When an existing purlin spacing doesn’t meet current code requirements, then the use of crisscrossing hat sections in a grid pattern can provide the additional support needed. Such grid systems are designed to go directly over existing sloped roof systems but many require the use of specific metal roofing panels to work properly.

5. New Framed Roof System

In some cases, a full, steep-sloped roof (greater than 4:12) is sought to be installed over a low-slope roof (1/4:12 – 4:12). In this case, a full metal framing system is available that will create the desired roof slope and transfer the new roof weight to the existing roof deck above the existing roof structural members. As such, the coordination of the new system with the existing roofing, insulation and structure needs to be addressed by licensed engineers. Once designed and installed, an “attic space” will be created by the new retrofit roof system, which should include proper venting in accordance with applicable codes, allowing trapped moisture to escape. It is also recommended that such “attic space” be reviewed by other building, fire, or insurance-related officials for possible sprinkling or extension of existing fire walls to the bottom of the “new” roof system. Regardless, a minimum of 3″ vinyl-faced roll insulation between the retrofit panels and the retrofit purlins will help prevent condensation and roof noise in addition to improving energy efficiency.

To find out more about which retrofit options, or options, are best suited to a particular building that you are involved with, contact your local MBCI representative.

Planning for Metal Roofing Retrofits

The decision to retrofit an existing commercial roof with a new metal one is usually based on the very real appeal of creating a long-term (50-60 years) roofing solution, achieving better energy efficiency, creating better aesthetics, or all of the above. Prior blog posts discussed these benefits in more detail and talked about different types of metal roofing retrofits. Here, we will focus on where to start in terms of planning to undertake a roofing retrofit based on covering a membrane roof with a metal-framed, low-slope, metal roofing system.

Existing Building Assessment

A successful retrofit is based on the new metal roof system working with the existing building structure and local conditions. Each of the following should be looked at first when starting the planning and design process:

  • Existing Roof Geometry: The shape (length and width) of an existing roof is important to determine the square footage of the roof, but so are the actual dimensions, since those can impact the height of the new metal roofing. The minimum recommended slope for new roofs is between ¼:12 and 3:12 , depending upon the roof system chosen for the new roof. Existing roof details such as overhangs, parapets, and the existing roof slope itself all need to be documented in order to determine how best to address them with the retrofit system.
  • Existing Roof Type: In many cases, the existing roofing does not need to be removed, but there may be ballast such as stone or other materials that are no longer needed. Oftentimes, the removal of this ballast will compensate for the additional weight of the new roof and framing system. The materials of the existing roof may also pose compatibility issues with new materials, so they should be documented to plan accordingly.
  • Existing Roof Substrate: Under the existing roofing, some type of substrate material is holding it up. It may be rigid insulation resting on a metal, wood, or concrete deck, or it may be an uninsulated substrate that has insulation below it. The specifics here need to be established, since the new metal framing will need to connect through this material. If insulation is in fact part of the substrate, then its effectiveness should be determined—has it gotten wet and been compromised, or is it still in good usable condition? Either way, how much is there?
  • Existing Roof Structure: The structural system of the building includes framing or other components that support the roof. This is what the new metal framing will anchor to and transfer structural loads to. Hence, the specifics in terms of type (steel joists, concrete beams, wood joists, etc.), the size, and the spacing are critical. Further, the carrying capacity of this system should be assessed and analyzed by a structural engineer, since the retrofit system will add 2 to 4 pounds per square foot of dead load to the roof structure. Further, this weight, plus any live loads from the roof, will typically not be distributed uniformly, but in a series of point loads. Therefore, the engineered capacity of the existing structure needs to be known to determine if any structural enhancements are needed.
  • Existing Roof Equipment: Many commercial buildings use the roof to locate mechanical, electrical, or elevator equipment. In some cases, that equipment can be moved to the ground or elsewhere, but in other cases it can’t, or would be too costly to consider. Hence the details, location, and height of such equipment needs to be known so a determination can be made on whether it can be covered and enclosed in the “attic” of the retrofit system, or if it will need to be raised to the top of the new roof.

New Retrofit Roofing Goals

With an assessment of the existing conditions in hand, the focus now becomes identifying the primary objectives of the new roof. These should be clearly articulated so the final design can address and include each of them:

  • Appearance: What is being sought in terms of shape, height, visibility, color, improved curb appeal, or other visual considerations?
  • Performance: What is the new roof being asked to address related to operations or performance issues? Common elements could be improved drainage, less maintenance, greater longevity, or more resistance to damage.
  • Energy Efficiency: Replacing a roof is the ideal time to improve energy efficiency in a building by adding new or more insulation. This could be done simply to meet current energy code requirements or to contribute to an overall energy-use reduction project at the building. In some cases, the new roofing system could enhance the ability to include energy generation, such as solar panels mounted to the new roofing system.

With proper planning and goal setting, a metal retrofit system can meet or exceed all expectations. This was the case recently at a water treatment facility in Dallas, Texas. Here is a photo of the existing built-up roof that was experiencing problems and needed replacement. It was assessed, analyzed and determined to be an excellent candidate for a retrofit metal roofing system.

Retrofits

 

Metal Roofing Retrofits
Here is a photo of the light-gauge metal framing installed to create the new low-slope planes and transfer loading to the existing building structure.

 

Planning for Retrofits
And, finally, here is the completed metal roofing, which looks better and is expected to perform better than the original roofing.

 

To learn more about MBCI retrofit metal roofing systems and how they might work on a building you are involved with, visit http://www.mbci.com/products/retrofit-products/.

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