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/.

Level of Development (LOD) BIM Specifications for Metal Buildings

When designing and constructing metal buildings, an increasing number of professionals are using a computerized building information model (BIM) as their primary tool. This allows for detailed, three-dimensional computer models to be created, not only to develop the design, but to identify material lists, coordinate details, avoid conflicts between building systems and streamline the design and construction process.

Problem: BIM Coordination

Of course, design is a process that requires some back-and-forth between multiple parties to arrive at the best final solution. So, when a metal-building supplier or manufacturer is asked to provide their information to be incorporated into a BIM process, the question that naturally comes up involves the level of detail. This is common across all trades, and fortunately, there is an organization that is addressing this issue. Known as the BIMforum (www.BIMForum.org), is is the not-for-profit United States chapter of buildingSMART International, and its mission focuses on improving BIM technology, collaboration, education, innovation and open information exchange. As they describe themselves, “Co-sponsored by the Associated General Contractors of America (AGC) and the American Institute of Architects (AIA), BIMForum seeks to lead by example and synchronize with counterparts in all sectors of the industry to jointly develop best practice for virtual design and construction.”

Solution: Level of Development (LOD) Specification

A flagship publication of BIMForum is the 2016 version of Level of Development (LOD) Specification. Having evolved over several years, this publication is “a reference that enables practitioners in the AEC Industry to specify and articulate with a high level of clarity the content and reliability of Building Information Models (BIMs) at various stages in the design and construction process.” Coordinated with other industry standards, it “defines and  illustrates characteristics of model elements of different building systems at different Levels of Development.”

Essentially, it defines and standardizes how much detail is expected in a building information model at different stages of design development. Therefore, if a metal-building manufacturer or any other trade is asked to supply its BIM information, then it needs to ask “What Level of Development?” so that is it providing the right amount of information to coordinate with the larger computer model for the building.

How LOD Works:

The LOD Specification is based first on the familiar Uniformat specification sections used by most spec writers. Metal Buildings commonly fall under Special Construction in Section F1020.40 in the Uniformat approach, or 21-06 10 20 40 in the Omniclass approach, and are found that way in the LOD Spec. From there, five levels of detailing are described by the numbers 100, 200, 300, 350 and 400, as described further below.

  • LOD 100 – This is the most basic of model, described as “Generic mass of special structure with system typically noted with a design narrative for conceptual pricing.” It is likely that this level of BIM is already developed by an architect or engineer and given to a manufacturer or supplier as a starting point.
  • LOD 200 – This level calls for basic primary structural member sizing, generic representation of secondary framing, and general cladding and exterior trim to be provided, including openings.
  • LOD 300 – More-specific sizing of all needed primary frame structural members, web tapers, frame connections and similar details are called for at this level. Similarly, secondary framing needs to be shown, including purlins and bridging, girts, subframes and base conditions. Exterior panel and trim with actual profiles, actual openings and all significant trim and accessories are shown here.
  • LOD 350 – This level starts to show coordination with other elements or building systems. Therefore, for the primary structure, things like base plate locations, bracing/gussets, clips and any reinforcement all need to be included. Secondary framing elements need to include similar details, such as nested members, connections to primary structure, any miscellaneous or secondary steel members, bridging, etc. Cladding and exterior trim would include all actual profiles, closures, downspouts and all minor trims shows at least generically.
  • LOD 400 – This is the full-fabrication level equivalent to shop-drawing level of detail. As such it includes all final details, including welds, bolts, holes, cinching and all other details of fabrication and assembly for primary and secondary framing, plus all cladding and trim.
BIM
Level of Development (LOD) Specification Example – image courtesy of BIMForum.org

By using these standardized Levels of Development, all design and construction professionals can proceed in an orderly sequence to provide the appropriate information, receive coordination feedback and then move on accordingly to the next level.

The full 2016 LOD Specification can be downloaded for free at http://bimforum.org/log/. The specific information for Metal Building Systems can be found on pages 177–186. For information on how to work with a manufacturer to provide the appropriate BIM information, contact your local MBCI representative.

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