Managing Metal-Over-Metal Retrofits

All over the U.S., there are metal roofs on buildings that are aging and have exceeded their expected service life. An independent study conducted in the late 1990s with permission of the Metal Building Manufacturers Association found that there was nearly 12 billion square feet of metal buildings shipped in the U.S. between 1970 and 1990. With billions of square footage of metal roofs needing to be replaced, metal-over-metal retrofits are a cost-effective solution that allows a new metal roof to be installed on top of the existing metal roof without having to do a tear-off.

With billions of square footage of metal roofs needing to be replaced, contractors and owners need to do their due diligence

By Marcy Marro

Photo courtesy of Roof Hugger

Mark James, marketing consultant at Lutz, Fla.-based Roof Hugger LLC, says to compete in the retrofit marketplace, the focus should be to educate the building owners, designers, contractors and specifiers about the ability to put a new metal roof over an existing metal roof with a structural sub-framing system.

Due Diligence

Metal-over-metal retrofit projects can lend themselves to a number of common issues or errors. Some common errors include not meeting current wind loads, adding too much weight with the new roof system, or not evaluating the suitability of the existing structure prior to doing a retrofit project.

Charlie Smith, national recover manager at McElroy Metal, Bossier City, La., adds errors can also include improper connections, such as attaching a hat section to the top of the ribs of the existing roof panel instead of attaching to the structure below. Or, “installing retrofits over an existing standing seam roof that was installed with high floating clips and not compensating for the space between the existing panel and the top of the purlin.”

To avoid many of these common errors, Tim Lane, president of TopHat Framing System LLC, Ravenna, Ohio, says the most important thing a contractor can do is the due diligence when working on a metal-over-metal project. “It’s important to spend enough time understanding the existing building and how they’re going to transform it to meet the owners’ needs.”

Contractors need to take their time looking at the roof from different directions, Lane says, otherwise they’re likely to make a mistake that will either cost them money, or they’ll end up burying it and the owner will pay for their mistake down the road. These types of mistakes include not putting in the correct screws, or using the right amount of screws. “The biggest mistake a contractor’s going to make is not dealing with somebody who can provide a little deeper expertise.”

The concern, Lane says, is that a contractor thinks they can just buy a frame, bolt it on, put a new roof on it, and everything will be OK. “From an engineering standpoint, the chance for a screw up is pretty big,” he explains. “Are they looking at what the existing building is capable of? Are they looking at the codes? Are they taking enough time to make sure that when they’re done they’ve done the right thing?”

Photo courtesy of Roof Hugger

Wind and Snow Loads

Completing a metal-over-metal retrofit can also bring an existing roof up to current code requirements. The current wind uplift pressures may be significantly higher than they were when a building was originally built, and by increasing the purlin load capacity of an existing building with additional brace points, a metal-over-metal retrofit system can bring a building back into code compliance. “What’s important is that the retrofit sub-framing system has been engineered to meet the design tested wind speed loading that the new metal roof has been tested for,” explains James.

In addition to looking into the current building codes, Lane recommends seeing about any newer codes that may be adopted soon. “Owners should be concerned about the long-term ramifications of what they are buying,” he explains. “If they allow a contractor to build to a code that is in the process of expiring, maybe there’s some relevant information in the upcoming new code that has an important meaning to the owner. If not then, maybe five, 10, 15 years down the line, whatever that service life of the new structure is, does have meaning to the owner. I think the contractor has a responsibility to do all of that stuff, and certainly the owner should do some of that due diligence, and have those options spelled out to them before they buy anything.”

In addition to increased wind loads, contractors need to be aware of snow loads in certain climates. For example, “In northern climates,” James says, “because we have this ability to increase the weight-carrying capacity of the existing roof, we can actually increase the snow load. And that has changed in codes as well.”

Photo courtesy of Roof Hugger

Structural Loads

It’s important for a qualified structural engineer to inspect a building for suitability prior to starting a metal-over-metal retrofit project. When it comes to a building’s structural load, Smith says, “There are provisions in the International Existing Building Code (IEBC) that allow for the addition of up to 3 pounds per square foot of dead load when doing a retrofit, but that does not mean you do not have to inspect the structure.”

But in many cases, the existing purlins may no longer be good. In that case, Lane notes that putting a high-performance roof system with a super high uplift and load rating on a frame that was put in 40 years ago under a different code setting won’t help the roof do what it’s designed to do. “Just because you put a high-strength, state-of-the-art metal roof on a building, doesn’t mean you’ve improved the purlins you’re expecting to handle this new capacity,” he says. “Is it really that much stronger, when what [the new roof] is attached to certainly isn’t? The point is that the purlins that are holding the roof are under-designed. They were designed properly when the building was built, but the codes have outrun the structure’s capacity, and they are under-designed for today’s standards.”

As Smith explains, “In certain instances, structural enhancement of a metal building can be accomplished by using a notched purlin. There has been quite a bit of testing done to demonstrate that rigidly attaching a notched purlin over an existing purlin on a metal building will increase the load carrying capacity of the existing purlin. There are many factors in play here. First, this only works when retrofitting over an existing exposed fastener system or a standing seam installed with low floating clips. If the existing roof is a standing seam installed with high floating clips, no enhancement is produced.”

However, if the current structure is under-designed or over-stressed, Smith says retrofitting with a notched purlin can bring the structure up to code without working on the inside in many instances. “It may be necessary to mirror or extend the existing purlin laps with the retrofit purlin,” he says. And, he notes, structural enhancement of the purlin lap from the outside is the newest development in metal-over-metal retrofits.

Photo courtesy of Roof Hugger

Increased Energy Efficiency

From an energy efficiency standpoint, metal-over-metal retrofits allow owners to ventilate the space between the two roofs, which decreases the roofs heat gain. “Metal-over-metal retrofit will always provide an opportunity to add insulation or an insulating air space between the two roofs,” Smith says. “If no insulation is added, the air space should be ventilated.”

Metal-over-metal retrofits also allow an owner to increase the thermal resistance value of the overall roof assembly. For example, James says, a R-4 or R-5 thermal value has been increased all of the way up to R-50 with added high-performance insulation.

However, Smith says one of the advantages of doing a retrofit is it is not required to bring the building up to current energy code requirements for additional insulation.

Additionally, metal-over-metal retrofits give an owner a chance to add photovoltaics, solar thermal and hot water systems, or even rainwater harvesting applications to a building.

Photo courtesy of Roof Hugger

Overall Benefits

Choosing a metal-over-metal retrofit to replace an aging metal roof can help reduce overall project costs due to its faster construction times and less required labor. Since the original metal roof stays on the building, there is less disruption, allowing the building occupants to normally continue working while the project is being completed.

As long as the original metal roof panels are not deteriorated from environmental conditions, a metal-over-metal retrofit can put virtually any type of metal panel over any type of existing metal panel.

“Metal-over-metal retrofits provide so much opportunity for an architect, design professional and a metal roofing contractor,” James notes. “There’s so much opportunity, and it is the absolute easiest way to get into the retrofit market.”