Only a fraction of the 40 billion square feet of roofing annually installed in the United States involves retrofit projects with sloped metal systems. However, replacement and repair continue to account for approximately 75 percent of all roofing work, and industry sources suggest that 30 billion square feet of roofs will be in need of major repairs in 2009.
Most roof retrofit work includes adding slope to an existing flat roof, a decision that can generate a strong return on investment with lower energy costs and little or no maintenance for decades. In most circumstances, a new roof can be installed without having to remove the existing flat roof.
In today’s market, the cost of adding a sloped metal roof system over an existing roof is, in most cases, less than the cost of removing a flat roof, and replacing it with built-up roofing or a modified bitumen roof with tapered insulation.
A slope as low as one-quarter of an inch per foot is sufficient to satisfy most metal roof warranties and can be achieved by installing light-gauge steel columns in varying lengths. After the columns are installed, steel purlins are positioned between each of the columns and the necessary bracing is installed. A new metal standing seam roof panel system is then placed on top of this sub-framing system. When the job is completed, it’s almost like having a small metal building sitting atop the original roof.
If properly maintained, the exterior surface should last at least 30 years and reflect up to approximately 80 percent of the solar heat that would normally penetrate the building. Adding un-faced fiberglass insulation in the newly created cavity can further increase energy conservation and limit heat transfer to interior spaces.
Environmentally conscious building owners and managers will relish the fact that metal framing systems, roof panels and trim are manufactured from recycled materials and are more than 80 percent recyclable at the end of their life cycle.
Old Meets New
Owners who may not be looking to add slope but simply want to replace their existing sloped metal roof can do so without the expense and hassle of removing the original roof. It’s simply a matter of positioning a light gauge structural member — notched to span over the original roof’s ribs or corrugation — directly over the building’s framing system.
The member is attached to the roof purlins through the bottom flange of the structural member and the existing roof sheet. A new standing seam metal roof is then attached to the new member. The cavity between the old and new roofs can be used to add insulation, which should allow the retrofit process to begin paying for itself quickly through energy savings.
Another reason to consider re-roofing over an existing sloped system is that the existing roof often fails to meet current code requirements for wind uplift. For metal roofs installed on pre-engineered buildings, the standard 5-foot purlin spacing often will not satisfy panel clip spacing requirements in edge and corner conditions in order to meet design loads established in current building codes.
In metal roofs installed over solid metal decks, the panel’s clips are often mispositioned to satisfy uplift loads and panel capacities. Placing the new structural members properly can correct these deficiencies without the need to remove the existing roof.
A metal-over-sloped retrofit also introduces the possibility of utilizing the newly created cavity between the old and new metal roof surfaces to provide convective cooling. By providing a continuous air gap from the eave to a ridge and venting the warmer air, the energy efficiency of the new roof assembly can be improved. Tests at Oak Ridge National Laboratory have demonstrated that this natural ventilation can reduce heat flow into the building by up to 30 percent. Above sheathing ventilation costs practically nothing yet yields significant savings.
Other systems can be added to metal-over-sloped retrofits to reduce energy consumption. One system that works on a similar principle as the above sheathing ventilation is solar thermal heat recovery, which integrates air heating and ventilation collectors into a photovoltaic system. The collectors use air as the heat transfer circulating fluid.
Building owners who install such systems are eligible for federal solar energy tax credits valued at up to 30 percent of the entire roof system, with no dollar limit. When combined with a special accelerated depreciation, the tax credits can pay for more than half of the retrofit improvements.
Another energy saving system that can be integrated into a metal-over-sloped retrofit is solar water heating, which can help to meet the hot water requirements of the building and reduce energy consumption. In fact, solar water heating can be incorporated into a solar thermal heat recovery system. With either of these systems, the new metal roof can easily accommodate the necessary solar energy hardware.
Whether you’re adding slope to an existing flat roof or re-roofing an existing sloped metal roof, a professional engineer should be engaged to perform the necessary structural assessments. Choose someone who is familiar with light-gauge framing and metal roof structural components and testing.
With proper planning, metal roofing solutions can positively impact sustainability and the bottom line. When taken together, these factors can enable a metal roof to pay for itself quickly and to continue generating a return on investment for the building owner well into the future.
Chuck Howard is a professional engineer and roofing consultant to The Metal Initiative, a coalition of industry stakeholders created to provide information on the use of metal in construction.