SPECIAL REPORT PREPARED FOR THE METAL INITIATIVE
Retrofitting flat roofs with sloped metal roofs or replacing aging walls with new metal cladding is a concept that is tailor-made for these difficult economic times.
Of the 40 billion square feet of roofing installed annually in the United States, only a fraction entails retrofitting roofs with sloped metal systems. Yet replacement and repair continues to account for approximately 75 percent of all roofing work, and industry sources suggest that some 30 billion square feet of roofs will need major repairs in 2009.
Most metal roof retrofit work entails adding slope to an existing flat roof, a strategy that can generate a good return on investment in the form of lower energy costs and little or no maintenance for decades. In most circumstances, a new metal roof can be installed without having to remove the existing flat roof.
A slope as low as .25 inches 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, a metal roof’s exterior surface should last 30 to 50 years. Additionally, with a highly reflective coating, a metal roof can reflect up to 80 percent of the solar energy that would normally penetrate the building. Adding un-faced fiberglass insulation in the newly created cavity between the old roof and new roof can further increase the building’s ability to conserve energy. An additional environmental benefit is that metal framing systems, roof panels and trim are manufactured from recycled materials and are themselves more than 80 percent recyclable. 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, placing the removed materials in a landfill, and replacing it with a built-up roofing (BUR) or modified bitumen roof with tapered insulation. Tapered insulation is required by most code authorities to achieve a certain level of roof slope.
Facility executives who may not be looking to add slope but 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.
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 to meet design loads established in current building codes.
In existing metal roofs installed over solid metal decks, the panel’s clips are often positioned randomly, and not in compliance with codes. Placing the new structural members properly can correct these deficiencies without the need to remove the existing roof.
A metal-over-slope retrofit also introduces the possibility of using the cavity between the old and new metal roof surface to provide natural convective cooling. This phenomenon is known as above sheathing ventilation (ASV). 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. It costs practically nothing yet yields significant savings.
Other systems can be added to metal-over-slope retrofits to reduce energy consumption. One system that works on a similar principle as ASV is solar thermal heat recovery. This strategy uses a solar thermal system known as an air heating and ventilation collector integrated into a photovoltaic system. The collectors use air as the heat transfer circulating fluid and can be used in systems that provide heat or domestic hot water. Facility executives 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 supplement the hot water requirements of the building. 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 adding slope to an existing flat roof or re-roofing an existing sloped metal roof, a professional engineer should perform the necessary structural calculations. Choose someone who is familiar with light gauge framing and metal roof structural components and testing.
Adding a structural metal roof to an existing building is a smart decision because the system will last for decades. It is also an environmentally responsible choice because metal roofs are manufactured from recycled materials, and the system itself is almost 100 percent recyclable. Moreover, a metal roofing system allows the use of additional insulation and the installation of solar panels that reduce reliance on electric energy.
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
Metal Roof Retrofits can Offer Energy Efficiency, Durability Benefits
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