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 roof or modified bitumen roof with tapered insulation.

Most roof retrofit work entails adding slope to an existing flat roof, a decision that can generate a strong return- on-investment in the form of 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.

A slope as low as 1/4” 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 is almost like having a small metal building sitting atop the original roof.

Sloped Metal Roof Should Last 30 to 50 Years
If properly maintained, the exterior surface should last at least 30 to 50 years and reflect up to approximately 80 percent of the solar energy that would normally penetrate the building. Adding un-faced fiberglass insulation in the newly created cavity can further increase the building’s ability to conserve energy used to control temperatures in the interior spaces.

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 land-fill, and replacing it with a built-up roof (BUR) or modified bitumen roof with tapered insulation. Tapered insulation is required by most code authorities to achieve a certain level of roof slope.

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 themselves more than 80 percent recyclable. In essence, sloped metal roofs are the ultimate in “green” building products.

Retain Old Roof when Retrofitting with Metal
Owners who may not be looking to add slope, but simply want to replace their existing sloped metal roof can do  so, again without the expense and hassle of removing the original roof. It is 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.

Adding a structural metal roof to anexisting building is a smart decision
because the system will last for decades.

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 preengineered 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 miss-positioned 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 ntroduces the possibility of utilizing the newly created 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-sloped retrofits to reduce energy consumption. One system that works on a similar principle as ASV is solar thermal heat recovery. It uses air heating and ventilation collectors integrated into a photovoltaic system. The collectors use air as the heat transfer circulating fluid. Building owners that 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 serve the hot water requirements of the building and thereby 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.

Professional Engineer Should Provide Structural Calculations
Whether 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 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 since metal roofs are manufactured from recycled materials, and the system itself is considered 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.

Chuck Howard is a professional engineer and roofing consultant who has specialized in metal roofs for more than 30 years. He currently provides commercial roof consulting to contractors, architects, building owners and The Metal Initiative, the educational arm of the metal roofing and wall industry in North America. For more information, contact The Metal Initiative at www.themetalinitiative.com or call 847.375.4785.