RE-COVERING AN EXISTING roof system with metal includes three basic scenarios: steep-slope metal over a steep-slope roof system, low-slope metal over a low-slope roof system and steep-slope metal over a low-slope roof system. Many similarities exist for all three; however, there are unique differences, as well. Reroofing, by definition, includes re-covering and existing roof system or tearing off and replacing an existing roof system. This article discusses re-covering an existing roof system with a new metal roof system that includes installation of metal panels, metal shingles or metal shingle panels over many types of existing roof systems.
LOADS AND FRAMING SPACING MUST BE EVALUATED BY THE DESIGNER TO ENSURE THE BUILDING CAN SUPPORT THE NEW SYSTEM
BUILDING CODES
Most importantly, roofing professionals need to follow the requirements in the applicable building code. The 2006 International Building Code and 2006 International Residential Code have general requirements for reroofing and specific information when re-covering an existing roof system. Roofing professionals should review the requirements in IBC Section 1510-Reroofing and IRC Section 907-Reroofing before beginning any reroofing project.
A roofing professional's first decision when reroofing is to determine whether a complete tear-off is required by the code. To install any type of re-cover roof system, the following requirements need to be met, according to the IBC and IRC;
> Re-covering is not allowed if the existing roof or roof covering is water soaked or deteriorated to the point it is water soaked or deteriorated to the point it is not a stable base,
> Re-covering is not allowed if the existing roof covering is wood shake, slate, clay, cement or asbestos-cement tile.
> Re-covering is not allowed if there are two or more applications of any type of roof covering.
If any of these scenarios occur, the existing roof must be removed before installation of a new roof system. However, the IBC and IRC have exceptions to the above three requirements. In both codes, two exceptions relate directly to re-covering with metal.
EXCEPTION 1
"Complete and separate roofing systems, such as standing-seam metal roof systems, that are designed to transmit the roof loads directly to the building's structural system and that do not rely on existing roofs and roof coverings for support, shall not require the removal of existing roof coverings".
EXCEPTION 2
"…metal panel, metal shingle…roof coverings over existing wood shake roofs shall be permitted when the application is in accordance with Section R907.4" (in the IRC) or"…in accordance with Section 1510.4" (in the IBC).
IRC Section R907.4 and IBC Section 1510.4 are identical. Both state: "Where the application of a new roof covering over wood shingle or shake roofs creates a combustible concealed space, the entire existing surface shall be covered with gypsum board, mineral fiber, glass fiber or other approved materials securely fastened in place".
In Exception 1, the codes allow metal roof systems to be installed over existing roof systems if their attachment is directly to the structure and not to the existing roof system. In this case, uplift resistance of the re-cover system does not rely on the existing roof. Loads from wind and gravity are required to bypass the existing roof system.
In Exception 2, the codes provide prescriptive requirements intended to limit the spread of fire when certain re-cover roof systems are installed over existing wood-shingle or-shake roof systems. A fire separation layer, such as gypsum board, mineral fiber or glass fiber is needed between an existing wood roof and a re-cover metal roof system to limit the spread of fire.
NRCA GUIDELINES
The Reroofing Section of The NRCA Roofing and Waterproofing Manual, Fifth Edition, provides specific recommendations about roof system re-covering and replacement. The following only is a summary; a more thorough review if the Reroofing Section is recommended.
When re-covering with a low-slope metal roof system, the following general recommendations are provided:
> A re-covered metal roof system must be compatible with or appropriately separated from the existing roof system.
> A roof deck should be secure, capable of supporting the design loads and provide the necessary pull-out resistance for fasteners used with mechanically attached re-cover metal roof systems.
> Where moisture is present in the existing roof system, any wet or saturated materials, including membranes and insulation, must be removed and replaced with compatible materials.
> Any signs of dimensional instability, such as shrinkage, ridging, splitting or cracking, must be repaired appropriately. Some areas of the existing roof system may need to be secured or removed.
A RE-COVER METAL ROOF SYSTEM MUST BE COMPATIBLE WITH OR APPROPRIATELY SEPARATED FROM THE EXISTING ROOF SYSTEM.
An existing low-slope roof system can be re-covered with a low-or-steep-slope metal roof system. One method during installation uses a post and purlin system to support the new metal roof system. This generally does not include installation of a new roof deck, but certainly could. This often is called a "retrofit" metal panel installation. There are a number of issues to be considered:
> Positive slope must be achieved. The structural framing system should provide a minimum slope of ¼:12 (1.2 degrees) for low-slope metal panels and 3:12 (14 degrees) for steep-slope metal roof system.
> The framing system should transfer the loads directly to the existing structure. Loads and framing spacing must be evaluated by the designer to ensure the building can support the new system.
> Detailing of the new roof system is critical. Roof penetration need to be extended. Proper ventilation is required in the "attic" space created by the framing. Perimeter edges need to allow proper drainage. Additional insulation may be required at the level of the existing roof system; dew-point location should be considered so condensation does not occur. Sliding snow and ice may become a concern for steep-slope systems; snow guards may be needed. Existing through-wall drainage does not occur below the new metal panels.
Many existing steep-slope roof systems can be re-covered with new metal-shingle roof systems; however, because of the uneven surface of existing coverings, including laminated-asphalt shingles and wood shakes, the aesthetic appeal of the roof system may be compromised. Installation of a heavy base or synthetic underlayment may provide an adequately smooth surface. A rosin-sized slip sheet also may be necessary to prevent new metal shingles from sticking to asphaltic-based sheets.
Because there are numerous profiles of metal shingles, all re-covered options cannot be identified in this article. However, the following should be considered:
> A generally smooth surface is required.
> The roof deck must be able to support the added load and provide appropriate fastener pull-out resistance.
> Fire and wind resistance must be considered.
> Fasteners must be long enough to penetrate through the existing roof and into or through the existing deck.
> Edge details need to accommodate the additional thickness of the existing roof covering.
There are many considerations to keep in mind when re-covering with metal. Roofing professionals must know the applicable code requirements and be familiar with industry recommendations and guidelines before re-covering with a metal roof system.
James R. Kirby is associate executive director, technical communications, for the National Roofing Contractors Association, Rosemont, IL.
