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Reroofing |
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All roofing systems, whether new or recover, must be attached at some point to the building structure. It is recommended that a base sheet or
divorcing layer of insulation be mechanically attached through the existing roof and into the structural roof deck before the recover membrane
is installed. If the existing roofing system is securely attached to the building structure, consideration can be given to adhesively attaching the
recover system to the existing system. Insulation or a venting base sheet may be "spot mopped" with hot asphalt to a smooth surface or a
"spudded" gravel surface, and an appropriate bituminous membrane solidly mopped to that.
11.17.3 Flashing
All existing flashings on the roof must be completely removed and replaced even when the existing membrane is being recovered.
As with new construction, JM recommends an 8" (203 mm) minimum flashing height above the roof membrane. In many cases, this will require
raising existing curbs and counterflashing. New flashings must not cover existing through-wall metal flashings or weep holes in masonry walls.
Existing metal counterflashing can be reused if it is in good condition.
11.17.4 Compatibility of Roofing Components
With the wide variety of roofing materials available today, it is important to ensure that the materials used in the recover system are compatible
with those in the existing roof.
If a divorcing layer of new insulation separates the old and new roof, compatibility problems can be minimized. New insulation should be
mechanically attached through the old roof and into the structural deck whenever possible.
Systems that contain polystyrene insulation must be treated with care. Hot asphalt will readily melt polystyrene. A divorcing layer of insulation
over the polystyrene may not be sufficient protection as asphaltic or solvent-laden cements can easily flow through the joints of the new
insulation and affect the polystyrene.
Fresh coal tar pitch is usually incompatible with asphalt-based roofing systems and most single ply systems. Asphalt can generally be used in
conjunction with older coal tar pitch roofs in which the volatiles in the coal tar pitch have dissipated. Higher softening point asphalts are less
likely to be incompatible with coal tar pitch than low softening point asphalts. Extreme care should be taken when mechanically attaching new
insulation or a new roofing system through an existing coal tar pitch roofing system. If the fasteners are not properly insulated they can conduct
enough heat to cause the low melting point coal tar pitch to flow and drip into the building.
11.17.5 Recover Base Specifications
Determining the condition of an existing roof and the need for a new roof involves complex evaluation procedures. Each project has its own
specific problems that require individual assessment. The following are Recover Base Specifications for use in recovering existing roof
systems. The base specifications provide divorcement from the old roof. After application of the appropriate "Base" specifications, any
built-up or modified bitumen roofing specification shown in the current JM Commercial/Industrial Roofing Systems Manual may be selected to
be applied over the Base Specification.
Recover Specifications
JM Recover Specifications consist of two basic parts. A letter indicates how the existing membrane is to be treated, and one or two numbers
indicate how the base of the new membrane system is to be attached to the structure. Because of the complexity of recover conditions, it is
impossible for any specification system to cover every eventuality. A JM Technical Service Specialist should be consulted to determine
if a recover system on any specific building is eligible for a Guarantee. A general listing of normally acceptable recover specifications
follows the specification descriptions.
Remove Membrane (R) Local Agencies and Building Codes should be consulted regarding removal and disposal of potentially hazardous
materials. Remove only as much membrane as can be completely covered with a new roofing system in the same work day. If removal reveals
wet or damaged insulation or decking, suitable repairs or replacement must be made prior to installing the new system. Existing insulation
must be primed with JM Concrete Primer prior to the application of hot asphalt. Wood blocking/nailers must be replaced or added to
accommodate the new roofing system. Proceed with the installation of an approved New System Base followed by an approved JM roofing
system specification.
Sweep Surface (SW) Using a manual or mechanical method, remove the loose gravel from the surface of the existing roof system. Care
should be taken to ensure that the existing membrane and membrane surface is dry. Areas that are determined to be wet or damaged must
be completely removed and replaced with materials that are compatible with the new system. The existing membrane should then be randomly
cut with an axe or other device at an approximate rate of one 6" (152 mm) cut per 10 square feet (0.92 m2). Proceed with the installation of an
approved New System Base, followed by an approved JM roofing system specification.
