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Reroofing |
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Rigid roof insulation is a substantial part of the cost of any roof system. When the existing insulation is found to be dry and in good condition, it
is in the building owner’s best interest to reuse this asset. This can be a viable course of action if properly undertaken.
When the membrane is removed, the existing insulations may be checked visually for damage and moisture. All wet and damaged insulation
must be replaced. To be acceptable in a partial replacement system, existing insulation must yield a smooth, rigid surface for attaching the
new system. Some insulations and membrane systems are better suited than others for this option. Asphalt attached built-up and modified
bitumen membranes can be effectively removed from perlite-based insulations. A layer of new insulation should always be installed on top of
the old insulation prior to installing any type of new membrane.
Attachment of the new system to the building structure is a key element of a successful long-term roofing system. Removal of the existing
membrane exposes the existing roof insulation and allows it to be mechanically attached to the deck system, if it is not already attached.
A new layer of insulation must be applied over the existing layer to ensure a smooth, dry substrate for the new membrane. This new layer of
insulation will also serve to cover any insulation fasteners in the existing insulation. This not only protects the membrane from fastener
back-out, but also increases the thermal efficiency of the insulation system. No membrane should be adhered directly to the old insulation. The
old insulation surface should be primed and the new insulation should be adhered in hot steep asphalt.
In a partial tear off, additional weight will be placed on the structure. The effect of the replacement roofing system on the structure should be
reviewed by a qualified structural engineer to ensure that the additional weight can be supported.
Provisions for proper drainage must be made. This can be done by the installation of tapered roof insulation, crickets, saddles, sumped
drains, or any combination of these components.
Because a new layer of insulation will be added in most cases, all flashing must be checked for proper height. Existing curbs may need to be
built up to provide an 8" (203 mm) minimum flashing height. If at all possible, units should be lifted up during the reroofing so that they may be
flashed properly.
11.16.2 Flashing Systems
Whether a partial replacement or a complete tear off, old flashings should be entirely removed. The exception to this would be with some
single ply roof systems, where the flashings can be left in place provided that they are adhered tightly to their substrate. Walls and curbs should
be repaired and built up to an acceptable height above the membrane. An entirely new flashing system should then be installed. Additional
wood blocking may need to be added at the perimeter of the building and at all base flashings in order to accommodate increased insulation
thicknesses. The designer is urged to review the flashing specifications in the JM Commercial/Industrial Roofing Systems Manual.
11.17 Recover
11.17.1 It must be emphasized that a total replacement of a failing roof is preferable to either a partial replacement or recover. The test cuts
taken during the evaluation phase will determine if the existing roof is suitable for a recover. If the test cuts reveal extensive areas of wet
insulation, then total replacement of the roof is the only viable choice.
There are some circumstances, however, which may justify examining recover as an option:
- A non-insulated or insulated system which has been thoroughly investigated and found to be dry and in good condition except for
normal wear and tear.
- When the contents of the building are so critical that they can not be exposed to possible water damage for even a short time. Computer
facilities and museums might fall into this category.
- High rise buildings in dense metropolitan areas may make the removal and disposal of an old system impractical. The Environmental
Protection Agency regulates the disposal of hazardous waste products. The local EPA office should be contacted for guidelines before
removing and disposing of any existing roof system containing asbestos or other hazardous material.
- Unusual construction techniques in the original construction may make it impossible to remove an existing roof system without damaging
the building’s structural support system.
11.17.2 Considerations
There are three basic considerations that must be taken into account before any recover system can be a success.
1. The structure must be able to safely support the weight of the new roof.
2. There can be no entrapment of moisture in the new or old roofing system.
3. There must be some means of positive attachment of the new roofing system to the building structure.
A structural engineer should inspect and certify that a building can support the added weight of a recover system prior to the start of work.
Local building codes should also be checked to ascertain that a recover is permitted.
The existing roof should be carefully checked to ensure that it contains no moisture that will adversely affect the new roofing system. Moisture
in the existing roofing system can be a cause of blistering of the new system and can cause premature failure. Prior to installing a recover
system, the old roof should be cut in a random pattern. This will ensure that if the new roof leaks, the water will not be trapped between the old
and new roofs, seriously degrading the new roofing system or becoming a safety problem. Cutting the old membrane will also allow moisture
vapor to escape from the old system. Roof vents installed in the new system will allow the moisture vapor to pass to the atmosphere. The
installation of one roof vent for every 1000 sq. ft. (92.9 m2) is recommended.
