In the 2007 CBC, SECTION 1510 specifically addresses reroofing.
For new installations, Section 1507.2.3 "Underlayment" would apply. Unless otherwise noted, required underlayment shall conform to ASTM D 226, Type I, ASTM D 4869, Type I, or ASTM D 6757.
For reroofing applications, the code does not appear to provide criteria in regards to underlayment between the existing and new asphalt shingles.
REROOFING 1510.1 General. Materials and methods of application used for recovering or replacing an existing roof covering shall comply with the requirements of Chapter 15. Exception: Reroofing shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2-percent slope) in Section 1507 for roofs that provide positive roof drainage.
1510.2 Structural and construction loads. Structural roof components shall be capable of supporting the roof-covering system and the material and equipment loads that will be encountered during installation of the system.
1510.3 Recovering versus replacement. New roof coverings shall not be installed without first removing all existing layers of roof coverings where any of the following conditions occur: 1. Where the existing roof or roof covering is water soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing. 2. Where the existing roof covering is wood shake, slate, clay, cement or asbestos-cement tile. 3. Where the existing roof has two or more applications of any type of roof covering. Exceptions: 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. 2.Metal panel, metal shingle and concrete and clay tile roof coverings shall be permitted to be installed over existing wood shake roofs when applied in accordance with Section 1510.4. 3.The application of a new protective coating over an existing spray polyurethane foam roofing system shall be permitted without tear-off of existing roof coverings.
1510.4 Roof recovering. 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.
1510.5 Reinstallation of materials. Existing slate, clay or cement tile shall be permitted for reinstallation, except that damaged, cracked or broken slate or tile shall not be reinstalled. Existing vent flashing, metal edgings, drain outlets, collars and metal counterflashings shall not be reinstalled where rusted, damaged or deteriorated. Aggregate surfacing materials shall not be reinstalled. 1510.6 Flashings. Flashings shall be reconstructed in accordance with approved manufacturer’s installation instructions. Metal flashing to which bituminous materials are to be adhered shall be primed prior to installation.
I think persuant to section 1510 condition no.1 if the existing roof covering is not adequate as a base for additiional roofing then they would be required to remove the existing and recover as a new roof system. Remember the building official should be looking at these things in order to make them work for both the applicant and the code.
Answer: Depends. The existing roof generally acts as the vapor barrier on a composition shingle roof. In every case, if the existing roof is completely shot, steps should be taken to prevent the old roof from damaging the new roof.
On a built-up roof, a base sheet should ALWAYS be securely fastened to the existing deck prior to roofing.
Mike Martins Building Inspector City of Half Moon Bay email@example.com
and Charles J. Voos CBO Chief Building Official/ Public Works Manager City of Half Moon Bay
elaborating on Don Myshrall's comment above--No you do not need underlayment as it is already there under the old roofing. Good roofers when reroofing will typically but the top edge of the new shingle into the bottom edge of the old. As long as the existing old roof was laid-out evenly, this results in a nice clean tight reroof job. Building felt would only tear and crinkle up and muck up the job. But, heh, if the manufacturer's specs require it, then you are stuck.
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