When a re-cover is barred (IRC R908.3.1.1)
Under 2018 IRC R908.3.1.1, a new roof covering may not be installed over the existing one, so a full tear-off to the deck is required, when any of three conditions applies:
- The existing roof is water-soaked or deteriorated so it is not an adequate base.
- The existing covering is slate, clay, cement or asbestos-cement tile.
- The existing roof already has two or more applications of any type of roof covering.
The layer count
"Two or more" existing applications bars a re-cover, so a finished roof never has more than 2 layers.
- Three or more would allow a third layer: the most common error.
- Four or more is looser still.
- One or more would forbid every re-cover, but a single sound layer may be re-covered.
One sound layer of asphalt shingles on a dry deck triggers no condition and may be re-covered. Slate or clay tile may not; allowing that is a named common error. A water-soaked, deteriorated roof comes off to the deck.
Slope is not a condition
Slope is not one of the three conditions. 4:12 is the OSHA boundary between low-slope and steep roofs (29 CFR 1926.500), not a reroofing rule.
Low-slope commercial reroofs (IBC 1511.1 Exception 1)
New construction under IBC 1507 needs a minimum design slope of 1/4 unit vertical in 12 units horizontal (2 percent). IBC 1511.1 Exception 1 says roof replacement or recover of existing low-slope coverings need not meet it, provided the roof has positive drainage.
Traps: rebuilding or tapering the deck to 1/4:12 ignores the exception; the exception covers both replacement and recover, not only a re-cover; and a 4:12 minimum is the OSHA boundary, not an IBC reroof slope.
Finding the section
In the 2018 IBC, the edition referenced for the exam, reroofing is Section 1511. It became Section 1512 in the 2021 IBC, so 1512 is the wrong edition. Section 1507 holds new-construction roof covering rules, and R908 is the 2018 IRC reroofing section, not the IBC.