01 May 1999
REROOFING CONSIDERATIONS AND CRITERIA
11-1. OVERVIEW OF REROOFING. Eventually every roofing system will reach the end of its
economic life. Reroofing options include removal and replacement, re-cover (where a new system is
superimposed over what is already there), and partial replacement. Reroofing decisions begin with a
careful survey of existing conditions. This may include visual inspection, moisture surveys, and roof cut
analysis. Structural analysis may also be in order especially if a different type of roof is contemplated
which changes dead load, drainage, or seismic behavior. Reroofing is both a problem and an
opportunity. It is a problem because work has to be done on an occupied structure. There are concerns
with noise, fumes, access, and interruption of use. It is an opportunity as there is no better time to
upgrade the roof. These changes may be mandated by revisions to ANSI/ASCE 7-95 for calculating
loads or by needed improvement in drainage or thermal performance. It is an opportunity to elevate or
redesign problem roof elements, to install deck supported curbs that are flashed independently of the wall
problems or thermal insulation deficiencies.
a. General. Reroofing of poorly draining low-slope roofs may take the form of adding structural
members (and a new deck if needed) and converting to a steep roof (refer to USACERLTechnical Report
M85/05 Steep Roof Conversions) for small, flat-roof buildings. However, slope conversions are often
more expensive than in-kind reroofing. There are a number of pressures that push for leaving a
troublesome existing roofing system in place. It is important to remember the best thing to do technically
is remove the problem rather than build up on it. It is wrong to re-cover over wet or deteriorated
(1) SSSMR for Slope Conversion. Metal roofing is frequently selected for steep roof conversion
since new lightweight structural members easily accommodate increased slope requirements; the new
metal roof does not need a new deck (figure 11-1). FM Data Sheet 1-31 expresses concerns related to
fire hazards when creating an attic space with metal roofing. Metal roofing can be installed over existing
(2) New Trusses and Decking. Adding trusses or rafters and a nailable deck permits the use of
conventional steep roofing. Steep roof conversions can be both costly and cost effective.
(3) Recovering Steep Roofing. Re-covering of steep roofing such as asphalt shingles is easily
done. Building codes permit either one or two direct recovers with missing shingles replaced first and
curled shingles flattened. Generally, underlayment is not required in shingle re-covers. Wood shakes are
not a suitable substrate for new shingles or shakes but wood shingles (which are flatter) can be
acceptable if they are still sound.
(4) Recovering Existing Bituminous Flat Roofing. Re-cover of existing built-up and modified
bitumen roofing may take the form of mechanically or spot attaching a base sheet (to aggregate-free
substrates), or mechanically or spot attaching re-cover insulation when aggregate is present (where only
the loose aggregate is removed). Full attachment to an old troubled membrane is never recommended.
(5) Recovering Existing Single-Ply Systems. When re-covering old single-ply systems it is
preferred that the old membrane be removed especially if it is shrinking or otherwise experiencing
dimensional instability. Otherwise, it may drag and distort the new membrane. At this time most plastic
and rubber membranes are not recyclable.