26 September 2006
appropriate authority. When increasing slopes of existing roof decks, clearances
for rooftop structures such as curbs, base flashings, access doorsills, etc. must
be increased to the minimums recommended in the NRCA Manual. Do not use
coal tar pitch specifications above :12 slope.
Recovering Bituminous Low Slope Roofs.
Re-cover of existing built-up and MB 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
Recovering Single-Ply Systems.
When re-covering old single-ply systems, the old membrane should be
removed--especially if it is shrinking or otherwise experiencing dimensional
instability. If not removed, it may drag and distort the new membrane. At this
time most plastic and rubber membranes are not recyclable.
Re-roofing Poorly Draining Low Slope Roofs.
Poorly draining low slope roofs may require use of tapered insulation to improve
drainage or may be converted to a steep roof (refer to Construction Engineering
Research Laboratories (CERL) Technical Report M85/05 Steep Roof
Conversions). Slope conversions are often more expensive than in-kind
reproofing, but may offer the opportunity to convert to a system with lower
Tapered Roof Insulation Systems.
Tapered rigid board insulation should generally be used only when the existing
facility has inadequate slope, and other means of providing adequate slope are
more expensive over the lifecycle of the facility. Tapered rigid board roof
insulation systems are more expensive per square unit than non-tapered
insulation. Do not use wet fill to accomplish slope in existing systems. Use
tapered rigid board in crickets and saddles to ensure positive drainage.
Metal roofing is frequently selected for steep roof conversion since new
lightweight structural members easily accommodate increased slope
requirement, and the new metal roof may not need a new deck. The fire hazard
concern about the newly created attic space must be addressed (FM Global Data
Sheet 1-31). Ventilation concerns must also be addressed. Watertight sealing of
penetrations of new framing connections through and old roof and maintaining
adequate drainage of the old roof during the construction period must be