21.1
Design low slope roof systems in accordance with the
recommendations of the National Roofing Contractors Association
(NRCA) Roofing and Waterproofing Manual and this manual.
21.2
Design all roofs with slope to roof drains or gutters.
21.3
Where alterations or repairs are required to over 40 percent of
flat roof areas, reroofing the entire area should be considered.
21.4
Reroofed areas shall conform to this Article.
21.5
Low-slope roof systems include but are not limited to the
following roofing membranes with roof insulation:
1.
Bituminous built-up roofing systems
2.
Modified bituminous roofing systems
3.
Single-ply sheet roofing systems
4.
Fluid-applied roofing systems
21.6
Design low-slope roof systems with a positive slope a minimum of
1:50 (0.25 inch per foot) up to a maximum of 1:12 (1.0 inch per
foot) to drains:
1.
Use tapered insulation, sloped structural systems, or level
structural system with sloped fill to achieve the required
slope.
2.
Do not use NRCA defined "One-way slope" (Sloping to a level
valley). See NRCA - "Tapered Roof Insulation Systems."
3.
Use NRCA defined "Two-Way slope" (actually sloping in four
directions).
21.7
Use a uniform square grid to lay out roof slopes to drains. Roofs
shall not slope to level valleys, but may have one-way slopes to
gutters at gravel-stop edges.
21.8
Locate drains at points of maximum deck deflection; generally at
midspan of the deck between supports where possible.
21.9
Design single-ply ballasted roofing systems using Factory Mutual
Criteria for wind force resistance. The following are Factory
Mutual Criteria:
a.
Loss Prevention Data 1-7, "Wind Forces on Buildings and Other
Structures"
b.
Loss Prevention Data 1-28, "Insulated Steel Deck"
c.
Loss Prevention Data Technical Advisory Bulletin 1-29,
"Loose-Laid Ballasted Roof Coverings"
d.
Loss Prevention Data 1-49, "Perimeter Flashing".
21.10 Anchor insulation to deck. Loose laid insulation is not permitted
except for protective membrane roof insulation system.
21.11 Specify quality of ballast material in VA Guide Specifications.
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