26 September 2006
A slope of :12 may be acceptable for large roof areas in certain conditions. On Navy
projects a slope of :12 may be acceptable for large reroofing projects in certain
conditions such as where existing construction constraints (e.g. windows sills) limit the
ability to increase slope. Prepare calculations to determine the minimum drainage
requirement using the calculation method as specified in the Sheet Metal and Air
Conditioning Contractors National Association's (SMACNA) Architectural Sheet Metal
Manual (current edition) in addition to recognized standards and applicable Code
requirements. Obtain the rainfall component for the calculation from the building official
local to the project site or UFC 3-400-02, Engineering Weather Data.
Roof Curb Heights.
Curbs must be high enough to not void the roof system warranty: 200 mm (8 in.) above
the finished roof surface is the minimum for military construction. Roofs with slope may
need additional crickets to assure positive drainage around equipment.
All buildings over two stories must have a roof hatch or building-attached exterior ladder
for roof access. Roof hatches are recommended for all buildings over one story.
Coordinate with UFC 4-010-01 DoD Minimum Antiterrorism Standards for Buildings.
Snow and Ice Considerations.
Most steep roofs drain over their eaves. Some low slope roofs also drain to eaves, but
most drain internally. Over-the-eaves drainage in cold climates can be problematic if
snow and ice issues are not considered early in the design process. Issues that
designers should consider are contained in the Cold Regions Research & Engineering
Laboratory (CRREL) report MP-01-5663, Minimizing the Adverse Effects of Snow and
Ice on Roofs.
Determine roof pressures and pressure zones in accordance with ASCE-7, Minimum
Design Loads for Buildings and other Structures. Coordinate component selection and
required attachments to obtain the required wind resistance rating.
For new construction, the minimum slope noted in paragraph 2-2.3 must be
accomplished in the structural deck.
Types of Roof Decks.
Steel Decks. Steel roof decks are common on military facilities. When
properly designed, they provide an economical and dependable roof deck. They are
lightweight and are particularly useful where relatively large clear spans are desired.