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 NEW NAVFAC ROOFING DESIGN CRITERIA
1. Minimum roof slope design requirement is changed from l/4 inch per foot to
l/2 inch per foot. Roofs designed with l/4 inch per foot slope often develop
poor drainage and ponding. This is a major cause of premature roofing
failure. Allowable design deflection and construction tolerance makes l/4
inch per foot roof slope impractical.
2. Use perimeter roof drains in lieu of interior roof drains when possible.
Interior roof drains should not be used on buildings having a width of 100
feet or less. When interior roof drains are used they should be located at
mid-span of roof deck between supporting walls and columns.. Also, provisions
must be made to accommodate differential movement between roof drains and
leaders. The roof drain should be designed to move with the roof assembly
when subjected to deflection. If this movement is restricted it can break
flashing connections between the roof drain and roofing membrane, allowing
water to enter.
3. Eliminate the use of parapets, copings, or similar types of raised
peripheral walls where practicable. Where their use is unavoidable, design as
follows:
a. Parapets for standard firewalls through combustible roofs shall be a
minimum of 36 inches in height. When not used as firewalls parapet height
should not exceed 18 inches.
b. Design masonry parapet walls with sufficient horizontal joint
reinforcement and control joints to control cracking. Control joints should
be designed with backer rods and elastomeric sealant to prevent water from
entering the cracks.
c. Assure that coping design includes sufficient attachment to parapet
walls to withstand local wind and seismic forces.
d. To prevent roof collapse in the event the primary roof drainage system
fails, provide scuppers through the parapet walls to serve as emergency
overflow drains.
4. Design roof flashing details to accommodate differential movement and
provide the most maintenance free design concepts. Specific examples are as
follows:
a. Minimize the number of penetrations through the roof by grouping pipes
and conduits into fewer strategic locations. See attachment one.
Avoid placing penetrations at low points of the roof.
b.
c. Do not use pitch pockets. See attachments 2 and 3. Pitch pockets
require frequent inspection and maintenance to maintain a waterproof seal,
Use flashing collars with umbrella flashings. See attachment 4.
Enclosure (1)
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