scope:

This handbook sets forth guidelines for design of roofing and waterproofing systems. It includes low-slope and steep roof systems, design of new roof systems, and reroofing of existing buildings.

Roof-system design requires a synthesis of the following design factors:

a) Suitably firm and stable platform for roofing;

b) Slope for drainage;

c) Fire resistance (Refer to MIL-HDBK-1008, Fire Protection for Facilities Engineering, Design, and Construction);

d) Wind-uplift resistance;

e) Thermal resistance;

f) Vapor control;

g) Life-cycle costing of available systems;

h) Ice dams and sliding snow.

The roof designer should confer with his technical consultants. The structural engineer should be consulted on the first four items of the foregoing list (deck, slope, fire resistance and wind-uplift resistance), and the mechanical/electrical engineer should be consulted on the next two items (thermal resistance, vapor control), as well as location and support of rooftop equipment (HVAC condensers, mechanical vents, etc.).

Design of roofing systems requires a constant balancing of advantages against disadvantages. This requirement is illustrated by the conflict between fire ratings and insulating efficiency. Thickening the insulation of a fire rated roof-ceiling assembly beneficially reduces heat loss through the assembly, but it raises ceiling-space temperature. This increased space temperature could reduce the roof assembly's time-temperature fire rating, e.g. buckling steel joists or accelerating the burning of other combustible structural members.

Rooftop location of mechanical and electrical equipment should be avoided whenever practicable. Penthouses and covered ground level locations are preferred. Rooftop equipment creates many roofing problems:

a) Difficult flashing details;

b) Obstruction of drainage paths;

c) Increases in hazardous roof top traffic by repairmen;

d) Accelerated corrosion;

e) Weathering of the equipment itself;

f) Potential vibration annoyance.

Varying climatic conditions have a big impact on roofing design. Vapor retarders are usually needed in cold climates and for buildings with high interior relative humidity. In warm, humid climates, however, -- e.g. Hawaii and Guam -- vapor flows downward through the roof toward an air-conditioned interior. Placing a vapor retarder on the deck and below the insulation is harmful in a tropical climate. It prevents the beneficial venting of water vapor to the interior, entrapping it in the roof sandwich where it may condense and wet the insulation.(refer to Section 4).

This difference in predominant vapor-flow direction may influence the proper choice of roof system: conventional assembly with insulation sandwiched between the deck below and membrane above, or a Protected Membrane Roof (PMR) assembly, with membrane and insulation positions reversed: i.e., with insulation above the membrane. In a cold climate where the predominant vapor-flow direction is upward from a heated interior, the membrane in a PMR doubles as a vapor retarder. This is an efficient arrangement. In a humid tropical climate, however, where the predominant vapor-flow direction is downward, the conventional roof assembly's position with the membrane above the insulation may be more efficient, with the membrane again functioning both as waterproofing and vapor retarder.

Solar radiation embrittles asphaltic materials and thermoplastic sheets, which may loose plasticizer under combined heat and sunlight. Shielding of the membrane and flashings is thus important, with aggregate, mineral granules, or heat-reflective coatings required on various membranes, both to increase membrane service life and to cut cooling-energy costs.

Windspeed has an impact on roof design apart from the obvious requirement of better anchorage (or heavier ballast) in high-wind areas. "Windstripping" of aggregate-surfaced built-up bituminous membranes has been noted in such diverse locations as Guam and Grand Forks, ND.