Fields BUR Systems Design Criteria Roof Decks

Where expansion joints are deemed necessary, they shall extend across the entire width of the roof deck; they shall never terminate short of the roof perimeter. Expansion joints shall be designed to accommodate contraction as well as expansion. Expansion joints shall be detailed and constructed to a minimum raised height of 10 above the finished roof system surface.

All expansion joints are required to be raised curb designs. Water shall not drain through or over an expansion joint.

Area Dividers

In the absence of expansion joints in the building’s framing system, or where the distance between expansion joints is excessive, area dividers shall be installed to assist the control of thermal stresses in the roof assembly. Area dividers minimize the transmission of stress from one area of the roof assembly to another by dividing the entire roof area into smaller sections. These sections shall be of rectangular shape and uniformly spaced. The designer shall specify the location for area dividers.

The separation of large roof areas provides locations for termination of the roof system, and aids in the control of stress concentrations and fatigue of the BUR membrane. Area dividers can also be installed to facilitate drainage and future reroofing.

Area dividers shall be designed as raised double wood curbs fastened to wood base plates that are attached to the roof deck. Area dividers shall extend a minimum of 8 above the finished roof system surface; shall be located at the high points of the roof assembly, with drainage away from or parallel to the area divider. Area dividers shall not restrict or impede water drainage. Elastic preformed control joints that are designed to be installed in the horizontal plane of the roof are prohibited.

Cants, Curbs, and Nailers

The most vulnerable area of a roof for water entry is where the horizontal roof deck intersects with a vertical penetration. Most water leaks occur at these flashing junctures. Therefore, extra attention shall be given to the design and installation of such flashings.

Cants: The bent radius of the roof and flashing membrane shall not exceed 45 degrees. Cants shall be installed at a angle changes such as those created by roof to wall, roof to curb, or other roof to vertical surface intersections. The installation of cants shall be to lessen the angle of these horizontal to vertical plane changes for the benefit of the BUR membrane.

Curbs: Curbs shall not be installed upon or fastened through roof insulation. All curbs, wood blocking, and penetrations shall be firmly secured to the roof deck before installation of the roof system. Drains, and vent pipes shall be secured prior to installation of the roof system so as to prevent damage to the roof system.

Support Curbs for Rooftop Installed Equipment: The design of the roof deck shall allow for concentrated loads imposed by rooftop installed equipment so as to control deck deflections from damaging the roof system. Vibrations from rooftop installed or joist installed equipment shall be isolated from the roof system. Poorly designed or installed equipment may allow moisture to enter the building either from the exterior or from condensation within the equipment. Therefore, equipment housings must be watertight and insulated. Condensation from rooftop installed equipment is required to be discharged through piping into roof drains, scuppers, or self draining roof perimeters.

Support curbs for equipment mounted on the roof with built-in metal base flashing flanges are prohibited. Only raised curbs that provide for the correct vertical extension of the roof membrane sheets; proper termination of the flashing membrane sheets, and installation of sheet metal counterflashings shall be installed.

Nailers: Well secured, preservative pressure treated, and decay resistant wood blocking and nailers shall be installed at all perimeters and penetrations for fastening the flashing membrane and sheet metal counterflashings. Wood nailers shall be installed on all prefabricated curbs and hatches for mechanical attachment of the flashing membrane.

Leave a Reply