The isometric roof drain detail shown here is known as an
interior drain detail because it always is used within the field of
a built-up roof (BUR) system and drainage is directed to the
interior of the roof area.
A typical interior roof drain assembly occupies about a 4- by
4-foot (1.2- by 1.2-m) square area or about the area of one piece
of insulation board. Drains should be located in square-shaped,
gradually tapered sumps formed in the insulation to facilitate
localized drainage at drains. Round or deeply recessed, sharply
sloped sumps are not recommended because they can create wrinkling
of a built-up membrane.
NRCA does not recommend metal sump pans unless the risk of
condensation on the metal surfaces recessed below the insulation
can be prevented and the membrane can be installed into the pan
Tapered board insulation is field-fabricated to ensure positive
slope to drain and installed on top of the structural deck. The BUR
membrane then is installed over the tapered insulation into the
roof drain bowl. To ensure the ply felts lay flat, you may have to
carefully slit the felts so they conform to the taper. It is
advised to protect the drain bowl assembly from bitumen buildup by
installing a plug or damming up the assembly before a BUR membrane
is installed. (Be sure to remove the drain plug or dam at...
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