Untitled - National Roofing Contractors Association

Membrane application to these decks should follow the
deck manufacturer's recommendations. Generally, attachment
of the base ply with approved mechanical fasteners
is recommended. If insulation is used, a nailable
base ply is required as the attachment layer. Insulation
should then be solidly adhered to the base ply with hot
asphalt only. Adequate ventilation of the underside of the
deck during the construction process is mandatory. In
no case should the first ply of the roof membrane be
mopped directly to precast planks.
XIV.
PRECAST/PRESTRESSED
CONCRETE DECKS
Precast concrete units are used as structural roof decks.
The manufacturing process of precast concrete units may
produce units that vary in joint elevation when set in
place. If variations in the elevations of adjacent units exceed
1/4 inch, the deck should be leveled with a fibrous
cementitious grout that has been feathered to a slope
of 1/8 inch per foot prior to roofing. Venting for this fill
should be provided. A vapor retarder may be required,
depending on the fill material used. (See Section X, Vapor
Retarders.) The joints between adjacent units should
be filled with a compressible material to prevent drippage
of the cementitious fill that is used to level the units.
Recommendations of the Precast Concrete Institute for
deck installation should be followed. Weld plates should
be provided between other structural components and
the deck units and between the deck units themselves.
The top surface is sometimes neglected in the casting
of precast units. A smooth surface, free of voids and
depressions, should be provided for proper adhesion of
the roof system. In order to provide adequate drainage,
the designer must consider the "bowing" (or vertical
camber) of precast units that occurs in the manufacturing
process of the units. Depending on structural design
and drain placement, the camber may assist or restrict
drainage. (See Section VI, Slope and Drainage.)
The designer should carefully detail all wood blocking
provisions at roof edges and penetrations to ensure pr0per
attachment of the membrane and sheet metal. In no
case should the roof membrane be attached directly to
precast/prestressed concrete decks. Preformed roof insulation
(preferably in two layers) should be installed on
top of the deck before application of roof membrane
materials.
When precast walls are used, the designer should carefully
consider the flashing provisions required to properly
secure the roof to the precast wall units. Cast-in reglets,
which are frequently used for this purpose, are difficult
to align properly. When they are not properly aligned,
they can hinder the proper installation of counterflashing.
For this reason the use of cast-in reglets is not recommended.
In all such flashings situations, consideration
should be given to camber and creep.
When precast walls are used, it is recommended that pr0-
visions be made for IaN parapet walls. The base flashing
should be fastened to a vertical wood upright whose
horizontal base is attached to the deck only. After the
base flashing has been attached to the wood upright, the
metal wall cap flashing may be installed. Then the counterflashing
may be attached to the wall cap, extending
down over the top of the base flashing. This method allows
lateral movements of the wall without damage to
the base flashing.
XV. REINFORCED CONCRETE DECKS
Reinforced concrete is used as a structural deck. The
pouring process requires large quantities of water, and
some moisture will remain in concrete decks even after
the surface is dry. Prior to the application of roofing
materials, the concrete surface must be smooth, level
and free of moisture. The deck contractor is responsible
for removing sharp ridges or other irregularities in the
deck surface. Wood bk>cking nailers should be designed
and provided at all roof perimeters and penetrations for
22 23