320
Ceiling: Generic Design Options
The application of the finished ceiling should be delayed until it has
been determined with the aid of a moisture meter that the gypsum
deck is sufficiently dry.
Some minor dusting may be caused by abrasion of the walk-on
surface; this is not considered to be a problem.
The disadvantages of poured gypsum are not considered to be critical
if the ceiling system is properly detailed and if the scheduling
implications are recognized.
2. Treatment of Walk-on Surface. The application of a sealant against
water penetration is not considered necessary. A hardener may be
applied to the top surface of the poured gypsum deck to increase its
wear resistance; this is mainly dependent upon scheduling. Any
product used should be of the type which allows the gypsum deck to
breathe. An example of such a product is Pensco True-Seal, an
acrylic resin normally used on exposed aggregate concrete to prevent
dusting. (Manufacturer: Federal International Chemical Company, San
Carlos, California).
U.S. Gypsum also manufactures a breather type hardener for gypsum.
It is a sand and gypsum product called Mastical, which is thicker,
heavier and more costly than the acrylic resin. U.S. Gypsum is
presently developing a breather-type sealant against water penetration
for use over Mastical. This product may be applicable for use on the
top surface of the gypsum deck.
3. Specific Design. An example of a specific platform design using
poured reinforced gypsum is shown in Figure 320-3. This specific
design assumes a structural beam spacing of 5'7-1/2", half of which is
33-3/4". The truss tees have therefore been laid out on a module of
33-3/4" instead of the standard 32-3/4". According to U.S. Gypsum,
there would be no premium for this increased formboard width when a
minimum quantity of 40,000 square feet is ordered, and there would be
no other adverse effects other than the fact that a deviation in detail
might affect the existing fire rating.
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