Q When hanging gypsum board, what happens if
you hang the walls first and then the ceiling?
A GA 216 and ASTM C840 recommend installing
the ceiling first, though they don’t explain why
A couple of reasons come to mind. First, as Bob
Wessel at the Gypsum Association explains, the joists that the ceiling
is attached to will inevitably move, which in turn is likely to
result in cracking at the joints. Also, getting a good clean joint Q where the butt end of the ceiling board meets the wall is likely to
be more of a challenge for the finisher than if the edge of the horizontally
installed wall panel is butted up tightly against the face
of the ceiling.
Q We’re doing a repair of a building constructed
in the early 1900s that has a “Caen Stone” finish. We have the original documents with the
plasterer’s “mixture, but something doesn’t look right. It looks as
if the plasterer was trying to pull a fast one on the architect to protect
a trade secret. The recipe calls for both Keenes Cement, which
is white, and gray material, perhaps portland cement, but that
seems like an odd combination. What is the correct mixture?
A Caen Stone is a faux stone look achieved with
plaster that was popular from the late 1800s to
the early 1900s. The plaster mixture is either used
as the finish coat or in all three coats of plaster and can be applied
over either lath or masonry. In high visibility and high traffic areas,
the mixture is used for all three coats so that chipping and gouging
of the finish do not reveal the different colors as would be
found if only the top coat were tinted.
According to Dick Engbrecht of USG, an article containing the
mixture for Caen Stone can be found in a November 1912, magazine
called “The Architect and the Engineer.” This recipe
instructs the plaster to mix five parts plastering Keenes Cement,
five parts Manti Utah Stone, three parts “yellow” stone, and one
or two parts mixture of white and gray (portland cement?).
But there are several recipes out there calling for a variety of aggregates
and differing amounts of lime to be mixed with the Keenes
cement, depending upon the desired hardness of the finish.
Q It is conventional wisdom in our area that a one hour
fire-rated wall does not require fire tape
over the gypsum board joints if they are backed
by metal. However, the Underwriters Laboratories Fire Resistance
Directory indicates all assemblies are tested with one coat
of joint tape. Is the tape necessary?
A The 2001 UL Fire Resistance Directory says,
“Unless otherwise specified in the specific
design, all gypsum board systems except those
predecorated or metal covered surfaces have joints taped and
joints and fastener heads covered with one coat of joint compound
(fire taped). Base layers in multilayer systems are not
required to have joints or fastener heads taped with joint compound.”
I asked UL if their omission could be interpreted to mean that
they were not used and was told that “‘unless otherwise specified”
means one must assume the compound and tape are present unless
the language specifically spells out their omission. I suspect
the confusion about the metal backing grew out of the instruction
exempting metal covered surfaces. However, having metal
framing behind the wallboard is not the same as having a metal
covering on the front of the surface. The passage about taping
only the top layer of multilayered systems suggests to me that
the idea is to keep hot and or toxic gases from passing through
the outside surface of the wall.
About the Author
Lee G. Jones is AWCI’s director of technical services.
