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RE: Finishing a curved ceiling - need some advice

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I recommend that you call USG at (510) 792-4400, (818) 956-1882 or (800)
874-4968.  They are the experts and they are eager to help.  I would check
with them about using layers of 1/4" board on a ceiling.  They may have
minimum support spacing recommendations that are closer together than what
you currently have.

Roger Davis
SDS Architects, Inc
205 N. Dewey Street
Eau Claire, WI 54703

-----Original Message-----
From:	Seaintonln(--nospam--at) [mailto:Seaintonln(--nospam--at)]
Sent:	Wednesday, October 13, 1999 2:27 PM
To:	seaint(--nospam--at)
Cc:	MKA97(--nospam--at)
Subject:	Finishing a curved ceiling - need some advice

I was the EOR of a custom home here in the desert. The roof was designed as
double radius GLB structure that looks something like the letter S on its
The great room is adjacent to a split-level. The downward curve of the GLBs
occurs in the Great Room while the Upward radius reaches it's peak above the
split level.

Structurally, the roof is performing as expected, however, the creative
aspects of the roof have caused a few problems in cracking of the gypsum
ceiling attached to the bottom of the GLB framing.

The roof is lightly loaded, but is still very flexible. The top of the roof
is built up using expanded polyurathane to create slope and drainage - which
is a very lightweight material.

The GLB's are deflecting under dead load (no assumed live load is present)
and I would expect the deflection to occur until a point of equalibrium due
to the dead load weight has been reached.

I believe that the contractor installed the ceiling finish incorrectly for a
radius roof. He used one layer of 1/2" Gypsum to form the ceiling. The
is (from my memory) around 45 to 55 feet - so the curvature is not extreme.
There is continued cracking both at the seams of the drywall and in the
of the drywall.

The owner has asked me to help him resolve the problem. Here are some of the
suggestions I had to offer:

1. Remove and replace the ceiling to allow the radius to be formed from two
layers of 1/4" gypsum board which is intended to allow the layers some room
for movement. I believe that the USG book covers special conditions where
curved surfaces are covered.
2. Remove and replace using metal "Hat" channels between the joists and
and the Gypsum to allow for flexibility in the ceiling without transference
of cracks in the gypsum.
3. I also suggested that the owner might consider finishing the ceiling in
elastomeric finish which would stretch with the movement in the roof.
4. I have heard that the ceiling should have been finished in lath and
plaster which would have been more forgiving for movement and creep as the
lath would allow some play in between the materials.

Do any of you have some specific suggestions as to how this problem can be
corrected. I don't feel that this is a structural issue since the roof was
properly designed and is within the allowable code limits. However, this
not help the client whose frustration level is high and can't seem to
the problem.

Please feel free to address me privately at SEConsltnt(--nospam--at) if you do not
feel that this is appropriate for the list.

Thank you,
Dennis S. Wish PE