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Finishing a curved ceiling - need some advice

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I was the EOR of a custom home here in the desert. The roof was designed as a 
double radius GLB structure that looks something like the letter S on its 
side.
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 radius 
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 field 
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 beams 
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 an 
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 does 
not help the client whose frustration level is high and can't seem to resolve 
the problem.

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

Thank you,
Dennis S. Wish PE