"Goedjn" wrote in message
...
On Wed, 15 Mar 2006 19:36:53 GMT, "Pete C."
wrote:

Goedjn wrote:


By putting a large quantity of unprotected fuel in direct contact with
the rated wall?

Pete C.

A sheetrock wall covered in plywood takes longer to
burn through than a sheetrock wall not covered in plywood.

Quite possibly, however the burning plywood fuel attached to the wall
will in all probability burn through the sheetrock on the ceiling and
ignight the joists or rafters above far more rapidly than say some
burning boxes on the floor of the garage.

My impression is the opposite.
If the plywood on the wall isn't constantly heated by an external
flame, it's likely to just go out. Plywood backed
by sheetrock doesn't burn worth a damn, and the fire-rating
of the ceiling assumes direct contact with an open flame anyway.
My guess is that the plywood will improve the performance of
the wall, contribute toxic gas and smoke to the room it's
in, and not noticably affect the ceiling.

1/2" exterior grade douglas-Fir plywood has
a flame spread rating of about 140. (ASTM-E-84)

I don't know what the numbers mean,
(Inches per hour, maybe?)
but according to this site:
http://www.awc.org/Publications/dca/dca1/DCA1.pdf
450 is a limiting number.

Finally got a reply from local government on this. Plywood actually makes
the wall have a better fire rating although it does add to available fuel
in case of a fire. plywood attached to sheetrock does not burn as readily a
a piece just stored in the garage. Anyway installed the plywood Saturday an
first thing I noticed was how much quieter it was when the HVAC came on,
not just in the garage but in the whole house.