It is always nice if you can get a screw or two into the ceiling joists.
Most times you can't. 2-4 toggle bolts 1/8" size will hold it just fine. 2
will hold most 2 tube fixtures and the only reason I would use 4 would be to
pull it flush to the ceiling all around.
my recent experience:
tenants will tear it out if the fixture is hung from the box plus two large
toggles (thru the
sheetrock). the toggles leave large holes in the sheetrock.
i'm going to mount the next fixture using the box plus two of the retrofit
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