Celotex is a brand name for a wood fiber sheathing board. It comes in 4 foot
x 8, 10, 12 foot lengths, sometimes for roofing it is available in smaller
sizes. It is made in various versions, in the early 50s it had a very smooth
surface and was used for wall covering before drywall was popular. I still
see the tar impregnated version use as exterior wall sheathing under siding
or brick veneer. It is generically known as "insulboard" and a common brand
name of "ten-test" in many areas and is used in many cheaper building
techniques but has been replaced by many newer products. Its original name
of "insulboard" implied it added insulation value, but it was miniscule.

"Una" wrote in message ...
Under the very old asphalt shingles on my house is 2 layers of 1/2 inch
sheet board, marked "Weyerhaeuser" "insulation sheathing board" "1B"
"R1.92" and "COMBUSTIBLE MAY SMOLDER OR BURN IF IGNITED" (and indeed it
does burn easily). It appears to be wood fiber composition board. It
may be 30+ years old. A small patch of it is rotted and below that is
good tongue and groove board, which may or may not be the very same T&G
boards that make up the exposed rafter ceiling of the room below. I have
no idea yet if there is one or two courses of T&G, but I do know there
is electrical wiring in the ceiling so it is either sandwiched between
courses of T&G or it is lying between the T&G and this sheathing board
stuff.

The roofer says this sheathing board is not up to current building code
and should be removed. What is this sheathing board called? The roofer
calls it celotex (?) but the celotex I find on the web seems to be an
entirely different product...?

Una