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