I am primarily a kitchen and bathroom remodeler, but have, recently come across a situation with which I am unfamiliar, and in need of advice:

A friend asked me to look at his roof, with the idea of correcting multiple leaks. The house is post-and-beam construction, with 8" x 8" beams at each bent, the bents being approximately 7 feet apart. Over the rafter beams is solid 2" x 6" tongue-and-groove roof decking, with beveled corner edges, which constitute the interior finish of the vaulted ceiling(s). [Potlatch, or equivalent]. The solid insulation above this appears to be 3" of a non-vented Homasote-like material, tan in color, with exterior roof edges built up all around with framing, to create a nailing surface for the exterior rake and eaves facia boards, and trim. The tan color is indicative of having been treated, somehow, since the product is normally gray in color, being compressed cellulose. The "homasote" insulation is overlayed with roofing felt paper, attached with staples, over which is a layer of organic 3-tab shingles, nailed with 1 1/4" roof nails, with the normal aluminum rake and drip edge roof trim. By measuring up from the interior ceiling to the under side of the skylight glass, adding 3/4" for the glass sandwich, then measuring down from the top of the skylight to the roof surface, and subtracting, the overall thickness of the roof appears to be about 5". Subtracting the approximately 2" thickness of the "Potlatch" roof decking gives the thickness of the non-vented "homasote" insulation of approximately 3".

The roof is about 25 years old, is leaking, and the organic shingles are curled and brittle, in need of total tear-off and replacement. The question has to do with the "homasote" insulation: does it support mold and mildew after it has been wet from a roof leak? If so, then the replacement of the insulation is also indicated. If not, then my thought would be to vertically strap the surface of the insulation with 1" x 3" furring strips, 16" O/C, fastened through the "homasote" to the "Potlatch", with 4" screws, to create a vent space above the insulation, then sheath with 1/2" CDX, which holds roofing nails better than does the "homasote", and roof over conventionally, with new trim, Ice-N-Water, felt, and shingles. If the insulation needs replacement, then the thought would be to substitute foil-faced polyisocyanurate, strap, sheath, vent, trim, and roof.

Does anyone out there have any experience with this construction? My main concern is with whether or not the tan-colored "homasote" needs removal and replacement.

I've used a Homasote product that was a layer of the Homasote material (basically made from recycled newspaper, I believe) plus a layer of isocyanurate foam. It was installed over plywood sheathing with no venting provisions at all. The roofing was an adhered sheet, because the roof slope was minimal.

As far as I know, venting is not necessary with either isocyanurate or Homasote, or the combination of the two. There are billions of square feet of commercial roof applied over such insulation with no venting provisions whatsoever. If it were me, I would screw the plywood through the Homasote, without the venting strips.

I know the Homasote material is treated for fire retardant capabilities. I don't know if it is treated to resist mold, or whether it can sustain mold growth. You might try the Homasote web site for some information about this.

It is true that there are billions of square feet of unventilated roof systems like this. However, some shingle or other system manufacturers will void their warranty if certain substrates and venting aren't installed prior to their systems. I would first check with the manufacturer of your shingles. Also, code officials in different areas will also require venting under certain situations. I would verify with a local code official that venting isn't required before going this path.

What would be the purpose of the small vent space that would result, especially since the space would not be vented to the outside at all. The type of construction decsribed is a standard method for post-and-beam buildings, and I can't imagine that either roofing manufacturers or building officials would have any problem with it.

I don't see the point of replacing the insulation if no mold is present, nor do I see any advantage in creating a non-vented "vent" space.

Perhaps the insulation is a Celotex porduct...I seem to remember that it was a tan color. It was probably cellulose in nature, and I don't know if it's still made in that form, although the Celotex name is still with us.