Depends a lot on the condition of the boards. Even though they may look and
feel really solid, the effects of being in contact with hot slate (the sun
can realy haet the shingles up) for 75 years hasprobably really dried out
the wood. this can make nailing into it fairly difficult - the wood will
splinter, crack, and break, especially when you need to nail close to the
edge. Of course, if there's rot anywhere, at least some of the boards may
need replacement.
By adding a plywood layer, the nailing problem goes away, and the number of
seams to worry about also goes down a lot.

I'd get estimates from at least 3 reputable roofers. I don't know how big or
complex your roof is, but 10K doesn't sound too unreasonable (at least for
New England), especially because your going to need to remove (and dispose
of) the old slate. Disposal is usually done by weight, and that slate is
heavy -especially if there is more than 1 layer (there may be as many as 3).

Something that may be worth looking into would be to see if a local
renovation/restoration/salvage company would be interested in trying to
salvage some of the slate. If they are, they may actaully pay you for it, or
may at least remove it and haul it away for free, leaving the roof ready for
the roofer to put a new layer of shingles.....

Another thing to keep in mind is that roof estimating is an art. Unless
your roof is in really bad shape, its impossible to give a solid estimate. A
roof that looks OK can end up needing a complete replacement of the sub
roof, and may even need some of the rafters replaced. Any roofing contractor
that doesn't explain this to you should be avoided.....

good luck
--JD





"TomCAt" wrote in message
...
I have an old house. It has a slate roof. The roof is about 75 years
old.
I have decided to replace the old roof.

A roofer with good references has given me a price of $10,000 to replace
the
old roof with an asphalt shingle roof. Now for the question:

Under the slate roof is boards. The boards are tight, as in they are
butted against each other. The roofer wants to put plywood over the
boards
before applying the new roof. Is this necessary? At the price of
plywood,
this is very expensive.

In other thoughts about taking off the old and putting on the new, please
post.

Thanks, Thomas