Now that everybody's weighed in on quarry tile terraces over roofing
membranes, and I'm home rather than at work, here's my two cents worth.

We have run up against quarry tile on setting beds in many cases in NY,
dating from the 1920's to the 50's.  Also had slate pavers on setting bed,
over coal tar at Chris' good friend and fave architect Rosario Candela's 740
Park (where John D Rockefeller Jr moved when he abandoned the old man's house
on W 54th Street).

As Ken noted, water gets in the setting bed, freezes, and turns the setting
bed back into sand and non-cementitious cement.  If the membrane beneath is
OK, I suppose this is just a nuisance, other than being a tripping hazard,
providing soil for growies to grow, etc.  However, if the membrane is bad, or
has gone bad, you're in deep roofing, since the water runs all over the damn
place, all below the level of the drains which are typically set flush with
the pavers; so the whole system has to be waterlogged before anything
substantial flows into the drains.

We have had a few cases where people insist on quarry tile, and I don't know
that they ever really come out right.  In one case, we did one with our good
friends at Apple for one of our patronesses of the arts, where the setting
bed was placed over drainage mat, which theoretically should have drained all
the water out beneath the setting bed.  As I remember, we opened it up later
and found low spots ponded and setting bed destroyed by freeze-thaw action.

What we recommend, and have done with success (as far as I know) is put down
coal tar, then drainage mat with waffles to conduct water to the drains which
have to be set at (or below) membrane level; rigid insulation board, filter
paper, plastic pedestals and then precast 2' x 2' pavers (or better yet,
Leland's bluestone pavers.

The pavers let water run through the open joints between them, and into the
space beneath the pavers, where it eventually finds its way to the drain.  It
is essential to eliminate ponding in the course of placing the membrane, to
assure that you don' t form mosquito breeding grounds in areas where the
water just lays there and is periodically replenished by rain, idiot tenant
shareholders watering their stupid plants (they don't call them vegetables
for nothing), and idiot tenant shareholders above them watering their stupid
plants and flooding your terrace.

Probably more raving to follow.  I'd still go with slate on a pitched roof.

Ralph