Now I'm (hopefully) back on the list with a new ISP, some more thoughts on
roofing.

I have a client who believes that he knows the best way to do flat roofs.
This involves the following buildup (from the bottom):

If reinforced concrete slab previously laid to fall, then the first two are
optional:

slip sheet - usually polyethylene sheet, but can be building paper or
geotextile

reinforced concrete topping slab laid to falls

then:

fully bonded membrane - he has a preference for a proprietary sprayed
bituminous system, but I am comfortable with well-applied torch on (multi-
layer essential) and even traditional hot kettle bituminous

membrane must turn up at perimeters to above finished paving level, and
turn down into drains, which have two stage outlets ... bottom stage at
membrane level and top stage at paving level

separation sheet and possibly geotextile and drainage mat

rigid insulation board

geotextile as filter sheet and separation layer

screed or mortar bed

adhesive

paving slabs, tiles or stone units

I also have a lot of time for open jointed suspended paving using chairs,
either adjustable or on mortar pads, installed over the membrane.  In this
case, an insulation board to the membrane is critical in colder climates
(such as NYC) because the membranes typically become brittle in cold
temperatures ... and will therefore crack and fail much more readily if
there is any sort of movement.

Somewhere in my reference collection I have a great book on flat roofs
produced in the UK.  I will see what it has to say and post more on that
later.

Another reference which is probably worth checking out is the William Allen
book called "Envelope Design for Buildings" reviewed in the most recent APT
Bulletin.  According to David Woodcock it has a chapter on flat roofs.

By the way - this system works equally effectively on plazas and
forecourts!!

Cheers

david