Hi,what is the acceptable/or the best procedure
to lay tile in 90 year old house.
The person that look at the job wants to use
1 1/8 toungr&groove plywood which i think is overkill,not only that but is quite expensive
I think 3/4 inch plywood would be just as good
as we are thinking of going down joists or can we still get good floor by laying3/4 inch plywood on old floor.
I think the reason this person would use 1 1/8
plywood is to keep tile from cracking from floor flexing,thank,s

First off, no matter what, unless your joisting is 19.2" on center, and you're replacing the subfloor, there's no need for 1 1/8" plywood. It does nothing for you, unless it's in layers.

Now, before we go any further, what do you have for subfloor right now? What do you have for joisting? Size of the joists (2x?), unsupported length, and on center spacing?

HI,this is whisky,i have 2x10 joists,right now
i have taken the flooring off and put down 3/4
inch plywood we are working on an area of 6x9
for bathroom with a corner shower,toilet
pedistal sink.
it has been broght to my attenion that we
should not 3inch screws to hold down plywood
as that will weaken the joist by going in too
deep is this true or old wifes tale,also we are
going to put ina recirulating pump for the hot
water because we are about 70feet away from
hot water tank and do not want to put in hot
water tank because of the way the house is
built thank,s

Depending on the thickness of the screws you could weaken the beams. But if your using drywall type screws most likely your not going to do any damage.
BUT, Do not use drywall screws as they can rust out and break off. Use stainless screws. Also use liquid nails on each beam as well as screws that are two inches in length. Any more would be a waste of time and money.
AS far as using the pump. Good idea. Be sure to use insulation on the hot water pipes where ever you can get it on them. These pumps although work well waste energy. Dont forget to install a timer on the pump and set it for the times you would most likly use the bathroom. No sense running the pump in the middle of the day when no one is at home.