Most garage roofs and roofs in general are not built to carry a live floor load. This means it is not meant to be walked on other than for occasional maintenance. Please make sure it can structurally hold a live load full of people. Code requires living area floor systems inside and out to be able to carry a live load equal to a party full of people standing together or about 40lbs per sq.ft. So a 22’x22’ area would need to be able to hold 5000 lbs of uniform live weight. Guardrails are also required per code.

If it is strong enough to be a deck the next question is the roofing. The tar roof is not meant for repeated use and using it will cause leaks. I don’t recommend painting it or putting tar paper over it for use.

If you are not worried about leaks and the structure can hold a party full people you could use decking tiles of wood or concrete, or any other thing to minimize direct foot contact with the tar roof.

This seems like a great suggestion.

One manufacturer of decking tiles suggests " laying down an EPDM sheet under the tiles as an extra precaution" over the existing flat roof tar or rubber membrame. My roof appears to be a mixture of fiberglass rolls and tar. Should I re-roof in seamless rubber membrane of just go over what I have and use an EPDM sheet. Also my rafter beams are very hefty 4x9 's but the sheeting appears to be only 1/2 inch treated plywood - is this a concern? can it be reinforced if needed from below to leave the roofing intact?

Tile is not a waterproofing. It is not in a shower and is not on a roof. You need a membrane.

For the membrane we are not using EPDM but rather 80mil PVC. I would suggest removing the existing roof down to the plywood. Then put a 1/2" decking matt over it. So it be layered tile to decking as:
1. tile
2. Schluter tile isolation matt
2. mortar set base
3. Schluter drainage matt
4. PVC membrane and flashing as required
5. roofing matt
6. decking

What is the span and spacing on center of the 4x9's? That is what determines if the 1/2" plywood is big enough to handle the dead and live loads of a roof deck.

I suggest you consult a structural engineer to make sure the roof stucture is strong enough for a deck. Then hire a roofer experienced in roof top decks, and low slope pvc roofs. Then hire an experienced tile layer. The roof top also needs proper drainage flow and outlet.

We live in Brooklyn and have a deck on our roof. We used tiles by great mats.com. The commercial ones are made with channels for drainage and protect the roof.