Is the flat roof structure over the garage designed to support whatever could potentionally be up there (people, furniture, etc.) ?

Is the flat roof surface designed for foot traffic?

What will keep people from falling over the edge or on other roof surfaces, and meet both code and common sense?

We had a spiral staircase from our second floor porch to our courtyard. Worked fine. You want at least the 5’ spiral (which is minimum code anyhow)

Will you be doing any adult beverages on your deck? Drinking and a spiral staircase might be a recipe for disaster.

Nothing wrong with a spiral stair but there are many other considerations.

Will the stair be inside or outside the house? What is the climate? What loading is the roof structure designed for?

This is an unusual situation but normally an egress path cannot pass thorugh a garage.

I would consider an alternating tread stair, perhaps a metal one.

A sprial stair might look good. Other stairs might look good too. Depends on what the house and deck look like. And to an extent, how your deck support is built and what kind of railings you want to use.

This is often the choice for roof gardens but there are many other things thta need to done too to even have a roof graden All of the issues Mark brought up. If you really want help we need to see your house and garage . We also need answerrs to quetions we have asked . All posted here in comments DO NOT start another post .

I always feel like I am chasing my tail when I use a circular stair, and I fear if I trip and fall the jaws of life will have to be used to get me untangled.

How do you plan to get your furniture up to the roof deck if not by the spiral staircase? Would it have to be removed in winter?

An elevator is the option that works for sure.