An alternating tread stair is allowed and takes up half the floor area. It's as steep as a ships ladder but because of the alternating treads it is much safer to climb.

Thanks Renovator8, do you know where I could find this information?

You need to check your code to department to see how they deal with this issue.

Under the International Residential Code, a fix stair to an attic generally makes this 'a habitable attic', and this stair must be constructed just like any other stair that is part of the means of egress from your home.

That is, no alternating treat devices or ships ladders, but a full 36" wide stair that conforms in every way to typical residential stairs.

See the link below.

But even this will vary with what version of the IRC your state may have adopted or enforces or whether your state or province adopts their own Code.

Here is a link that might be useful: Means of Egress 2009 IRC

Thanks for the link!

Fine Homebuilding provides a good explanation in the link below.

Per IBC 1009.10, alternating tread devices are limited to an element of a means of egress in buildings of Groups F, H and S from a mezzanine not more than 250 square feet in area and which serves not more than five occupants; in buildings of Group I-3 from a guard tower, observation station or control room not more than 250 square feet in area and for access to unoccupied roofs in accordance with Section 1009.9;

and as per local amendment to IRC R311.5 may serve as an exit from an area not to exceed 200 square feet within a dwelling unit.

Alternating tread stairways shall have a minimum projected tread of 8.5 inches (216 mm) with a minimum tread depth of 10.5 inches (267 mm). The rise to the next alternating tread surface should not be more than 8 inches (203 mm). The initial tread of the stairway shall begin at the same elevation as the platform, landing or floor surface. An approved handrail shall be provided on each side.

A shipâÂÂs Ladder stairway may be used in a dwelling unit for areas up to 200 square feet as per local amendment to R311.5 The pitch shall not exceed 60 degrees with a minimum width of 24 inches. Treads shall not be less than 4 inches in width and the rise between treads shall not exceed 10 inches. An approved handrail shall be provided on each side.

But the issue may depend on how the building official defines the use of the space served. If the stair is not used for egress, the stair may not have to meet section 311.

Here is a link that might be useful: alternating tread stair

The problem with this is, if they don't fold up and close (which creates a seal), and they are not traditional stairs with a landing and door at the top, there is no fire seperation. In most, if not all areas, you need to have 5/8" fire rated drywall on the ceilings, and walls between the garage and home. Not to mention fumes from your vehicle can get up into that space and into the home as well.