The Rough Opening
The rough opening for attic stairs is measured out or traced from a manufacturer's template on the ceiling below the attic after a suitable location has been chosen. Usually, existing joists are used as edges if possible.
Ceiling material is cut out, leaving joists and beams. If joists and beams are in the way, they are removed after the ceiling material has been removed and a separate frame is built.
Depending on the model and style of stairs, the rough opening is usually cut slightly larger than the stair unit itself (15 inches to 3 inches).
Framing the Opening
When the ceiling within the rough opening has been removed and joists cut back to expose entry to the attic, the joists are reinforced by a hog frame. A hog frame is a frame around the rough opening a few inches back on the attic floor.
Four (usually 1 x4s) boards are laid face down and nailed to the joists around the perimeter of the opening. Boards on edge are nailed down perpendicular, creating an "L-" shaped ledge (see Image).
Headers are also installed to connect joists on each end of the opening.
Installing the Stairs
With the rough opening framed and secure, the stair unit is set from the attic. Before setting the unit, 1 x4s are usually installed at each end of the opening to help support the stairs before they are nailed an screwed.
They are removed after the stairs are installed. The hinge end of the unit is placed at the rear of the opening so the stairs will unfold to the front when being used and collapse backwards when the hatch is closed.
Before it's nailed and screwed to the headers and ledge, the unit is leveled and shimmed accordingly.