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Remove the access panel, and pry off any trim pieces. Make sure the area between the joists is free of obstructions such as plumbing or electrical wiring.
Remove any framing between the joists. Use the reciprocating saw and a flexible metal-cutting blade to cut through any nails holding the framing in place. You may need to pry the framing away from the joists to provide a gap for the blade.
Tip: Ceiling joists are placed on 16" or 24" centers, which means the stair assembly will be 14 1/2" or 22 1/2" wide. Check the measurements before you purchase attic stairs.
Measure the dimensions of the stair assembly, and mark them onto the ceiling (Image 1). Make sure the marks are between the joists.
Cut along the lines with the reciprocating saw. Support the edges of the dry wall while cutting to prevent it from breaking off and tearing. You may want to cut the dry wall in small sections, and cut from above the ceiling to keep a lot of dust out of your face.
Cut a 2" by 4" block to fit between two joists. Dry-fit the block at the cut end of the new opening so it's flush with the cut surface. Mark along the edges of the block where it butts against the joists. Drill two pilot holes (Image 2) in each of the joists so the block can be secured with screws.
Set the block in place, and secure it by driving screws through the joists and into the ends of the block. Use dry-wall screws to fasten the cut edges of the dry wall to the ceiling joists and the new support block (Image 3).
Tie the stair assembly together and lift it through the opening. Rest it on the joists temporarily.
Install two boards across the opening, one near each end. These will hold the stair assembly in place so while it is being secured.
Rest the stair assembly on the support boards. When it's positioned properly, use long screws to fasten the frame to the ceiling joists. For additional support, use lag bolts instead of screws.
After the stair assembly is secure, have a helper remove the support boards from the ceiling. Then lower the stairs and come down from the attic.
With the stairs unfolded, begin securing the rest of the frame to the joists. Use shims in the appropriate places (Image 1) so the frame stays square. The shims will remain in place and be trimmed flush later. Attic stairs are usually long enough to accommodate different ceiling heights, so you'll probably need to trim them for a custom fit. The stairs are too long if the hinge for the lower section won't close all the way (Image 2).
Fold the lower section into the assembly so it's out of the way. Hold a yardstick along the next-to-last section (Image 3), and measure the distance to the floor from both the front and back edges.
Unfold the last section, and transfer the measurements. Place the stairs on a stable surface, and cut them to size with a circular saw.
The roofs of many older homes are supported by 2" by 8" or 2" by 10" trusses. If that's the way your house is built, don't worry about the trusses' ability to support the weight of a floor. If the house is newer, however, it might have been constructed with engineered 2" by 4" roof joists, which are self-supporting and strong but weren't intended to support a floor.
Most engineered joists will support about 10 pounds per square foot, including the joist's own weight and the weight of the plywood floor. The floor system will probably weigh about 5 pounds per square foot, so you won't be able to store heavy items in such an attic.
It's time to begin installing the attic floor. Because wires probably crisscross the joists, you won't be able to lay the plywood sheets atop the joists. Instead, the plywood will need to be raised above the wires by using 1" by 4" furring strips. Start by grouping the wires together. Measure the distance from the flooring area to the grouped wires (Image 1). Transfer the measurements to a furring strip, and cut the strip to size.
After cutting the furring strips to size, dry-fit them on the joists. Each grouping of wires should fit neatly between the strips. When the strips fit properly, secure them with screws or nails.
Attach nail stops over each grouping of wires (Image 2). The nail stops will prevent you from accidentally nailing into a live wire.
Install furring strips and nail stops on each joist where the flooring will be installed.
Cut plywood sheets into 2' by 8' strips so they can fit through the door. It's okay to use low-grade 3/4" sheathing plywood for the floor. Don't use particleboard for the floor. It's too heavy.
Snap a chalk line down the center of the area the floor is being installed. The chalk line will serve as a guide to help install the boards straight.
Align the long side of the plywood with the chalk line so the ends fall in the center of a furring strip. This will make the flooring more stable.
Drive nails along the ends of the plywood to hold it in place. Slide the next piece of plywood against the first one, and attach it to the joists in the same manner. Repeat the process until the floor is complete. Install the sheets along the center line first. This will allow for more room to work in when you start installing the sheets where there's less headroom.
Make some additional cuts on one of the sheets to form a landing for the stairs. Make sure to attach the landing securely: it's the spot that will get the most traffic.
Use finish nails to install trim around the edges of the stair opening. Miter the corners of the trim for a more professional look. Leave 1/4" clearance between the trim and the stair panel so that the stairs open easily. Set the nails, fill any nail holes, and apply touch-up paint if necessary.
Tip: If the attic doesn't already have a light, consider adding one. A simple pull-chain fixture should be adequate.