Things You'll Need
3-inch deck screws
2-by-6 inch boards
Handrail and decorative posts
Safe access to a loft area is required to make maximum use of the extra space in your home. If the loft area is used only occasionally for storage, then a simple loft ladder is sufficient. If the loft area is used frequently or as a living space, loft stairs are a safer and sturdier alternative to a ladder. They are installed in a fixed position and include stringers that provide additional support to handle heavy loads and daily use.
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Finding Rise and Run
Measure the height from the floor to the landing level of the loft. This gives you the total height needed for the stairs.
Divide the total height by the desired height for each stair, usually 7 to 9 inches. Round this figure up or down to the nearest whole number to determine the total number of risers needed.
Divide the total height by the total number of risers needed to determine the actual height, or rise, needed for each step.
Multiply the total number of risers needed by the desired depth, or run of each stair tread; stair treads are typically 10 to 12 inches deep to provide sufficient room for foot placement. This figure is the total run, or the total horizontal distance, needed to complete the staircase.
Subtract 1 from the total number of risers to determine the actual number of treads needed for the stair stringers. The total number of risers includes the height to reach the loft landing, so you will need one less riser on the actual stringer.
Draw a pattern onto a 2-by-12-inch board, using the calculations for total number of treads, risers, riser height and tread depth; use a framing square to ensure precise angles and straight lines for the stringer. Remember that you will have one more riser than treads.
Cut out the stair stringer with a circular saw, using a handsaw to finish the corners so you don't cut beyond the marked lines.
Measure and cut from the bottom of the stringer a measurement equal to the thickness of the stair treads you plan to use; this prevents the risers from being taller than your calculations as a result of the thickness of the treads. Use the actual measurement of the treads when determining how much to trim because a 2-by-6-inch board is actually only about 1 1/2 inches thick and not 2 inches thick as the name states.
Trace the finished stringer onto two or three additional 2-by-12-inch boards and cut out the pattern. You'll need a stringer for each side of the stairs, plus one or two support stringers in the middle, depending on the total width desired for the loft stairs.
Assembling the Stairs
Screw each stringer in place into the rim joist of the loft level with 3-inch deck screws; this is the exposed, outer floor joist of the loft. Insert the screws at an angle to ensure the screw goes through the stringers and into the joist and use at least three screws for each stringer.
Measure and cut a 2-by-4-inch board to the same width as the staircase and screw this through the back of the rim joist and into each of the stringers.
Screw a riser plate into place at the bottom of the stringers to hold the stringers together at the bottom. The riser plate must be equal to the width of the staircase and screwed into the front of the stringers. Riser plates can be used on each step, but loft stairs often omit these plates, instead opting to leave an open gap.
Screw the treads into the place on the stringers to create the steps of the staircase, leaving a 1/4-inch gap between each board on the tread. If the desired depth of each step is 12 inches, there will be a 1/4-inch overhang on each step; if a 10-inch depth is desired, you will have a 2-1/4-inch overhang.
Measure and cut two 2-by-12-inch boards the same length as the stringers, using a miter cut on each end so the boards fit snugly against the bottom floor and the rim joist of the loft level. Screw these boards in place through the stringers on either side of the staircase. While this step is optional, the straight edge of these boards frames in the steps and conceals the exposed tread and riser joints on either side of the stairs.
Install posts and a handrail to either side of the stairs if desired. While some may prefer an open staircase with no handrail, this is the safest option in homes with children.
You may be able to find precut stringers at your local building supply store. This takes a lot of the work out of building a set of loft stairs, but the stringers must exactly fit the total rise measurement between your loft and the lower level.
It may be best to break up stairs with more than 12 steps with a landing to reduce the strain on the 2-by-12-inch stringer boards. A 4-by-4-inch post secured to the stringer on each side of the staircase at the halfway point of the total run also helps to add support to the structure.