Cutting stair jacks, or stringers, is a challenging construction project. There are software programs designed to ease the job of making mathematical calculations for design of the stair jacks. However, once the technical considerations of stair design are understood, anyone who can do simple mathematical calculations can lay out a stair jack. Local building codes govern specifics of staircase design, so contact your local building inspection department for information before cutting stair jacks. Avoid making mistakes in cutting stair jacks by using a story pole, suggests Tate Baynor, a 25-year veteran of the construction industry.
Find the total rise of the stair case. Measure the distance from the bottom floor to the top surface of the upper floor. Write it down. This measurement is the total rise of the stairs.
Calculate the number of steps -- treads -- needed. Divide the total rise by 7, the usual height of a stringer rise. The quotient of this division problem represents the number of steps needed to equal the total rise. If the number is a fraction, round it up or down to the nearest whole number. Write it down. This is the number of steps, or risers, to cut into the stair jack.
Calculate the exact height of a single rise. Divide the total rise by the number of steps needed, according to your calculation in the previous step. If the number is a fraction, turn it into a decimal. Write it down. This is the rise height.
Use the rise-to-tread ratio to determine the width of each tread. Subtract the single rise height from the rise-to-tread ratio, which is 17 or 18. (Consult your building inspector for local codes.) Write it down. The difference is the width of each stair tread.
Subtract 1 from the number of risers (R) and multiply by the width of a single tread. The number 1 is subtracted because there is one more riser than treads because the top tread is the upper floor. Multiplying R by the tread width (TW) equals the total run, or length of the staircase. Write this number down.
Create a story pole. Using a piece of 1-by-2 lumber, measure from the bottom of the pole and mark the height of one single riser. Place a piece of tread lumber, narrow side down, against the mark so that the tread lumber is above the rise. Mark on the other side of the tread. This now locates the first tread. From the last tread mark, measure the distance of a single riser and mark. Mark a tread, as before. Continue to mark risers and treads until you reach the end of the pole.
Lay out a pattern jack using the story pole as a guide. Lay your story pole on a piece of 1-by-12 lumber that is the length of the total run plus several inches. Copy the marks that represent the bottoms of each tread onto the 1-by-12 lumber.
Place clamps on your framing square with the clamp on the tongue, the long side, at the measurement representing the width of the tread and the clamp on the blade, the short side, at the measurement representing the rise. Place the tongue at the mark representing the bottom of the first tread and the blade at the mark representing the bottom of the second tread. Mark with a pencil inside the framing square. Continue for all steps.
Cut along the lines with a circular saw or a handheld cutting tool, being very careful not to over cut. Finish cuts with a hand saw if necessary. This is your pattern jack, cut from less expensive material. Position the pattern jack in the stairwell. Adjust the top and bottom of the pattern to take floor covering options into consideration. When satisfied with the pattern jack, use it to mark a set of matching stringers.
Tips & Warnings
- The top tread is the upper floor, especially on interior staircases.
- Wear eye protection when cutting wood to prevent injury from airborne debris.
Things You'll Need:
- Measuring tape
- 1-by-2 lumber
- 1-by-12 lumber
- Carpenter's square with clamps
- Circular saw
- Handheld cutting tool
- Hand saw
- Safety glasses
- Fields Home Center: Stair and Landing Construction
- Hammer Zone; Designing Stairs and Laying Out Stair Stringers; Bruce W. Maki
- Landscape Architecture Resource; Stair Riser and Tread Formula; June 2008
- Central Kings Rural High School: Stair Construction
- Tate Baynor; Construction Worker; Pinetown, North Carolina
- David Sacks/Lifesize/Getty Images