Alternating tread stair design

Also called a ladder stair, a Jefferson stair or a loft stair.

Adjust the riser height to the space between floors. For a 9’ floor to floor height there would be 14 risers of 7.714”. Start with this and adjust the angle of the carriage from there.

Pen sketch of an alternating tread stair seen from the side. Callouts read 14 R at 7.714 inches for 9 foot floor to floor, sides and treads from clear 2x12 or LVL lumber, hand rail at 34 inches above the nose, full height 1x notched in 2x12 for attachment, 4 foot 9 inches to the wall (will vary), 24 inches minimum clear inside, countersink 3 to 3 and a half inch screws and cap with buttons, 1x4 kickplate, 1 and a half inch nose, max rise equals 8 inches equals R, run equals 10 to 11 inches equals T.

Photograph of a finished alternating tread stair in pale wood standing against a wall. Its treads step out to the left and to the right in turn between two stringers, with a handrail on the open side.

Pen sketch of one tread drawn in perspective, its front edge stepped back across half the width. Callouts read 10 to 11 and a quarter inches, kick plt., adjust for nose, and 24 to 28 inches across.

I think I would add a non-slip surface material to the step portion of the tread.

The sizes on the sketch

Every size below is written on the drawing above. Your building department has the final say on all of them.

Part	Size on the sketch
Sides and treads	clear 2x12 or LVL lumber
Risers for a 9’ floor to floor height	14 at 7.714”
Max rise (R)	8”
Run (T)	10 to 11”
Clear width inside	24” minimum
Hand rail	34” above the nose
Nose	1½”
Kickplate	1x4
Tread attachment	full height 1x notched in the 2x12
Tread screws	countersink 3” to 3½”, cap with buttons
Floor space out to the wall	4’-9” (will vary)

On the tread detail: 10” to 11¼” front to back, 24” to 28” across, with “adjust for nose” marked at one end.

(sketch problem 6-8-04)

This type of stair saves lots of space and can have standard rise and run dimensions to meet code. However, it is quite steep and a bit scary to go down head first (most people turn around, like you would on a ladder). Some building codes will allow this type of stair into lofts of 250 sf or smaller. Here are some suggestions about how to build this type of design. It is sometimes called a Jefferson stair as Thomas Jefferson was an early promoter of the space efficient design.

I hope this gives you some useful ideas. John Raabe, www.countryplans.com. For a better printing PDF file click HERE

Where to go from here

Print or save the PDF of this sketch, which prints better than the page.
Read how building codes and permits actually work with stock plans.
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