You should always check with your local building codes!
The International Residential Code (IRC 2018) says:
"R322.214.171.124.1 Winder treads. Winder treads shall have a tread depth of not less than 10 inches (254 mm) measured between the vertical planes of the foremost projection of adjacent treads at the intersections with the walkline . Winder treads shall have a tread depth of not less than 6 inches (152 mm) at any point within the clear width of the stair. Within any flight of stairs, the largest winder tread depth at the walkline shall not exceed the smallest winder tread by more than 3/8 inch (9.5 mm)..."
"R311.7.4 Walkline. The walkline across winder treads and landings shall be concentric to the turn and parallel to the direction of travel entering and exiting the turn. The walkline shall be located 12 inches (305 mm) from the inside of the turn. The 12-inch (305 mm) dimension shall be measured from the widest point of the clear stair width at the walking surface.. "
Please pay attention to the meaning of "tread depth". IRC means by that "Going" (withoiut nosing).
- Step 1 . Choose the number of winder steps – 2 PCs, 3 PCs, 4 PCs, N PCs ( default N PCs ).
- Step 2 . Select suitable units of measurement – mm, cm, m, inches, feet.
- Step 3 . Specify the drawings colour – monochrome, color.
- Step 4 . Define the type of stairs – Left-handed staircase (left turn), right-handed staircase (right turn) .
- Step 5 . Determine the horizontal dimensions of the stairwell opening.
- Step 6 . Choose t he drawing mode of staircase, depending on the method of calculation of the width of the tread or "Going" (without nosing overhang) – the upper and lower stairway are pre-selected and fixed or calculated automatically; goings are equal throughout the flights (rectangular treads) or not.
- Step 7 . Input stair dimensions – staircase width; Total Rise - the height from finished floor level to finished floor level (FFL-FFL). I f the top step at the level of the 2nd floor, the height of the opening = height of the stairs.
- Step 8 . Set steps parameters: minimum width of the winder steps located in the corner of turning part of staircase - “dimension MW” and “dimension MF” - minimum going of winder treads (see detailed pictures below); preferred height of the steps or rise (the closest value will be selected), tread thickness, stair tread nosing (overhang of the step edge), the top step below the upper floor level or at the level (optional).
- Step 9 . Specify the Risers - you can install risers in your project (optional), define risers thickness.
- Step 10 . Input the parameters of the stringer — the thickness and the minimum width of the stringer (minimum throat depth), as well as their number per flight (3 by default).
- Step 11 . Slabs and walls measurements - these are used only for 3D-visualization and do not affect the final results.
- Step 12 . Confirm the entered values and the begin the calculation of the L-form stairs with N-winder steps using the " Calculate " button.
Design features in the drawings
To create the most comfortable stair structure it is recommended to select the steps dimensions on the basis of the universal formula of a “perfect staircase” by French engineer François Blondel. If the step width decreases, it is necessary to increase its height, and vice versa - if the step width increases, its height must be reduced, while maintaining the step length in the range of 630-650 mm.
Blondel's formula: 2 × Rise + Run = 63 ÷ 65 cm
- Rise – the height of the step;
- Run – tread width or going;
- 63 ÷ 65 cm average stride length of a person. The IRC limit gives a 2r+R of 25 ½
One more rule "Rise + Run" says that rise plus run should equal 18 inches (17 -19). The IRC limit gives an r+R of 17 ¾ ”
Separately, it should be noted again the dimension MF, which determines the shape of winder steps set and the area of the supporting surface of the step, in particular. If you reduce dimension MF, the staircase will be more curved and compact (tending to the shape of a spiral), however, at the same time, the strength of the tread will decrease, since the contact area with the stringer will be reduced. Therefore, when specifying an MF of less than 10 cm, measures should be taken to strengthen the structure: the number of stringers per march should be 3 or more, and the winder stair tread should be made of hard wood.