In the design of residential stairs, the main considerations are giving sufficient headroom, providing enough space for moving furniture and large appliances, and providing for the safety of the inhabitants. Another consideration is accessibility, for which wheelchair ramps may be required. There are different types and configurations of stairways, the main ones being a single, unbroken straight run between floors, a set of two or more runs which change direction and have a landing in between them, and curved or spiral stairways.
When designing multi-run stairs, the best practice is to place a landing at each change of direction. Landings are flat platforms serving as resting points and safe spaces for changing direction; their length should be equal to the width of the stairs at a minimum.
In some stairways which change direction, landings are omitted in favor of winders, stair treads set at an angle midway between the two adjoining stair run directions. These radiating steps should be at angles of either 30 degrees, in which case 3 steps are needed for a 90 degree turn, or 45 degrees (less preferable), needing two for a right angle turn. Winders are less desirable from a safety standpoint, and should not be used unless a landing is not possible due to tight space.
Rise to Run Ratio in Stair Design
The slope of a stairway is called it’s rise to run ratio. Rise is defined as the vertical height of each step, and run is defined as the horizontal distance from a step’s vertical face to the next step’s vertical face. Good stair design follows established rise to run ratio rules to achieve safe and comfortable stair climbing for the house’s occupants. The optimum ratio has been shown to be a rise of 7 to 7 ½ inches to a run of 9 ¾ to 10 ¼ inches, in other words a rise-run ratio of roughly 7:10.
If the optimum ratio cannot be used, due to space constraints, then follow these guidelines:
1. Maximum rise of stairs should be 7 7/8 inches.
2. Run of stairs to be at least 8 ¼ inches
3. Tread width to be at least 9 ¼ inches
4. Rise to be no less than 5 inches
5. Run maximum to be 14 inches
Calculating Rise and Run
In order to determine the exact rise of the stairs, start with the Total Rise, which is the exact vertical distance from one finish floor to the finish floor above it. Divide the total rise by the riser height you have selected; this will give you the number of risers needed, when rounded up to the next whole number.
Example: Your total rise measurement is 8 feet 4 1/2 inches, or 100 ½ inches. For 7 ¼ inch rise, 100.5 divided by 7 ¼ = 13.86, or 14 risers required.
The tread run is calculated similarly, dividing the Total Run (exact distance measured horizontally from start point on first floor to end point on next higher floor) by the required number of treads needed (14 in the above example).
When determining the total run and placement of the stairway, remember to allow enough clearance to the ceiling. At no point should the minimum headroom be less than 6 feet 5 inches, as a rule of thumb.
Once the tread rise and run has been determined, the stair stringers supported the risers and treads can be designed. Stringers are placed at each side of the stairway, with an additional stringer in the middle if the width of the stairs is more than around 35 inches.
The above deign rules apply to main stairways in residential buildings; stairways to unfinished basements and attics can relax requirements somewhat, but only if these stairways will not be in everyday use.
Photo by tandemracer, Creative Commons Attribution License