Rough-Framed Stairs - 05

Rough-Framed Stairs - 05



So, let's complete the stair calculation part of this blog.

In the following example we be working inside a structure and will use a standard 8’ high wall with 2x10 joists and ¾" subflooring.  

It is assumed that the lower floor is level.  (This is critical since the actual elevation length from the top of the stairs to the bottom, is not directly vertical below where the top of the stairs Connect to the floor system. If the lower floor is level you can measure straight down Andy confident that you will be close, however if the floor is not level or you are building stairs outside from a deck to the ground, then you must extend the upper level elevation out to a point where you can measure down directly over where the bottom of the stairs will be placed)

The 8’ wall is comprised of:

(3) plates that are 1 ½” thick each, plus
(1) precut stud that is 92 ⅝” tall. 

The total height of an 8’ wall equals 8’-1 ⅛”.

The floor system is comprised of:

(1) 2x10 joist @ 9 ¼” tall plus,

(1) plywood subflooring @ ¾”.

The sum of these lengths are from the top down:

¾” Subflooring

9 ¼” Joist

1 ½” Double Plate

1 ½” Top Plate

92 ⅝” Stud

1 ½” Sill Plate

8’-11 ⅛”  Floor - to - Floor Height **

** If all these lengths are added together, this would be the theoretical or mechanical distance from the lower floor surface to the upper floor surface (minus any specific floor coverings).  It's important to account for floor surfaces if they differ between floors. For example, if the lower floor is hardwood or tile and the upstairs is carpet, the thickness of the carpet may not exactly match the thickness of the hardwood or the tile. These differences should be taken into consideration when performing stair calculations.  For this example we will have uniform floor coverings throughout.

** Also it is important to note that there are many different combinations of materials with different sizes that can make the Floor-to-Floor height different than the example used here.  

To get a rough idea of how many risers will be needed, using a calculator that can operate in feet and inches, take the floor to floor height of 8’-11 ⅛” and divide it by 7 ½".  (Using 7 ½” is a good optimal riser height to shoot for.)

8’-11 ⅛” ÷ 7 ½"= 14.2833 repeating

This will give you a number and a remainder.  This means that 14 risers at the size that we divided by Will fit in this stair Plus the remainder multiplied to our riser divisor.  So now we know that we can have 14,15 or even 16 risers in our stairs.

We round the number down and also up to the nearest whole numbers and then divide those number into our floor to floor height of 8’-11 ⅛”. This will give you two different riser heights.  

8’-11 ⅛” ÷ 14 = 7.65178571429
or 7 ⅝”  ( or in F-I-S  0-7-10)

8’-11 ⅛” ÷ 15 = 7.14166
or 7 ⅛”  (or in F-I-S  0-7-2)

A riser cannot be any taller than 7 ¾” for general construction and must be less than 7” to comply with ADA regulations, so we will have to divide our floor height by 16 risers to be in compliance with ADA code.

8’-11 ⅛” ÷ 16 = 6.6953125   
or 6 11/16” (or in F-I-S  0-6-11)


Sloped Nosing Stringer Pattern

To determine how far the run will be for the stairwell, (if we choose to start the stairs with a dropped stringer) subtract one from the number of risers and multiply that by the individual tread length. If the tread length is not specified on the plans, they are specified in code (IRC and IBC) to be no less than 10 inches in length. ADA and commercial applications require an 11 inch tread.

14 risers (13 treads)
= 13 x 10” = 130” stairwell run
= 13 x 11” = 143” stairwell run

15 risers (14 treads)
= 14 x 10” = 140” stairwell run
= 14 x 11” = 154” stairwell run

16 risers (15 treads)
= 15 x 10” = 150” stairwell run
= 15 x 11” = 165” stairwell run



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