If the riser and tread dimensions are not given on the plans, then you need to calculate them. To do this you should consider the following points:
• You want the steps to feel comfortable.
— When walking up steps, a person’s mind determines the height of the riser based on the first step. Make sure all risers and treads are equal, so the stairs will not cause people to fall.
— The lower the riser, the longer the tread needs to be to feel comfortable.
• Common dimensions for riser and tread are 7" rise and 10%" tread.
• Use the following three rules to check to see if your stair dimensions are in the comfortable range.
— Rule 1: Two risers and one tread added
should equal 24" to 25".
— Rule 2: One riser and one tread added
should equal 17" to 18".
— Rule 3: Multiply one riser by one tread and
the result should equal 71" to 75".
Important Stair Code Regulations
The following guidelines for stairs are according to the 2006 International Residential Code (IRC) and 2006 International Building Code (IBC)
Width — 36" minimum — with occupant load of
49 or less (measured in clear, to finish).
44" minimum — with occupant load of
50 or more (measured in clear, to finish).
Rise — 4" minimum.
7%" maximum for IRC — 7" for IBC, with residential 7%".
Tread — 10" minimum for IRC — 11" for IBC.
Riser height and tread length variance —
3/8" maximum variation between the treads within any flight of stairs.
Headroom — 6′-8" minimum, measured vertically from a line created by connecting the nosing of the stair treads to the soffit above.
Check for headroom by finding the distance vertically between two lines that represent the distance between the stair treads and any obstruction in the headroom. One line is a straight line that connects the nosing on the stairs. The second line is one that runs parallel with the first line but 6′-8" in a vertical direction above the first line.
To create the stair noising line, first measure up the height of one riser and back from the riser the distance of the noising and make a mark. From that mark, measure parallel to the subfloor a distance equal to the combined width of the number of treads. Then measure perpendicular to the subfloor
the combined height of the number of risers and make a second mark. Chalk a line between these two marks. From this line, make the second line that is parallel and yet a minimum of 6′-8" vertically. This is your headroom and if anything protrudes into this space you do not have minimum headroom. Remember this distance is to the finish, and so if you are putting carpet or drywall on then you need to allow for their thickness.
Measure treads and risers using framing square.
Subtract thickness of riser from top.
Notch bottom for ledger or top plate. Notches differ. (See illustrations in this and following section.)
Care must be taken when marking the top and bottom steps. The thickness of the stair tread and the type of finish flooring on both the tread and the floor must be considered so that all the risers will be the same.
5. For the top tread, be sure to figure in the riser so that the treads and nosings are all equal.
