Stair headroom calculations

[Greg Blocklayer]

I've added headroom calculations to this online stair calculator
http://www.blocklayer.com/Stairs/StairsEng.aspx
(Check the 'Show Headroom' box to the right)
The way it's set up now, the measurement is taken from the top of the stringer to the floor opening. Is this correct, or should it be from the tread? If so, and the next step is close to the point, wouldn't that be a problem? How do you work out headroom?
Thanks for any input.

[Steve Schoene]

Here's what the IRC code (R11.5.2 says: "The minimum headroom in all parts of the stairway shall not be less than 6'8" measured vertically from the sloped plade adjoining the tread nosing or from the floor surface of the landing or platform." Emphasis added.

Remember that the floor above the stairs is generally going to contain floor joists--in frame construction--so that the minimum thickness is likely to be at least 8" (2x8 joist + subfloor & finish floor + ceiling drywall, if any ) and could easily go to over 13".

[Ben Grunow]

Greg- tack a piece of plywood on the wall below where the headroom looks to be the tightest. Now draw, in full size, the stair you plan to build on the plywood (usually this is the bottom 4 steps at most). Include the 1 1/4" overhang of the stair tread nosings past the riser faces and draw it all out level and plumb very carefully. Now draw a line, using a straightedge, across the tops of the treads just touching the rounded edge of each step. After this you must draw a line parallel to this line. To do this you simply make two arbitrary marks as far apart as possible on the line you just drew over the stair drawing and then draw a plumb line through each of them. Measure 80" up from the intersection of the two lines () and make a mark. Now connect these marks with another line. That line defines your minimum headroom. Go larger if you need to move queen sized beds (or larger) up there because they barely fit.

[Greg Blocklayer]

Thanks for that info. The calculator now shows the headroom measurement in red with a ! if it's under 6'8".
What is the spec for min and max rise and run in the US? At the moment the calculator shows rises over 8", and runs shorter than 9" in red in the dropdown selectors. Also, if you check the 'calculate Run' radio button, it caluclates Runs = 2'1/12" - (rise * 2). Is this correct?. I've seen a few diferent numbers for this.
Thanks again for the help.

[Steve Schoene]

For residential stairs under the IRC there is a maximum rise of 7 3/4", with a minimum tread depth of 10". Treads shall have a nosing with a minimum of 3/4" and a maximum of 1 1/4", except the nosing is not needed if the tread depth is 11" or greater.

Not all states use the IRC, some codes have allowed 8 1/4" rises in residences, but I am not sure they do now. In some states, different communities may modify the codes to their own particular choices. The code requirements for commercial stairways also different, with some codes requiring a maximum rise of 7" and a minimum run of 11".

As far as formula rules of thumb there are several. The 1997 Standard Building Code called for 2R + T = 25, where R is rise and T is tread depth. There are other rules of thumb around that I can't lay my hands on at the moment.