stairs cut stringer sizes

[DudleySquires]

I need to make a stair case. Straight, nothing fancy. A wall / closed stringer will be required on both sides until about 2/3 of the way down, when one of the walls stops. At this point I want the stringer on that side to become and open / cut stringer.

Obviously to make it open, the stringer needs to be notched and in doing so it is weakened. I want to know what regs / guidance there is for what the remaining height of the stringer should be (at the apex of the notch, as it were). But I can't find anything online. Nothing in Part K of the Regs either that I can see.

As ever, a picture paints a thousand words:

Cheers

 

[tony1851]

The minimum thickness will depend on the unsupported span of the stringer, ie the distance measured up the slope from the point of contact with the floor, to the point where the stringer is supported off the wall.

 

[ree]

Using 2" x 12" or 2" x 10" stringers you will have plenty to spare for the margin. Anything in the ball park of 50mm is sufficient.

Rod out (draw) the whole flight, in reduced to scale size, on a sheet of ply or plasterboard - from FFL toFFL.

Come back here if you need any more assistance with doing the job.

 

[pinenot]

Is the stop point of the concrete wall, returned under the proposed stair, if so
you could consider stopping the short wall stringer at the closest tread (+ whatever structural required) put in a steel bracket to support the last "open treads", this would perhaps look better???, just a thought...pinenot

 

[DudleySquires]

Thanks folks! I have since found that the term for what I am describing above is "throat size", which has allowed for increased googling efficiency

1.5" * 11" is now on its way, and in this particular case should allow for a 4" throat - plenty I think for the four-steps-or-so of cut stringer that I desire.

Mine is a timber-framed house and the wall return around which this open / closed stringer conundrum revolves does not yet exist - I will be constructing the full-height-stud-wall next to, and the wall-return-under the stairs myself. Indeed, as mentioned, I may also support the staircase on this mini-wall.

Many thanks

 

[ree]

I've just told you above what the commonly used term is: "the margin", and in residential stair work its built into the stringer - no need for any calculating.

I have never heard it called anything else.

Google certainly shows it being called the "throat size" but carpenters and joiners on site dont defer to google.

FWIW: one google web site required a min of 3 1/2" throat size, for a stringer taking four treads. Go figure.