Decking stringer - throat query

[Deansplit]

Anyone know if a 70mm throat is too small for a stringer?

I'm building a staircase to a raised flat roof. The stringer will be 5.3mm long (run = 4.25m, rise - 3.3m). That makes 17 steps of 260mm run with 200mm rise) but the largest my supplier does for a stringer (9x2") leaves only 70mm for the throat. The stairs will be only 900mm wide so I could use 3 stringers if that compensates for the small throat? There will also be a single block wall 1/3 of the way to support (as making a shed underneath).\

Any thoughts

Thanks

Dean

 

[r896neo]

Its far from ideal, your main danger is cutting so much out of a 9 inch joist that it puts a lot of pressure on the corner of the tread/riser notch which is liable to crack.

Normally sistering some 25mm ply along it screwing and glueing to clamp the throat well together with the deep sections would help but outdoors that is a bad idea.

Screwing and glueing some 3x2 along instead would help and perhaps a third stringer or just a double stringer on the outside for good measure.

Removing as much bounce as possible will make the biggest difference to its lifespan