Hi Duayne,

The section of the stringers will depend on the type boxed or cut.

Boxed or housed strings can be 30mm or more thick and minimum width will depend on the rise and run plus de nosing distance + the under carriage width.

In your case the if we give a width above nosing and under carriage as 1″ we get 10″ wide string.

This string could be made using a minimum of 1 or 1 1/2″ thick stock.

For a free standing string it’s usual to make it at least 1 3/4 or more depending on the esthetiques of the stair.

On the other hand a cut string widths  will depend  support needed under the steps.

For a free standing stair the cut strings width under the step/riser cut outs will need to vary according to the total length of the string and it’s thickness.

This is usually 5″ or more for strings that are 2″ thick.

I suggest you post your StairDesigner project file and I’ll tell you what would be the best size to use according to your project.

All the best,

Ness