Width of Satircase String 32mm/28mm?

[antony6]

Hi,
I recently got a staircase to my cellar delivered by a joiner.

In our discussion when he came to do the measurement, he agreed to deliver a staircase with a string thickness 32mm, but after it is fitted, I measured the strings on both sides, they are only 28mm.

I contacted the joiner, but he is NOT responding to me.

Is 28mm standard size?
Is there any safety issue if I use a staircase with 28mm?

Strings are wood and looking yellowish colour.
Joiner said it is redwood/softwood. I don't know how to find out whether it is redwood or something else.

Thanks for reading my post.