The Natural Philosopher wrote:

Personally, I'd pay them to take the rest of the pine out altogether.

Having made a set of strings out of 1" MDF once (which due to an
architects inability to count never got used), I would not use it again
for the strings a staircase with rebates. You are putting it under
stress in its weakest direction, especially when the treads and risers
are well wedged. It would only require a damp atmosphere for a couple of
days and the supporting part of the string would simply de-laminate from
the rest of it.

So pine for the strings works well, MDF for treads, and MDF or ply for
the risers.


--
Cheers,

John.

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