STephen wrote:

What are the arris blocks? Are they the triangular blocks fixed between
riser and tread?

Yup. Arris (or somethimes aris) is a generic term for a corner of
something or the angle formed by the meeting of two surfaces in
architecture. Arris rail is typecally 2x2" ripped across the diagonal.
Stairs usually have small blocks of it under the riser to tread join (I
used longer strips rather than just short blocks on my stairs)

Also, did you see the pictures I posted up on the website?

Yup, looks like someone has made an attempt to stick some mastic on the
gap. It may look better, but won't add any strength.

The other likely problem you may have is that the risers may be painted
into position and hence stuck there. I expect you would need to work
from the underside, hammer the treads back into position, take out the
old wedges and re-glue them, then hammer those tightly into place.
Finally screw the base of the risers to the back of the treads.

--
Cheers,

John.

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