Top of stairs detail:
Note that this is the
current standard - earlier stairs aren't always the same and the one thing often missed (including in some modern installations, especially where the walls are not masonry) is the side fixings into the wall. In effect your stairs are supported by the stringers being notched over floor at the top and held in place with some fixings through the top nosing into the floor trimmer, joist (in this case the "I" beam shape) or floor. At the bottom the newel post is mortised out and the stringer has a matching tenon cut into it, but fundamentally the bottom of the stringers is generally shaped like this - so your stairs sit on two small rectangular ends:
To prevent damage to trhe stairs you need to support the structure whilst you remove the floor joists beneath the bottom end. I think trying to do this with a stack of loose bricks is potentially dangerous. The support structure needs to be rigid, fixed to the stair stringers (90/100mm nails or 5 x 100mm screws), becuse at the end if the day you could be supporting 200kg of stairs, bannister, underdrawings, framing, etc which may pivot from the top end of supported on anything loose. Sketch of arrangement below:
When you have got the staircase adequately supported, the floor and framing beneath the bottom end can be removed and the bottoms of the stringers
and the newel post tied together (permanently with a piece of timber screwed upwards into the bottoms of the stringers and newel. These are not new stairs (probably 80 to 90 years old from the style) hence my caution in recommending that the bottom of the stairs is tied together
It is up to you how that sits in place, but you could either fix two vertical legs beneath the tie block which sit on your (new) DPM, then just remove the temporary support underneath the stringers, install the insulation then add the screed:
or you could do without the feet but leave the temporary support in place whilst you install the insulation and run in the screed (screeding around the tie block) and once the screed iis fully cured remove the temporary supports, back-filling the resulting holes with more screed (basically a small amount of sand and cement knocked-up on site)
The glue blocks and wedges that hold your stairs together can be clearly seen in this photo from below, taken before the
under drawings (generally a plasterboard, plywood or hardboard applied to the underside of the stairs, as in the case of your stairs) are applied:
It is hopefully apparent why I would take the precaution of adequately supporting the stairs and fixing a tie block beneath the bottom ends of the stringers and bnewel post; I have seen people being a bit slapdash doing this in the past and having parts of the stairs pull away - not an easy thing to repair in situ
Apologies for the ropey sketches, but I am on the train and it's a bit of a bumpy ride - and this laptop doesn't have any graphics software on it!
