Victorian bay window subsidence crack (1.3-3.7 mm); should I pull out?

mattheus

@Phil4432 It is totally plausible that the crack is due to subsidence, as claimed by surveyor (differential movement). The question is more -- so what -- why is bay subsidence bad -- it's not like the house will sink into the ground or I will not be able to sell it. Looking at British Geological Survey data the house is actually in a low risk zone; although 50 m from it high risk zone boundary starts.

I paid a ton of money on this supposedly top ranked surveyor -- realistically will not be rolling dice again.

mattheus

stuart45 said:

The weight from above is going down the stone jambs and putting a point load on the wall which is pushing the wall out a bit. If the weight from the jambs was spread onto a stone or concrete bill it would be better.

What weight could there be from the light roof of the bay window? Could it be that it is the weight of 1st floor walls no longer properly supported by the bressummer bream, coming down? They obviously removed bay window sill/bill in the past (as seen in photo), replacing it with some hideous tiles -- but I don't see how this would have helped spread the load, considering it seems to not have gone directly under the stone jambs? Do you think it is the bressummer beam failure or missing sill/bill that have ultimately caused the crack in the bay window?

stuart45

The weight from the first floor won't be transferred to the bay.
The weight will be from the bay roof, ceiling, masonry above the jambs, and jambs themselves.
The main issue is probably a poor foundation for the bay, but the point loads on the corners don't help. Jambs often sit on top of cills to help spread the load.

Section62

mattheus said:

Looking at British Geological Survey data the house is actually in a low risk zone; although 50 m from it high risk zone boundary starts.

If you are that close to the boundary then you can't rely on the data to be accurate enough to say one way or another - mapping of soil types/risk levels isn't precise, the boundary lines are plotted based on extrapolation of known points, similar to the way contour lines are drawn on maps.... there's an element of artistic licence involved.  If I was looking at a site within 50m of a 'high risk' area I'd assume the site was also high risk until confirmed otherwise by more localised surveys.

mattheus said:

What weight could there be from the light roof of the bay window? Could it be that it is the weight of 1st floor walls no longer properly supported by the bressummer bream, coming down? They obviously removed bay window sill/bill in the past (as seen in photo), replacing it with some hideous tiles -- but I don't see how this would have helped spread the load...

In theory if the bay rafters are securely fixed into the front wall, and the front wall drops, some of the load from the 1st floor wall and above could be transferred diagonally down through the rafter(s).  If the lower end of the rafters are securely attached to the bay wall plate then that could impart both horizontal and vertical loads into the top of the bay walls.

If (and it is a big if) that scenario was the case here, the most likely thing to expect would be signs of vertical cracking as the bay wall(s) are pushed away from the front of the house, starting at the top.

That's only a vague possibility, and I don't disagree with stuart45.  But the combination of cracks in the bay and the sagging of the brickwork on the main front wall together suggest a need for a structural engineer to take a proper look and advise.

Regardless of what the surveyor charged, the fact the sagging brickwork on the front wall was not picked up in the survey, and a 'relaxed' approach to the cracking, would ring very loud alarm bells....

stuart45

That's a good.point about the loads from the first floor wall being transferred down through the rafters.

mattheus

Thank you for extra details. Purely based on the information so far, what is my worst case scenario?

FreeBear

Worst case, the bay is demolished, new foundations, and rebuilt.

Most likely scenario - A bit of underpinning, crack stitched with helibars, and rerendered.

mattheus

Since the surveyor is essentially useless your opinion about the back of the house would be much appreciated.

FreeBear

Cracks in photo B under the window - Do they correspond with a doorway below ?

Could be an indicator of missing/inadequate lintel - Further investigation required.

The remainder of the cracks look to be trivial and consistent with age of the building (just my unqualified opinion).