Yes, you should be concerned about lateral stability. There may not be enough shear walls to resist a wind that is perpendicular to the old party walls.

I'm guessing this thing is 3 tall storeys high and impressively narrow in plan. Are there front and rear masonry walls bracing those party walls? Are they in good condition? How big are the doors and windows? How deep is the house front-to-back? I would guess 600mm thick is stone, but you don't say. Brick? Are the floors some sort of reasonable diaphragm that is anchored to the gable walls? Too many questions.

I've literally seen an end wall of a 3-storey stone row house fall down. I had already told an owner of an end unit in a beautiful 1850s building that we had an emergency situation that required shoring and stabilizing, but he was arguing with the City about funding for a heritage building in a designated district and couldn't spend money because it wouldn't be eligible for a grant until they approved it. Then I got an email at 7:00 one morning saying the top of the wall fell off (above the roof line), and as I was typing a reply to him he called me, more stone was falling as he watched, and I got video from his phone of the top storey of wall collapsing into the churchyard next door. In the end we saved the building, shored and rebuilt it, but all on his dime because we went ahead and did it while the City dithered over funding and the building department of the same City issued orders to the owner. This was a 600mm wall constructed as an exterior end wall, you have one constructed as an interior wall.

Hi OldbldgGuy,

Crikey that was a scary story.

My building 4 stories, not too narrow about 6m wide x 7.5m deep. Yes front walls are bracing the party walls and there is also an internal wall mid way which braces both gables also. To be fair the walls seem in reasonable condition and are 600mm thick (mixture of rubble stone and brick). The internal buttress wall seems to be only about 150mm. See attached front elevation for window arrangement. The ground floor looks like an old shop front that was infilled with masonry. The joists are spanning front to back with T&G flooring but it is unlikely the joists are tied to the gables at each floor to achieve full diaphragm. Also a stairwell on one side also disrupts a full diaphragm.

• https://files.engineering.com/getfile.aspx?folder=71ae831a-9e74-4566-8ab3-e

I don't know where you are, but I'm in Canada and frost was a big part of the problem with the house I referred to above. Poor maintenance, water gets into the wall and freezes, and nothing good happens after that. The day the wall fell was a sudden thaw after a deep freeze, so really not unexpected once you knew what was going on inside the wall. If your joists are parallel to the walls in question, you need the flooring tied to the walls to brace them and that's easier said than done. I'm afraid I've got nothing but dumb questions or dumb suggestions without actually seeing it. A 10mm horizontal crack on the windward wall would concern me but if it has pushed in at the top and stopped, it may have only taken up a lack of restraint from being built protected and plumb, previously unexposed to wind, and now the structure is engaged so it's stable. Maybe crack repair is all you need. I'm sorry I can't be of more help.

OldBldgGuy....are you in the Toronto area and did you work with Trow at some point?

A Great Place For Engineers to Help Engineers

I'm self-employed in Niagara, and I used to know a couple of guys who worked at Trow, but that was a long time ago. My father and grandfather were contractors, I've never worked for long anywhere other than in the family business, but these days I'm a one-man-band, doing consulting only.
The townhouse I referred to above that the end wall fell off of was in downtown Hamilton.