Lateral strength of extended cavity wall

[benawhile]

My small 2 storey house, built on a simple rectangular foundation, is being extended half width at the front and full width at the back.

We have cavity walls existing and extended.

The builders had to make vertical saw cuts in the existing brickwork at the corners where the extensions butt on, for a vertical dpc to prevent damp being carried across along the old external rear wall, which has now become an internal wall.

Inside we are making openings in what was the external wall, but is now an internal wall, downstairs and upstairs.

Downstairs we are able to leave a 440mm pier supporting the steel beam across the opening, which appears to also give some lateral strength to the external wall, the strength of the corner having been weakened by the vertical saw cuts

 

However upstairs the boundary between existing wall and extension is required to be a flush join where wall which was external but is now internal is removed.

 

There will be effectively a simple butt join between new and old wall as in the upper part of the picture with the bricks marked 1st floor A, at what was a corner.

My concern is that as we have to continue the saw cut for the full height of the wall, there will be a weak spot at this join, as the new brickwork isn’t keyed in to the old, but furfixed, and the only way we can provide any strength is to continue the internal skin brickwork a bit further into the existing house thereby staggering the furfix profiles so that the internal one is set further back than the external. See the lower part of the picture, 1st floor B.

 

Of particular note is that every alternate course in inner and outer skin is weakened by there being just two half bricks abutting each other

There was a reason provided by the architect for specifying flush furfix joints rather than keying in as that would allow for more ground movement at the join

 

Will this staggering provide appropriate strength?

 

Obviously I will go back to the architect for more consultation and reassurance, but I wondered if I could get an objective view of whether this is a routine problem and solution.