Plaster/mortar cracking along joint of extended property

1 Mar 2013
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United Kingdom

We live in a 1950's semi which had a single storey extension to the ground floor in 1984. The extension extended two rooms (lounge and kitchen, so a party wall, internal wall and external wall).

Recently we noticed a crack in the plaster of the party wall which we hadn't seen before. There are also cracks in the interior and external wall along the of the extension joint. I also checked outside and there is a crack from top to bottom in the mortar which joins the new and old brickwork. The plasterwork cracks are sub-1mm wide but run the entire length of the party and external walls (I can't see all of the internal wall due to kitchen cupboards).

Like I say, these cracks seem to be recent, which would suggest some 'event' might have caused them. The only things I can think of are some works we did to the bathroom last March (we took down a non-load bearing wall although this wasn't near the joint) and the fact that local drainage issues have meant that the mixed drain on our drive has been overwhelmed and flooded the garden (this has occurred five times in the last three years). The house is set in on a concrete slab foundation which was extended to take in the extension.

I know that the modern practice is to run a bead of mastic between the two structures to allow for the differences in thermal behaviour/settling of the new addition - we don't seem to have this. The external wall is double bricked with a one inch cavity, not sure about the extension construction. It's concerning me because it's only just happened, on all three walls at roughly the same time.

Could it be a case of grinding out a chase and adding an expansion mastic to cope with the movement? Obviously this won't cure any movement so it will crack again?

Any advice would be very welcome!
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