Ok to fill this crack or is further investigation needed?

[DIYWarrior]

I'm currently renovating an ensuite bathroom in an extension that was added to our house by the previous owners more than 20 years ago. After removing the old plasterboard I uncovered the crack in the images. It appears the cavity blocks of the extension are not tied into the original house (the white wall is the original house gable wall).

On the outside there was some form of mastic I think applied to the gap and in the 10 years we've owned the house It's remained stable i.e. the crack hasn't widened.

Am I ok to rake any of the loose mortar out and fill the crack with something? (I'm thinking foam). The gap goes fully through to the outside render so I'm thinking foam for the thermal benefits. The wall will be battened, insulated and plasterboarded.

Or should I get it investigated further? When we bought the house the surveyor wouldn't have seen it but they would have seen the mastic repair. I found this line in the original surveyor report "There is some cracking on the rear of the property, which we believe has been caused by old settlement of the building but
this is not considered significant and no remedial work is necessary. Nevertheless, the cracks should be filled to prevent water penetration." - the only thing that doesn't make sense here is the crack was filled and I don't see any other cracks.

Thanks

 

[stevie888]

The crack is minor and totally consistent with the history. No crack in plasterboard before you removed?

The outside sealant looks fine, but if in doubt cut out and replace.

 

[blup]

Some sort of flexible sealant would seem appropriate, the gap looks to small/shallow for foam to work. It has been stable for a long time.

 

[DIYWarrior]

Yes, no cracks in PB that I can remember. So should I fill with foam or something else?

Also, any reason why the builders wouldn't have tied the new extension into the existing building or is it just poor workmanship?

 

[^woody^]

Extensions are normally tied with ties that allow for movement of the extension. The joint should have been a foam backer and mastic joint not a mortar joint, but for now, widen and deepen the crack a little and use a polymer or polysulphide mastic.

For the rendered images, I'm not sure of the relevance for those, but any mastic that would be painted should be suitable for painting and that means not silicone

 

[DIYWarrior]

For the rendered images, I'm not sure of the relevance for those, but any mastic that would be painted should be suitable for painting and that means not silicone

The reason I included the images of the outside was to show that the crack was previously repaired on the outside but appears to have remained intact as it was completed before we moved in 10 years ago.