Expansion Joint or Cock Up

[learner]

i have discovered the outer leaf of blocks in my extension has a few mm gap from top to bottom of the wall which looks like it was put for a reason as the blocks are cut.

I cant see if the same is on the inner leaf as there is another wall coming off at a right angle.

 

Is this an expansion joint or have they messed up.?

[learner]

The gap breaks the stretcher pattern of the wall

[Temp]

Sounds like an expansion gap. They are normally required in long walls over 11m. 

[learner]

The wall is just over 10m.

Should it be filled with some sort of mastic?

[Temp]

I think ours just has some sort of foam sheet in it but it's on the sheltered side of our house. I think a low modulus sealer is recommended..  

 

https://www.sealantsonline.co.uk/ProductGrp/Dowsil-791-expansion-joint-silicone-sealant