Expansion gap in thermalite external 7m+ wall

[dave1975]

We have an external thermalite block wall on our side extension which we have been told by building control needs to have an expansion joint retrofitted as it exceeds 7m in length. Can someone advise me how wide the gap needs to be and what to fill it with between the blocks. It will then be rendered after that and the remaining gap will presumably be filled with silicone. Thanks.

 

[^woody^]

In accordance with the recommendations of whoever made the blocks.

Normally 10-15mm with a polyethylene foam filler strip, polymer or polysulphide seal and expansion beads for the render.

Likewise for any inside joint but normally hidden by the board lining

 

[dave1975]

Thanks. The building inspector said to do the inside was preferable but as location is very shaded he could live with joint just on outside. Can the joint fillers be picked up from general builders merchants or is it more of a specialist place?

 

[jks7492]

10mm foam expansion strip finished with a polysulfide sealant.

You can get the foam strips from builders merchants, but if you buy a 100mm wide strip I’d trim 10/15mm off so that it finishes below the face of the block and gives you some room for sealant without it standing proud of the wall.

 

[dave1975]

Ok thanks guys. Would you recommend using the strips rather than backer rod ( which I have seen some people use ) ?
Is one easier to install than the other seeing as the wall has already been built?

 

[^woody^]

The building inspector said to do the inside was preferable but as location is very shaded he could live with joint just on outside

Yes, but the inspector wont be living with it in a few years if it cracks, you will. His justification is actually actually complete nonsense. No doubt you'll be cranking the heating up when it gets cold and letting in the humid summer air when its warm, and your wall will be expanding and contracting.

The inspector is not a designer and takes no responsibility if a building fails, and it's completely wrong to do stuff just to pass an inspection.