Hi All, apologies if you have already seen this question, I thought I had posted it but now can’t find it in the list…!
I’m looking at a BISF house that seems to be structurally ok, external rendering, external sheeting etc. The only concern I have is a crack in the floor slab that runs in a straight line, emanating from the fireplace and ending up underneath the living room window. The crack is a couple of mm wide and I can’t tell how deep it is. There is no change of slab level either side of the crack and it looks historical rather than recently happened. There are no tree’s nearby. Has anyone else had or seen a crack of this nature, is it serious? I have attached a pic where you can just make it out running across the room. Any advice appreciated.


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Hi Dougie
The good news is that your floor crack appears to be a simple case of historic settling and it is no cause for alarm.

The BISF house is built on strip foundations which run around the perimeter of the house. There are two further foundation strips running through the house from one gable end to another.

Once these foundation trenches have been set (some use an outer wall of brick) the centre area of the slab is filled with a thin layer of concrete on top of hardcore.

This inner slab has no structural value other to serve as a floor pad. The hardcore underneath appears to have settled over time causing the upper surface of the slab to crack.

I have uploaded a few images below to depict the structure in more detail for you but I certainly wouldn’t be too concerned from what I can see there.

Just check outside where the crack meets the outer wall and see if there are any recent vertical cracks in that area.

I hope this helps


Marc Buck

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    • 3 years ago

    Thanks Marc, great info. I hadn't appreciated the way the slab was constructed so thanks for attaching the illustrations.

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