Concrete Base Expansion Concerns

9 Jun 2013
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United Kingdom
Hi All,

I'm putting in a raised concrete base at the bottom of my garden for a medium size tongue and groove pre-packed home office (not brick built) and a large dog kennel. The size of the area to be concreted is 5m x 9.5m (this allows 0.5m all around it to the fence).

My original plan was to put in a ground level concrete foundation along the desired boundary and on top of that lay a line of the 7N concrete blocks to create in effect a frame. I have several old steel grids and an old iron fence that I was going to lay in the area (on top of some hardcore) to provide extra strength (and save me having to get rid of them). Then I was going to fill with 150mm of concrete. Because the access is really poor, I cant get a lorry load of ready-mixed delivered/poured and don't fancy trying to knock that amount up on my own in one go (I do own a concrete mixer), so I was going to use more the of the 7N blocks to divide the frame into 3 equal parts (obviously I'd tie the dividing line of blocks into the outer line frame of blocks). I could then mix/float the smaller areas much easier and am not worried about seeing the line of blocks when its finished as it will have buildings on it.

My question though is this - will this stay together in hot/cold conditions? I have done some research about expansion dividers, but don't know If this concrete area is really big enough to need something, and if so what? I have just had my mind put at rest about concrete expansion under a deck thanks to one of your helpful members, but am not sure if a much larger base of 47.5sq metres would be different and maybe expand/contract more, possibly pushing the blocks out. I have already purchased the blocks before this potential problem occurred to me. Thanks again in advance for you help.
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5x9.5m would be far too big to not have an movement joint anyway. You'd need at least one in the centre of a slab that size.

You can certainly split it into 3 bays if you like. I assume the finished floor of your office will be a timber subfloor anyway rather than this slab?

Your concerns about expansion are probably a bit over the top. it will not move enough to actually break the bond of the blocks or anything like that.

Your plan sounds fine apart from the old iron railings. Any form of reinforcment must be covered by at least 25mm of concrete on all sides so you need to flot it in the middle of your slab not just throw the concrete on top of it. Sitting on bits of paving slab is a common choice.
Thank you for the sound advice - I hadn't thought about getting concrete under the steel as well (which makes obvious sense when you think about it). I've got lots of spare bricks so I'll probably use 6 of them to lift the steels up. The office itself sits of legs that then bolt to the concrete.

Just one quick question though - what do you mean by 'movement joint' - do you mean a joint so that I don't have to mix and float the whole lot in one go (so I don't need any form of expansion joint etc)? Also, the steel/iron fence that I am burying is app 9m long so I was thinking if I ran it through the two dividers it should hold everything together super strong. If its going to sit 25mm of the hardcore then I'll have to space it such that the metal posts run between the bricks of the dividers. I would assume this wont weaken the base at all. Many thanks for taking the time to reply - its much appreciated!
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That's brilliant - have read the really informative link and ordered some of the flexcell expansion board from my local builders. Just one final question (which wasn't covered in the link) - if I am filling a concrete foundation trench to sit the block frame on - should the expansion board run across the foundations too? I was thinking a small cut out piece on the same line as the main expansion joints might be a good idea, but its not worth running it right across the area as 90% would be buried in soil then and just rot? However, my other thought is maybe the foundation is not a lot of concrete and should remain absolutely rigid. Looking on youtube, they are laying foundations for a small office and don't put expansion joints in the foundation trenches, so maybe not?

Thanks again for your help - it really has opened my eyes on an area I had never really thought about before!
No don't split the footings.

To be honest you don't need to buy proper board if you don;t want to. A piece of thin timber 15mm ish would suffice because in your situation the joint is purely to control cracking in the slab.

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