Extending a shed base

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I have a 7' * 5' shed sitting on an 8' * 6' base made from 2' * 2' flagstones (i.e. 12 flags arranged 4 * 3). It looks like these flags have been laid directly on the soil but the platform looks flat and level enough.

I would like to upgrade to a 10' * 8' shed. On the face of it the job looks do-able; add one row and one column of flags (with a sand base) to create a 10' * 8' platform, with flags arranged 5 * 4. But I'm not sure how to go about this. All the DIY guides I have seen do the job from scratch starting with bare ground. I'm going to assume (tell me if I'm wrong!) I don't have to pull up the existing platform and start again, i.e. I can dig out the soil on two sides of the rectangle, level up with sand, and add the eight new flags. On this assumption, a few questions:

1. The current shed is 5' * 7' so there is a foot of unoccupied flag on each side. The new platform will be exactly the same size as the new shed. Is that OK or do I have to lay more flags to allow for a 'border'?

2. Is the sand I require what B & Q sells as 'sharp sand', and what depth of sand should I lay? Any ideas what quantity of sand I am likely to need? (I'm thinking in terms of how many of B & Q's Large Bags)

3. I will assume I need not bother with spirit levels as the new flags must align to the existing platform whether it is truly horizontal or not. Do I need to have the existing shed removed before I lay the new flags, or is the one foot perimeter of visible flagstone sufficient to get the alignment right?
 
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I'm not a professional flag stone shed foundation layer by trade so one of them might come along later and disagree with me. But my opinion for what it is worth is

1. There doesn't need to be a border around the edge of the shed. If you were laying a full concrete base, you wouldn't want an edge as it would allow water to run under the shed when it rains. With flagstone you don't have that problem as rain water will disperse inbetween anyway.

2. Depending on the soil there, how compact it is etc, you "may" get away with levelling the soil and just laying directly onto there. You say the other flagstones are just on soil and they haven't been a problem, have they? I did this in my mothers garden years ago and they have stood the test of time brilliantly. If you do still want to use sand, it is impossible for anyone to tell you how much you need as it will depend on the depth you need it to, which will depend on how you dig out what is there.

3. Why would you not bother with a spirit level? If it is because you don't own one and don't want to invest £30 in one then fine, you "may" get away with it and your eye good enough to keep it all level enough, but if you have one in your shed, I would be using it. If you are doing it by eye, you will probably want to have the old shed down so you get a better visual on keeping it all flat. But if you have a spirit level to borrow/use then you should be fine running out from the existing border.

HTH
 
Thanks for that HTH. I do have a two foot long spirit level and I would have no problem spending £30 or whatever on a longer one. But I'm thinking it would be superfluous. The finished platform has to be flat. So if the existing flags have been laid at, say, half a degree off horizontal, the new flags must also be laid half a degree off horizontal. I was thinking of a long straight piece of wood to keep everything in a two-dimensional plane.
 

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