Garden Room foundation

Running Fool

So I'm getting a 10 x 10 garden room - picture of similar attached - and I need to sort a foundation for it.

I've been told to get 4 3m lengths of treated timber and tonne of 804, screw them together, put polythene down, pour gravel on it, wack it down, and get the eco base pieces to go on top.

Now I'm more or less up to a task like that.

The issue is that part - about a 1ft - of the shed will go onto the patio you can see it in the attached picture. So that means my foundation needs to be level with the patio.

To me that seems tricky to do. Won't it all eventually shift a little as it's gravel as a base? Won't it be difficult to measure and add everything so the eco base is not higher/lower than the patio?

Just wondering if it's safer to get someone to do this for me instead of making a mess of something that's going to be there for a good few years.

Thanks

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newfy

i would dig out some clay up to the edge of the patio,so the gravel has somewhere to sit, then fill it up flush to the patio and make sure to walk it down.

Running Fool

Thanks. Would walking it be enough to keep it compacted enough?

Lumen

I don't like the use of polythene here (for weed control?), or 804 for that matter. You're not pouring a slab, and these details will limit the ability of the area under the shed to drain. Drainage is often an issue in small gardens so you should try and keep as much percolation potential as possible.

I'd instead use Mypex or equivalent heavy duty weed control fabric, and instead of 804 use a couple of sizes of angular gravel (coarser underneath) and then use the rest of the finer gravel (e.g. 10-14mm) to fill the boundary of the eco base outside the perimeter of the shed.

Gravel will stay put if it's properly constrained at the edges. If the edges aren't constrained it can shift. Is that what the timber is for? Bear in mind that timber at or below ground (in contact with soil) will rot in a few years even if it's treated, and then it won't do whatever job it's supposed to.

edit: I suppose I should caveat this by saying if you don't follow the suppliers' spec it may affect your "warranty" or whatever. If the shed is going to be plastered inside then movement of the structure could induce cracks, in which case the most stable foundations (804 properly compacted with a wacker plate) might be justified.

Running Fool

Right, thanks, that all makes sense and is helpful.

That detail about what to do was just the advice from the guys making the shed.

Kind of in two minds myself. I think to be safe I might try and get someone to do it as it's a bit beyond my skills.