I have an area behind the house, 2.5m x 6.5m, where I plan to put various outdoor storage: bike shed, composter, raised beds, possibly IBC rainwater tank (inspired by another thread). Eventually it could even become the floor of an outdoor 'garden room' of some kind.

It feels sensible to pour a level hard standing here as a base for all of the above. My current thought is a concrete raft slab of 100-150mm thickness would be more than enough support, but I'm open to suggestions on this.

Three sides are enclosed by ~2m high concrete retaining walls supporting neighbour's gardens, so formwork along the remaining edge should be straightforward.

My questions are:

- Should I insulate and/or damp proof below it, anticipating the potential ultimate use as an outdoor room? Or are these generally raised up on pedestals anyway?

- Will I need reinforcement and or movement/expansion joints in this footprint?

- Anything missing? Ducting for electrics perhaps?

I realise this could spiral into a full-on house slab if I'm not careful, and I'm keen to keep things simple and affordable. More likely than not it will remain a concrete extension of the patio for many years - I just don't want to miss an opportunity for some future-proofing.

NB - I can't dig down much deeper for the risk of upsetting the retaining walls, so underground nuclear bunkers aren't going to happen!

Why not use hardcore + paving? Both can be reused if you need an upgrade to concrete for some other future purpose (and don't need to keep the hardcore).

But I might well add a duct with an electricity supply to a shed / barbecue area. Maybe even a drain.

Thanks all, random thoughts are what I'm here for!

Posted By: djhMy engineer lives in a northern mining town

Posted By: djhWhat type of soil do you have?

We're in a Southern mining town (Somerset coalfields), hence my terminology as the house is a raft slab to avoid similar issues! Soil is very heavy clay, but in this location there's a lot of MOT Type 1 gravel mixed in too as we stoned up around the house during the main build.

Posted By: Mike1Why not use hardcore + paving?

Posted By: WillInAberdeenThat's about 3-4 tonnes of concrete. Can you reverse a mini mixer lorry round there

Both very good points. We are going to hardcore and pave most areas, but I want the shed/bike store/workshop(?) to be secure (potentially with cast-in/bolted hoop) and weeding the paving cracks is a constant battle that I already have at the front of the house. I feel like having a dedicated area of decent hard standing to put things on will be much more use in the long term.

I am very aware of the embodied carbon though - hence questions on minimum thickness and what I can get away with. I'd rather invest in making one good 'long life loose fit' slab than have to revisit at more expense (carbon and financial) later.

Re: concrete trucks, we can reverse one within 12.5m, which was close enough for pumping a levelling screed when we laid flooring in the house a few years ago. I realise that might not be the same for a thicker concrete mix though.

That reminds me of another reason for doing a pour now - ultimately we're planning to reclaim the parking area as soft landscape, which would likely prevent a concrete truck from getting so close to the house in future. So it's 'now or never' in that respect.

Posted By: WillInAberdeenNeed 100-150mm sub base underneath, sand blinding and DPM good idea

Bare slab needs to be laid with a fall slope to shed rainwater towards one edge

Thanks, all very valid points. I'd forgotten about laying to falls.

Posted By: WillInAberdeenCement not good for CO2, can you use some different binder in the mix

Good idea. I suspect this is likely to be down to availability. I'll have a word with our local minimix suppliers to see what they can offer.