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I'm going to have a 10x8ft shed installed in my back garden. I'm wondering if I should pay someone to put in a concrete slab base or use one of those interlocking gravel filled bases you can get.

I've searched around the web and can't seem to find much information about the pros and cons of using one of the plastic bases - the pros are that I can install this myself and it'll probably be cheaper. Obviously concrete will be stronger, but would it make much difference? I'm not going to putting any really heavy equipment in the shed, but at the same time I want it to last at least 15 years (same as shed warranty).

Perhaps someone with some experience can help here (I have none).

Plastic base example product

  • Concrete foundation is the gold standard. It's the best for almost everything. Cost is the reason you would "step down" to a lesser product. Have you gotten a quote on the concrete? – JPhi1618 Oct 8 '19 at 14:26
  • Not yet, I'd also like to avoid it for ecological reasons. Concrete has a very high Co2 impact. Unless of course a plastic base really is terrible but like I say, I just don't know – John Hunt Oct 8 '19 at 15:32
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Concrete is impervious to insects and other critters, it is very clean once fully cured, and will not warp or buckle if installed correctly. Of course, concrete is also very expensive and can be upwards of $10 a sqft and should normally be professionally installed unless you're a pretty advanced DIY'er.

Gravel bases and gravel filled plastic can be a good alternative, but they have a few downsides. It's easier for moisture to get in. Gravel will keep out larger pests, but insects can find their way in. Dust will be a lingering issue but if you start with clean gravel it shouldn't be terrible. It will be more likely to move since it's not one solid panel of material.

The plastic panels can be a good choice for a light-duty shed and will give it a more natural feel. Which should you choose? Well, I can't tell you that, but hopefully this info helps.

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I would definitely recommend gravel for the primary reason that it will drain moisture away from the shed more quickly than concrete. As far as the plastic grid+gravel method, I don't really have firsthand experience with that style of foundation; I'm more familiar with installing a treated wood perimeter filled with compacted gravel to place the shed on. There's a pretty good how-to guide at https://www.siteprep.com/how-to-install-a-gravel-shed-foundation/ and they have a video guide embedded as well.

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One alternative to concrete would be soil cement. The basic principle is that you mix 5-10% of portland cement with the soil, add water then compact it. This reduces both cost and environmental impact.

Depending on the use case and type of soil, you could probably get away with just compacting the soil.

To get around the moisture problem you could use concrete plinth's in the corners and mount the shed on those to elevate it off the ground. If you have a small gap between the ground and shed, there should be sufficient ventilation to counter any moisture issues. You would add gravel in the places where the shed walls meet the ground.

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I did something similar awhile a back but I used concrete piers instead. Since I live in an area where the ground never freezes I didn't have to think about frost lines so I didn't have to very deep. This was kind of a go between the costs. The slab is nice as you'll have your floor already. With a pier system you have to build the floor as well.

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I have used 1-inch-thick flagstones under plastic sheds before; I find the flagstones secondhand at a good price on some neighborhood trading website, I can transport and place them myself with moderate labor, and they fit tightly together and give a pretty good feel under the shed's built-in plastic floor. I rake the ground bare and adjust the dirt for level/flatness before placing the flagstones. I dislike making permanent things unnecessarily, and the flagstones can easily be taken up and sold or given away when the shed is eventually removed.

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