Cellular Plastic Shed Base – Advice Please

[Nick ThorpeParticipant @nickthorpe64546]

We moved just under two years ago which is one reason I have been quiet on here. I would like to erect a 14 x 8' wooden workshop in our garden which has pretty stable sandy soil.

I am reluctant to put in a concrete slab partly because of cost and it has been suggested that I could install a cellular plastic shed base. I am a bit wary but the loadings look fine and the load is evenly spread. Cost would be under £250 with infill gravel which would be a big saving on concrete which I do not wish to lay myself.

More info HERE.

Does anyone have any experience of these systems please? Any advice would be welcome.

Many thanks, Nick

[Nick ThorpeParticipant @nickthorpe64546]

[Speedy Builder5Participant @speedybuilder5]

First thoughts:-

Weed control

Rising damp will rot the wooden? floor in time.

What will you bolt your machines down to?

[Steve Skelton 1Participant @steveskelton1]

I personally would stay clear of that type of base for a number of reasons:

1 It is likely to draw damp up from underneath even if you try and put a membrane down

2 It is likely to be a bit unstable if you do decide to put any loading on it

3 It would have poor insulation properties

4 It will hard on the feet and would be difficult to keep clean.

It may be more expensive (not sure if it really would be) but far more cost-effective, in my view, to create a reinforced concrete base.

I have done this myself and it is not a great deal of work.

[Clive FosterParticipant @clivefoster55965]

Basically these bases are shallow cellular structures to retain gravel beds so the gravel doesn't run away from under the load. Similar things are often used on gravel drives, especially sloped ones, for the same reason.

In principle its little different from concrete except that concrete binds the aggregate into a stable solid matrix whilst these merely retain loose material.

The issue will be getting a decent compaction and ensuring that all is stable. Even stable sandy soil may erode away under hidden drainage when covered. If you are going to put serious weight on it for long term use getting a stable, compacted layer of gravel that will stay put for many years will take a bit more than the shovel and level process used on driveways. We have several gravel driveways around here laid with this sort of separator and the gravel still goes walkabout. Just nowhere near as badly as plain shovel and roll.

I'd be inclined to put a bit of extra work into the floor so it can cope if any area becomes loose. I'd also put some kind of edging round to retain things when compacting.

As ever the devil is in the detail but, so long as you pay attention to any possible problems it should be just fine.

Of course some folks get away with murder, metaphorically speaking. I knew a guy who just dropped 2 layers of shuttering ply on the back lawn. Built a workshop on top and moved in. Heavy machines, eg Victoria U2 mill, got an extra layer of ply! Lasted over 20 years to my certain knowledge!

Clive

[Martin KyteParticipant @martinkyte99762]

The usual reason to use cellular with infil is to level a sloping site. It's quite normal to do this with garden buildings. A mate of mine has 2 Dunster Buildings on such a base. They do however have fairly heavy framing under the floor timbers. The shingle infil will drain well so pressure treated timber should not suffer and as it's under the shed that area really shouldnt see much moisture provided it's ventilated.

regards Martin

[Peter Cook 6Participant @petercook6]

I considered one for my workshop which has a wooden floor on bearers. I went with a concrete slab because the level of precision (flatness and level) needed in the surface on which the plastic stands needs to be very high if you want a long term stable base to put the bearers on. The one I looked at recommended a couple of inches of well tamped and carefully levelled sand on top of the soil. Any subsidence in the underlying soil will also cause problems.

I suspected getting the base level and flat would not be worth the effort. A concrete slab only needs the underlying surface to be roughly level, the shuttering round the edge so far easier to get level. Then the concrete is fairly self levelling.

If you do use a wooden floor get one constructed from treated timber.

[Dave HalfordParticipant @davehalford22513]

They use the stuff in modern touring caravan parks, so it gets 1500kg vans and heavier motor homes driven on and off it every 7 days during a season.

it's contained by bull nosed edging

[Neil WyattModerator @neilwyatt]

If you do decide to use one of these, keep a record – photos, etc. It might be interesting to write it up for MEW readers

Neil

[DC31kParticipant @dc31k]

A shed base is a completely different animal to a shed floor (or a floor for a shed).

Maybe the OP would clarify if the proposed shed comes with (wooden?) floor or not as some the answers above make assumptions one way or the other and the advice given will not be valid if the wrong assumption has been made.

[JohnFParticipant @johnf59703]

Plus one for DC31k’s comments, indeed the write up in the link is not clear whether the shed shown has a wooden floor or not !

As a way to create a plinth to erect a shed with a floor set on bearers I think this system would work well but not for a shed without a floor thus this being the floor .

John

[BazyleParticipant @bazyle]

Either way you need to remove the topsoil or it will gradually lose its organinc content and shrink. That's going to be 6-8 inches so needs at least a partial infil of MOT1 or 2.
One of the problems with concrete bases is the few inches just outside the shed wall on which the rain and perhaps roof draining fall and then into or under the shed. The gravel option avoids this.
An alternative might be paving slabs. If there is a worry about the slabs tilting and moving over time another option might be concrete gravel boards.

[colin branniganParticipant @colinbrannigan54160]

I used this cellular plastic for my shed base, 12 x 8 wooden shed to be used as my machine shop, the site was part of my lawn and I simply cut the grass as short as i could, laid down some membrane and the plastic squares and filled it with lime stone grit.

Been down six years and hasn't moved so very happy with it, my reasons for using it were cost and if I choose to remove it I can quite easily and recycle it.

Colin

[Nick ThorpeParticipant @nickthorpe64546]

Thank you everyone for your help and advice and if it goes ahead Neil I will happily do a piece for MEW.

Just to clarify, the workshop would come with a 20mm wooden floor and timber bearers over which I would lay flooring grade chipboard on top of a membrane. I had one of these workshops built for my last house by a company in Shropshire that were very good and were able to make modifications for me, so this time I could for example modify their 3 x 2” bearers to 4 x 2”. A weed barrier type membrane would also go on the earth before laying the tiles.

To add further clarification the site would be dug out to reflect the depth of the tiles, levelled and I was considering hiring a vibrating plate to compact the soil.

Regards, Nick

PS: Should note that I didn’t bolt my machinery down in my last timber workshop and it was fine. The mill, lathe and pillar drill were old school and weighed a ton – literally!

 

Edited By Nick Thorpe on 21/03/2021 20:07:55

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