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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Lean To & Concrete Floor https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1383027056 Message started by Lectrician on Oct 29^th, 2013, 6:10am

Title: Lean To & Concrete Floor
Post by Lectrician on Oct 29^th, 2013, 6:10am

Currently have a 600mm wide concrete path and then mud. This is going to be inside a lean to soon. I want to put a DPM down and then concrete sand cement screed?

The ground slopes from one end to other by some 4 inches. Ideally I will leave the path and the new concrete or screed will just go on top. What sort of depth do I need to go to with concrete or screed on top? What needs doing to the hard mud? Gravel and whacker?

Thanks.

Title: Re: Lean To & Concrete Floor
Post by woodsmith on Oct 29^th, 2013, 8:06am

I'd be worried that the new concrete would crack where there are changes in the sub base, ie along the line of the path. Can you not dig the path up, dig out the entire area level then maybe put some of the broken concrete back in as hardcore?

Title: Re: Lean To & Concrete Floor
Post by Lectrician on Oct 29^th, 2013, 8:37am

That was one of my concerns too TBH. It just seems like a lot of work :-(

Title: Re: Lean To & Concrete Floor
Post by CWatters on Oct 29^th, 2013, 8:50am

How much can you raise the level by? Could you go for a suspended timber floor (needs a ventilated void underneath)?

Will it be heated to allow use as a conservatory in winter?

Brick stub walls? Foundations for those?

I share others concerns about it cracking. Best take an angle grinder to the concrete and have it out.

Typical floor slab is normally something like..

150 mm of hardcore, 25-50 mm of sand, 1200 gauge membrane (plastic sheet), 100 mm concrete slab, >85 mm insulation, 60 mm screed

With some types of insulation you can do.

150 mm of hardcore, 25-50 mm of sand, 1200 gauge membrane, >85 mm insulation, >100mm concrete (& rebar/mesh).

If you really want to leave the path then you might get away with it if you made really sure the hardcore areas were well compacted and used rebar/mesh in the concrete? The insulation would take up some movement. Don't plan on tiles though just in case.

Title: Re: Lean To & Concrete Floor
Post by Lectrician on Oct 29^th, 2013, 3:49pm

It's only a shed, nothing homely. I only want to raise it for two reasons - to combat the slope (not really an issue I guess), and to provide a DPM to keep it a little drier in there.

Title: Re: Lean To & Concrete Floor
Post by CWatters on Oct 29^th, 2013, 9:24pm

Ok not so bad then.

Ideally remove the concrete path but you might get away without.

I think I'd dig out the soil and compact the ground with a wacker plate. Then say 4" of hardcore (crushed limestone is good) again compacted. the area of hardcore should be bigger than the slab by say 6" on all sides. Then a sand blind (also compacted) and a DPM. the DPM is also to stop water from the concrete soaking in and drying the mix before it sets. Then formwork for the concrete with the top level. I reckon you need at least 3" (4" is better) at the shallow end meaning 7" of concrete at the thick end.

If the base is bigger than the shed then you should really lay a coarse of bricks or sacrificial treated timbers with DPC on top to raise the shed off the slab eg to prevent any water that pooling around the edge rotting the floor.

Using hardcore to level the site instead of concrete might/might not be possible - Hardcore should be below ground or it can wash out from under the edges of the slab. One option might be to cut out parts of the path, lay small trench founds, build up in brick which will then both raise the shed and act to retain hardcore on the inside which fixes the level. You might then get away with less concrete. Say 3-4" all over.

lots of ways to skin a cat.