Self levelling over concrete slabs laid in a summerhouse to get a smooth floor

Hi - first post here so be gentle.....

We have a 7m x 3m summerhouse in our garden - built in around 2004, although not by us - we moved in in 2015.

The floor is laid with concrete slabs and is uneven (a pain when you want a smooth surface), plus in winter, it's bloody cold. The building is built on a large concrete sub-base but we're not sure if the slabs are laid on a DPM or straight onto the concrete - we think the latter. Either way, water is getting under the slabs - which given the very high water table we have now isn't all that surprising maybe - certainly the wettest we've seen it since moving in. This is creating some efflorescence on the slabs - where this appears I'm seeing a moisture meter (standard 2 probe one but with a masonry setting) reading of over 20%; in dryer areas it's less than 15%.

We'd like to make the surface smooth and lay some Karndean type vinyl tiles over which means one of 2 approaches.

1. dig up the slabs and start again. not something we want to do as it's messy, and there is a T-shape stud partition at one end which is built on top of the slabs, so that would need removing as well to get all the floor up.

2. lay a self levelling floor. this is our preferred approach as we can then make a smooth surface, although it will raise the surface (not sure by how much as I haven't fully measured the rise and fall of the current slabs). I can remove the current skirting easily enough; walls are plasterboarded and the door threshold strip is being replaced so will be higher (new door going in). I reckon an additional depth of up to 20mm before the floor is flat.

My main question is whether to lay a latex type DPM over the current floor before laying the self levelling, and perhaps after as well?? Can you get water resistant self levelling?? And is there any treatment I should look at to remove the efflorescence or just brush/hoover it off??

Someone also suggested using Jackoboard over the self level before laying the vinyl tiles - not a product I'm familiar with but it looks interesting as it will also add some insulation and warmth.

Picture of the current slabs and efflorescence attached.

Apologies for the lengthy post - be interested in comments and suggestions!

The proper way to do it

Take the slabs up, lay dpc poor concrete and smooth it, can even polish it if you want and then paint.

The dampness will kill the self leveling compound and solve nothing, the slabs don't give a good enough base and may move over time causing the self levelling to crack and fail.

Don't put the self leveling on top of doc with slabs in place for the same reason. Self levelling is only thin layer and it will break up under foot.

Now how about the walls of the summer house, do they get damp?

hi - thanks for the reply.

I hear where you're coming from and if I had the time and money to do the "correct" thing I would, but simply, I don't. I would also need to remove the stud walls to get all the slabs up and frankly I CBA to do that as well as dig the slaps up. So I don't want to go down the best route and looking for a decent alternative. The summerhouse is not a high use area so I can live with a 2nd best option - if in time it breaks down, then so be it, I'll accept my errors!. The walls of the building are not damp as I can see from outside that the timbers are laid over a Visqueen like DPC but I don't know if this plastic DPC continues under the slabs - I suspect not. most of the floor isn't damp, just a couple of spots where the efflorescence is showing.

bearing in mind the slabs have been there for about 15 years and haven't moved at all in that time, the chance of self levelling cracking is small - and if I put a good liquid DPM over the slabs then that should keep any damp getting to the self levelling, plus another DPM over the self level (or something like a 4mm Jackoboard) before laying either vinyl or laminate. the self level will have varying thickness due to the rise and fall of the slabs, but I would look to be laying around 20mm.

so - given that lifting the slabs ain't on, do you think my idea would be a good 2nd best option??

A cheap option would be to basecoat/cement over the slabs and then look at laying F-Ball Adhesives Isolator over the cement, LVT can then be glued to that.

It's not the 'right' way to do it, but an option worth exploring

thanks for the reply crazydaze. the “right” option is not something I want to do, just looking for a next best that will work without too much disruption/mess. the cheapness isn’t the key but a good suitable one - digging up etc would cost a bomb, so maybe something between your idea and that.

slipperywhenwet said:

thanks for the reply crazydaze. the “right” option is not something I want to do, just looking for a next best that will work without too much disruption/mess. the cheapness isn’t the key but a good suitable one - digging up etc would cost a bomb, so maybe something between your idea and that.

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I would look at the base coat and F-Ball isolator option, it’s the minimum of prep and should give yiu the finish that you require.

someone also suggested Jackoboard which I believe also has a built in waterproof membrane and will also act as an insulation layer. any knowledge of this??

crazydaze said:

I would look at the base coat and F-Ball isolator option, it’s the minimum of prep and should give yiu the finish that you require.

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what base coat would you recommend??

slipperywhenwet said:

what base coat would you recommend??

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Inwould have a look at the Arditex range.

crazydaze said:

Inwould have a look at the Arditex range.

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thanks - will look

Thin SBR bonded screed or asphalt if they still do such a thing.

You can get self levelling compound that is suitable for outside use, and one that can go down in a thick layer. If you do it cheap you may have to do it again. Get your spade out !
Take up the slabs - support the stud wall from underneath it. Dig a little more level off with sand- lay down DPM next lay 50 mm polystyrene insulation slab then concrete on top - as thin as can get away with.

I know someone who has just had an intergrill garage converted to a room and was interested to see that they put down kingspan on top of the original concrete floor and then a wood board then carpet.