Concrete base uneven for garden workshop

27 Jan 2010
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United Kingdom
Thank you for your help - Please save me!

In just a few days a new pottery studio is arriving for my wife, but the concrete base has gone wrong :(

The Concrete base is 5.5m by 2.4m (18' by 8'), steel reinforced, hand poured by an amateur friend on top of 10cm of compacted mot type 1.

The pottery studio is a timber building of the same size, containing a heavy pottery kiln, heavy clays, etc. The studio arrives in panels and is installed for free but you must have a proper, level concrete base.

The studio sits on a wooden pressure treated sub-floor/base, provided by the factory.
It is a frame of 44mm high joists spaced 300mm apart, with the flooring already connected to it. (flooring is 19mm pine + laminate on top)

Challenge 1: The left third of the concrete base is lower than the right :(

It is lower the full length across (2.4m) to varying degrees, dipping between 5mm and 30mm lower than the right side.

Challenge 2: It is impossible to put packings all the way along the joists when you can't see them through the flooring... I could put some packings in at the edges - but it feels like this new studio is starting with a bodged job :confused:


(A) Will the joists(+flooring, walls, etc) warp over time if the joists are only supported at the edges using packing?

(B) Is there a solution to level the concrete first? I've been told adding more concrete wont work...

(C) Any other ideas?

Thank you for your help! :notworthy:

My only idea so far is:
1 - Build a temporary frame of wooden joists
2 - Pack out all the dips under the joists (with whatever pressure treated bits I can cut up to fit or maybe damp proof course in layers??)
3 - Remove my joists and leave the packings in position, ready for the arrival of the real sub-floor
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First idea that comes into my head is lay rows of bricks/blocks where the joists will be, take your time and you can lay them level. You'd really need the floor there on site to do that, so you can know exactly where the joists will sit. So you will end up having to build the shed yourself.

Or you could use some decent sized beams like sleepers (ideally the proper creosote ones), sat on the concrete at one end, and packed at the other end with slate or whatever.
Both of those would raise the height of the shed, meaning a step up, but raising the shed joists off the concrete (and with a strip of dpc) will keep the shed timbers dry and rot free for longer.
Thanks @scbk I appreciate it.

Using DPC for packing is a brilliant idea. I like the idea of putting down slabs, but the studio is already at the maximum height permitted, which ties our hands...

I'm trying to gauge how bad the problem is - that is, how far can a joist go unsupported before it's an issue?
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You could lay 50 x 25mm tile battens (get the proper treated ones).

Lay them at 90 degrees to the floor joists at say 300 to 400 centres.

Pack up level with plastic wedge packers.

Where the gap is under say 12mm fill with some cheap silicone.

Where bigger use something like mortar.

You will need a laser level to get it really flat.

Alternatively you could screed it, but thats quite a bit of work for a diyer, fixing battens is easier for diy.
If this is basically a shed-style construction designed to lay flat on the concrete, you're best off getting some exterior grade self levelling compound and use that to level off the slab.
Thanks @Notch7 @ivixor @foxhole - I appreciate your inputs (y)

Here are the options we've been given from tradespeople who came to see it:

(1) Slabbing:
Put concrete slabs onto the existing base using sharp sand (apparently dries hard like brick mortar).
This will add approx 80mm-100mm in height. (which is a lot)
He insists that screeding is the wrong approach because it will eventually crumble.

(2) Screeding:
Put 50mm screen onto the highest point and fill in the rest to match that point.
He uses fibre screed (apparently different to latex?) to give it strength. He applies a concrete-wetting pva-like product to control the drying speed of the screed.
He insists it will be strong enough and will last our lifetimes.
He demonstrated his laser level and how he can deliver a flat base with a few mm tolerance.

We would prefer the screeding as it's much cheaper and only adds 50mm height. But the first person says it won't work :confused:

For completeness we were also given a third option:

(3) Brick frame:
Build a brick wall around the existing base that goes up 100mm higher.
Use lasers to get the brick wall perfectly levelly.
Fill up to the 100mm with concrete, on top of the existing concrete and up to the height of the bricks.
So that's 2 100mm concrete bases on top of each other.
(we aren't interested in this idea)

Thanks again for all your help and insights, I really appreciate it! :notworthy:
We would prefer the screeding as it's much cheaper and only adds 50mm height. But the first person says it won't work

Screed will work, I didnt suggest it because of the height increase and the cost.

Its certainly the easiest way to form a flat surface over an uneven area.

The screed is bonded, so it doesnt need to be as thick as a floating screed.

Bonded screed is 25mm to 40mm range, or down to 15mm if its a screed with sbr added.

Im not sure about the edges, it might need just chamfering off with the trowel.

Itll be pretty strong, afterall render is oretty rock hard on walls and thats bonded.

Its hard too imagine how the base was wrong to start with, was it done with shuttering? -my guess is it was done with a carp level, it shouldve been done with a laser really or a string line and 1800mm long quality level.

Always check the accuracy of a level -you only have to swing it around 180 degrees and check the bubblr is in the same place, you dont need a level surface to check.
Thanks @Notch7 @foxhole

I'm glad to hear screed will do it. :D We will have heavy stuff in the shed - a pottery kiln, many pottery clays, work benches, etc.

The story of how it was done wrong was exhaustion + a lack of care from the concrete guy. He started on the left, not noticing he hadn't filled up to the shuttering and used a thin piece of wood 3m long which of course bowed in the middle and depending on where he ended his 'sawing' motion we ended up with a dip or a rise. If I'd known then what I know now, I could have acted as quality-control... The guy was clearly amateur and everyone now tells me that a base 5.5m by 2.5m was a task for 3 people, and that one person had no chance of getting it right. (you cant have an eye for quality when exhausting yourself by manually mixing concrete on your own)

Sometimes you get tricked by those who can talk a big game... :(

Biggest lesson learned: Agree up-front what they consider to be 'within tolerance'.
Also thanks @ivixor - I'm getting mix messages about the self levelling products and no one I meets wants to use them (apparently it's too easy to get it wrong...?) I appreciate your message
Hi @Notch7 @foxhole - can you shed some further insight here? (many thanks!) :notworthy:

The builder (who is recommending slabbing it) strongly insists that 50mm screed will not survive a frost.

The screeder says it will be fine (as a side note he is recommending screed with fibres in it for strength)

Google has no information about "will a screed survive a frost" and product data sheets only mention freeze-thaw should be avoided during application:

Our screed will be installed this summer and is protected from moisture going forward because:
- the width/length of the screed is exactly the same dimensions as the shed
- the timber joists have decorative skirting that will overhang the screed (no way for horizontal rain to get under there)
- we plan on wrapping the damp proofing around the sides of the screed and under the shed

How do I get a concrete (haha - sorry - its laugh or cry time! :D:confused::cry:) answer on this?

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