Levelling old uneven shed floor before insulating

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alex robinson

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I am slowly in the process of rebuilding a fairly large shed (many thanks to previous posters for advice about insulated roof panels). The old concrete pad is mostly intact, but there is quite a lot of unevenness (up to 40mm in places where 2 different concrete pours have moved independently).

I would like to put down PIR insulation, followed by a DPM and then a floating MR chipboard floor. For flattening, I was thinking of a simple sand and cement? I have seen in lots of places that it needs to be quite thick to prevent cracking. Given that this is going to be under the floor, does this matter? So long as the end result is flat (not so concerned about level) is there any reason why I cannot go from a feather at the highest points to whatever depth required at the low points? My current plan is to use a laser to fit battens at the correct heights and fill between them. Having levelled a kitchen floor with self levelling compound, that is out due to cost.

Second part of the question - how thick would people suggest the PIR needs to be? I am currently thinking either 30 or 40mm. Obviously more is more, but it gets expensive and it is only an outbuilding..

Many thanks in advance!
 
I know precisely nothing about floor levelling, but at a theoretical level all you are trying to do is support the backside of the PIR board in a uniform way. If the uniformity of support breaks down over time then this could translate to floor unevenness. Your plan of a fill between battens could have two concerns of poor support.
1. If the battens and/or sand cement mix expanded/contracted differentially and overtime the battens became proud of the mix then then boards would be unevenly supported.
2. If the edges of the levelling mix are not restrained they could crumble away and leave an unsupported portion of floor.
Personally I think your plan sounds sensible, and I don't see that cracks would be an issue.

I would check if the selected insulation is ok to use in a damp condition without breaking down, if it is not then the DPM should go over the levelled base but under the insulation. You'll then want to decide if your insulation is contiguous enough to act as a vapor barrier of if you need a 500g poly sheet between insulation and boards.
 
I know precisely nothing about floor levelling, but at a theoretical level all you are trying to do is support the backside of the PIR board in a uniform way. If the uniformity of support breaks down over time then this could translate to floor unevenness. Your plan of a fill between battens could have two concerns of poor support.
1. If the battens and/or sand cement mix expanded/contracted differentially and overtime the battens became proud of the mix then then boards would be unevenly supported.
2. If the edges of the levelling mix are not restrained they could crumble away and leave an unsupported portion of floor.
Personally I think your plan sounds sensible, and I don't see that cracks would be an issue.

I would check if the selected insulation is ok to use in a damp condition without breaking down, if it is not then the DPM should go over the levelled base but under the insulation. You'll then want to decide if your insulation is contiguous enough to act as a vapor barrier of if you need a 500g poly sheet between insulation and boards.
Thanks for that. I was wondering if I had missed something massive when I kept seeing all these things about minimum thickness. I guess both those points could be fairly easily addressed. First by taking the battens out after it has set and filling in the holes, and the second by the brick mini wall the woodwork will be on to give support at the edge.

Think I am going to go for the membrane just to be on the safe side - seems mad to risk things for a small bit of plastic.
 
If the two slabs are moving how about tying the back together using concrete butterfly's or rebar rod secured with resin in channels?

Also 50mm is the standard thickness of celotex why not use this?
 
I did exactly what you were describing, in a smaller scale, a 16'x8' garage with pronounced subsidence (40mm) in one corner and fragmentation of the slab into three 'islands'. A skilled builder friend advised sand and cement to level, mixed "tight" i.e. minimal water, and laced with P.V.A. to give adhesion to the existing slab surface.

I then laid a 1200 guage D.P.M, doubled and a floating floor of Celotex, topped by chipboard, glued together on the joints and only a smidge of self-levelling on top - to just fill in the joint lines and a few scrapes and dinges.

I have a Wadkin AGS at 400lbs plus, a pillar drill of about 150lbs, bandsaw, large sander, ton weight bench, yada, yada... nae probs. My builder friend said: " As long as the subsurface is consisently filled and levelled, it won't matter a hoot if it fragments. The walls will contain the new and old concrete and the Celotex/floating floor will even out the loads across the whole lot. End of problem." Two years on, he's right.
 
I've just done this a few months ago so no long term information, but I did a 4:1 mix which I put down dry and then misted with water initially to form a skin, before giving it a soak. Was only around 20mm thick, but I'm compression and no sideways load has worked well. I put wooden bearers in mine between the celotex, but might well be overkill and I've moved the lathe in which weighs around 1.2 tons and this worked well
 
I had a similar problem with a very large old shed. I bought a breaker from screwfix for £150 (almost as good as a pneumatic drill) and broke up the old slab, and then used that as the base before pouring concrete over it. It gets heavy machines over it frequent and is still perfect.
 
as for the slab better to use a wash coat of Latex to the old CLEAN NON DUSTY/OILY floor then pour/lay the conc when still damp/wet.....
I was advised buy Sikkens to do this on an outside path that gets very wet....
In the cement mix I'd also add glass fiber strands, very cheap....then it wont crack again...
a skim coat of cement 40mm, this is really not quite enough...can u go 60-80mm.....?
it'll make for a better, longer lasting job.......
def do as Jameshow says in tying the 2 slabs together....it's quite easy using pre bent wiggley rebar or the ready available st/steel ties from the builders merch....the resin comes out of the tube pre mixed in the spout.....let it set for 24hrs...or get the mix n pour sort......
I have a cracked huge garden wall that going to get the same treatment....
never liked the idea of a wood floor for a workshop even if useing a damp membrane.....

ps, if the finished conc floor is not polished then use a latex floor leveling compound...easy to do but best done with 2 extra helpers....
it does add a little expense to the job but well worth doing.....then unskilled hands can do it.....
the level compound basically comes in 2 strengths.....get the type that will take traffic from forklifts.....not much more expensive....
then the floor will be level and easy to sweep....
 
Unfortunately replacing the slab isn't an option - I have to make the best of what there is. To be honest, given it is on incredibly heavy clay, that amount of movement is actually quite good. Looking back, current superstructure there for at least 60 years, and slab possibly since the 40s.

I did exactly what you were describing, in a smaller scale, a 16'x8' garage with pronounced subsidence (40mm) in one corner and fragmentation of the slab into three 'islands'. A skilled builder friend advised sand and cement to level, mixed "tight" i.e. minimal water, and laced with P.V.A. to give adhesion to the existing slab surface.

I then laid a 1200 guage D.P.M, doubled and a floating floor of Celotex, topped by chipboard, glued together on the joints and only a smidge of self-levelling on top - to just fill in the joint lines and a few scrapes and dinges.

I have a Wadkin AGS at 400lbs plus, a pillar drill of about 150lbs, bandsaw, large sander, ton weight bench, yada, yada... nae probs. My builder friend said: " As long as the subsurface is consisently filled and levelled, it won't matter a hoot if it fragments. The walls will contain the new and old concrete and the Celotex/floating floor will even out the loads across the whole lot. End of problem." Two years on, he's right.
I've just done this a few months ago so no long term information, but I did a 4:1 mix which I put down dry and then misted with water initially to form a skin, before giving it a soak. Was only around 20mm thick, but I'm compression and no sideways load has worked well. I put wooden bearers in mine between the celotex, but might well be overkill and I've moved the lathe in which weighs around 1.2 tons and this worked well

I am happy with a bit of a step at the doors, but don't want a huge climb. How thick was the insulation you used? Do you think it made a big difference? 50mm might be a bit too much, though it doesn't cost much more than the 40.
 
as for the slab better to use a wash coat of Latex to the old CLEAN NON DUSTY/OILY floor then pour/lay the conc when still damp/wet.....
I was advised buy Sikkens to do this on an outside path that gets very wet....
In the cement mix I'd also add glass fiber strands, very cheap....then it wont crack again...
a skim coat of cement 40mm, this is really not quite enough...can u go 60-80mm.....?
it'll make for a better, longer lasting job.......
def do as Jameshow says in tying the 2 slabs together....it's quite easy using pre bent wiggley rebar or the ready available st/steel ties from the builders merch....the resin comes out of the tube pre mixed in the spout.....let it set for 24hrs...or get the mix n pour sort......
I have a cracked huge garden wall that going to get the same treatment....
never liked the idea of a wood floor for a workshop even if useing a damp membrane.....

ps, if the finished conc floor is not polished then use a latex floor leveling compound...easy to do but best done with 2 extra helpers....
it does add a little expense to the job but well worth doing.....then unskilled hands can do it.....
the level compound basically comes in 2 strengths.....get the type that will take traffic from forklifts.....not much more expensive....
then the floor will be level and easy to sweep....
I like the idea of the glass fibre strands. Seems a good way of minimising the cracking.

Not sure if the floor levelling is going to work. I have given up entirely on the idea of level - 1 section has a 12cm fall over 3m... If I can get it (nearly) flat I will feel that is a massive success!
 
I did 50mm, but anything makes a big difference. And don't give up on level, just fitting out might at the moment and being able to roll around a cart of sheet material weighing around half a ton is lovely
 
Are you actually going to be heating the shed? Heat rises and unless the roof and walls are very well insulated thicker insulation in the floor won't be a significant benefit. Even 25mm will give you a thermal break between your new chipboard floor and the ground beneath and is plenty. Get the foil faced stuff and tape all the joints with foil tape.
 
50mm as well, plus 19mm tongue-and-groove chipboard flooring. Yes, it has made a whopping difference. But then, I put it up the walls and across the ceiling as well. Circulation gap in the ceiling, below the corrugated roofing.
 
Ok. Sounds as if anything is a lot better than nothing. Was going to put 90mm in the walls and the roof panels have 80mm.

Not planning on heating, but I really want it not to suffer from condensation and be bearable in moderate cold. Depths of winter things are going to be cold whatever.
 
I did exactly what you were describing, in a smaller scale, a 16'x8' garage with pronounced subsidence (40mm) in one corner and fragmentation of the slab into three 'islands'. A skilled builder friend advised sand and cement to level, mixed "tight" i.e. minimal water, and laced with P.V.A. to give adhesion to the existing slab surface.

I then laid a 1200 guage D.P.M, doubled and a floating floor of Celotex, topped by chipboard, glued together on the joints and only a smidge of self-levelling on top - to just fill in the joint lines and a few scrapes and dinges.

I have a Wadkin AGS at 400lbs plus, a pillar drill of about 150lbs, bandsaw, large sander, ton weight bench, yada, yada... nae probs. My builder friend said: " As long as the subsurface is consisently filled and levelled, it won't matter a hoot if it fragments. The walls will contain the new and old concrete and the Celotex/floating floor will even out the loads across the whole lot. End of problem." Two years on, he's right.
Planning to do the same in my new (old) garage. Even 25mm insulation under 18mm chipboard should make a big difference. I would expect it eventually to compress and collapse locally under heavy vibrating load (e.g. PT), so will put 25x25mm timbers in too, just in case.
 

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