Garage/workshop floor slab repair

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Woodypk

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Hi all,

Just wanted to pick thy brains for ideas on the best way to fix my double garage floor slab.

It’s an old prefab garage where the slab looks like it’s been laid in two parts (garage erected before we moved here) and is on two ever so slightly different levels.

The issue is that it’s not level and water comes in and pools all over the floor in different areas. I’m now wanting to raise the whole slab slightly (10-15mm?) and level it out and wanting to know the best and most cost effective way to do this.

Floor leveller will eventually be used to give the most smooth final finish but to raise it up by what I’d guess may be 10-15mm or more over the (approx) 32Sqm area would cost a fair bit using levelling compound.

Is there any way to lay a robust yet thin layer of material (screed or other material for this??) over the floor to take up the bulk of the unevenness/imperfections before I use a self levelling floor compound to finish the last few MM?

I’ll be doing the work myself and it doesn’t need to be a professional job that will last forever. Just good enough to be a smooth finish for castor wheels and to keep the water out until I decide if I want to rent a unit or rebuild the workshop from scratch.

Thanks in advance.
Tom
 

eribaMotters

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Tom, I'd be thinking of taking a different approach. I doubt a self levelling type floor screed will be hard enough to withstand workshop use, unless you go over the top with a very expensive 2 part floor paint as used in car showrooms, fire stations and the like. 4 years ago I priced this paint up for present garage floor of 10x5m and it was north of £600. How about a concrete speed bump type ramp into the garage to stop water ingress.

Colin
 

Woodypk

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Hi Colin,

Thanks for the reply.

The issue is, the ground surfaces surrounding the garage are all uneven all way around the garage. So water is seeping in through 3 of the 4 garage sides (one side is lower and grassed). Where the garage is situated, that sort of method wouldn’t work.

Ideally I need to raise the internal floor space some how.
 

Woodypk

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Looking at this another way… if I we’re to raise the internal floor space, what would be the best material to use that would be strong enough at the smallest thickness?

Would I not be able to just lay a new concrete floor over the existing one and put a chamfered edge where the doors are so it’s not a 90degree ‘bump’ to enter or exit the space?

And if this is possible, what’s the minimum thickness I could get away with that would be strong enough for a workshop floor?
 

MARK.B.

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Even if you go to the expense of raising the level of the floor, you will always have a damp problem unless you stop the water from entering in the first place . one way to do that would be to dig a trench and lay what i think is called a french drain, essentially a perforated pipe laid on and covered by pea gravel or similar,This will allow the excess water to be drained away from garage . Fitting a gutter and down pipes that take the roof water away would also help . 10/15 mil of floor leveling compound will imho not solve your problem,it may look OK for a while but if you have no DPM coupled with water coming in then the compound will break down and you will be back to square one. No quick fix but my advice would be to cure the water problem first then rip out and relay a new concrete floor with a good quality DPM :)
 

Spectric

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Not an easy problem to solve on the cheap, and how good is the existing floor because if that is in a bad state then whatever you do will probably also fail. The best solution is to start again but expensive, so to get you out of trouble the way I would go is to firstly use a damp proofing slurry like


to cover the floor and especially where the precast sections meet the floor. Then lay a heavy plastic membrane that comes up the sides all round and then a two inch screed using


You could just membrane and use the second product but the first one really prepares the floor and gives the first line of defence, but like any job where you are trying to repair an existing problem the outcome cannot be guaranteed and you pay the money and take the chance.
 
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eribaMotters

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When I built my previous workshop at 8x5m I had "contractors" in to do the concrete, otherwise known as cowboys. The mix they used was blind and moisture was rising through the floor. It was also a bit out of level.
I cleaned the floor and waterproofed it with 2 applications of a black bitumen dpc type pint.
Next I ran a row of thin 500 mm square paving slabs down the long edges and along the centres, bedded on sand and cement. This gave me the chance to sort out the levels.
A couple of days later a concreted between the two bays between the rows of paving slabs.
The overall thickness was about 50mm. The job was manageable by myself. I gave the floor a couple of months then painted it. In hindsight I could have just sealed it with a 50/50 pva and water mix as I've done in my current workshop.

Colin
 
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johna.clements

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If the ground around the garage can not be lowered dig a trench around the garage to just below the floor level with an outfall at the side where the ground is not higher. I would assume that the slab is not very deep so you do not want to go too deap and undermine it or let the frost get under it. Lay some dense concrete blocks, concrete gravel board, bricks, sleeper etc to retain the soil. Leave a gap between the blocks etc to allow air to circulate and of sufficent width to allow you to clean out any leaves. Maybe 75mm then you could use a rip of plywood or a bit of batten a couple times a year.

You could cast a new slab on top of the existing slab inside the garage, most likely cheaper than a thin surfacing. say 75mm thick of 40 newton concrete (1 in 4 with as little water as possible). The existing crack in the floor will induce a crack in the new slab unless you use steel mesh over the whole floor (even then no guarantee) and you need to provide cover to the steel; 35mm cover plus 6mm wires gives a 90mm thick slab minimum. Easier maybe just to put a joint in the slab above the existing crack.
 

Lorenzl

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According to youtube video's it can be done with spray foam :)

One I watched was raising path slabs; it raised the low slab and the high one which was not quite what was wanted. I suppose if there was a gap between the slabs that wouldn't have happened.
 

Woodypk

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All,

Thanks so much for the comments.

One thing I’ve taken in from reading these suggestions is that there is no cheap solution to this problem…

Now I suppose I have to weigh up the amount of work and cost I want to put into fixing an existing issue or paying even more to start again with a fighting chance of making something decent by ripping down the garage and starting again.

It seems obvious now that in reality, the cause of the problem should really be fixed before the other issues, even if it is just for a couple of years fix. The higgledy piggledy floor isn’t so much as an issue as I use big castors on my bases but the water ingress is… I suppose a few more winters of waterlog may have to be endured before I bite the bullet and rebuild properly.

Slightly off topic here, but has anyone here decided to build themselves a timber workshop (not a cheap shed type but something more robust) over going for a brick or similar built workshop and for any reason other than cost? I’d be interested to know and I ask because I suppose if starting again, ripping up the slab and all, that a timber framed building would need much less a substantial foundation than a brick built building.
 

Lazurus

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I have just finishes a 8m x 5m brick and block workshop. When I set out I looked at SIP`s, timber and timber brick hybrid. I went in the end for the traditional brick and block with just 50mm cavity insulation, as even having Easi span joists made it worked out less expensive and time consuming in the long run. in the end I have a great working space, and when I am too old for that will be a great annex to see my days out when SWMBO says I am getting too smelly for indoors. Simple pent roof with EDPM and associated trims, roof light and 3m bi fold doors made it a very pleasing building. Surprisingly well insulated with just 50mm rockwool in the cavity.
 

Spectric

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Now I suppose I have to weigh up the amount of work and cost I want to put into fixing an existing issue or paying even more to start again with a fighting chance of making something decent by ripping down the garage and starting again.
You need to do some cost and time calculations, work out how much to hopefully fix the existing garage base but bear in mind you still end up with a precast garage and then how much to lay a new base and build a garage that meets your requirements, you could use some of the old garage walls as hardcore for the new base but you will still have to get rid of some waste. Another point, what is the current garage roof made of as it could be asbestos which would add cost to dispose of but also not the best roof to have, so another justification to start again.
 

Lazurus

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Don't mention asbestos, we just had the cladding replaced on our bungalow, we knew it was asbestos based and the specialist company quoted £1k to remove and dispose of a complete side and gable end as it was low level stuff (see below). However under the soffits was some of the nasty stuff, required full positive pressure tunnel to be built, shower unit and space suits, went up to £6K, Wasn't expecting that!!!! Doing some research many councils will remove from site free of charge, if it is the low level stuff, paper overalls and a positive pressure or good quality mask is all that is needed, it is the disposal that can be an issue. Ours ended up in Peterborough from Norfolk.
Or find a friendly farmer and get him to dig a big hole!!!!!

White Asbestos​

Also known as chrysotile, this is the most commonly used form of asbestos. It was used in walls, floors, roof and ceiling tiles. It is still used in brake linings and other industrial uses.

Chrysotile is classified as a part of the branch of serpentine formed minerals. It is made up of curly fibres. This shape makes chrysotile fibres easier to breathe out and less damaging to the lungs than other types of asbestos.

Chrysotile was the last type of asbestos to be banned in the UK. Chrysotile was banned completely in the UK from 1999.
 

Jameshow

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How about a couple of coats of damp proofing bitumen paint and then 1" insulation / battens and a 18" mm plywood or chipboard?
 

mikej460

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I had this problem in my outbuilding (old stables with 114mm concrete block walls) and now my 1970s sectional garage. Fixing the water ingress and flood water getting into the outbuildings (we live near a river that floods every few years and the outbuildings are at a lower level than the house) was achieved by laying a 1200 gauge DPM then a bed of 120mm jablite insulation slabs (off eBay and cheaper than concrete), a vapour barrier then 100mm of concrete with fibres. It's now as dry as a bone and surprisingly warmer in winter. This lifted the floor by 220mm which required the wall heights to be increased by the same and a new roof installed (the old one was corrugated asbestos fibre cement and knackered). I also installed guttering to both sides of the apex roof. I'm using this same technique for my new workshop that will replace the old garage this year. However, this isn't cheap but it is very effective.
 

Ttrees

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I've pondered about this as I am in a similar setup regarding flooding, albeit a bit more drainage is about the only really affordable attempt I can fix as a tenant.
Just wondering what do you folks do about the wall if you fix the floor with a new slab and have DPC up the inside walls,
PC090011.JPG
 

Keith 66

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I had a similar problem with my garage. A cheapish fix that works to seal the Precast slabs & floor joins.
Rake out any mud, roots, dirt from all gaps, wire brush them out too, blow out with compressed air if you can, vacumn cleaner if you cant. The joint has to be clean. Hot air gun to dry it out in short sections if you need to.
Apply Arbokol 1000 Polysulphide sealant to the join, this stuff sticks like the proverbial to a blanket & is moisture cured so as the damp comes back it will help it cure. Run a thick corner bead with this stuff & it will keep the water out as good as anything. Dont bother with silicone as it will let go & leak.
Once you have done this screed the inside over a DPM if you need to.
 

Jonm

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Lots of good advice here. Some pictures would help, of the existing slab and around the outside particularly where the wall meets the ground.

Does the building have gutters, if not then fitting them should substantially reduce the water problem.

For a thin screed of 10-15mm the main things are making sure it sticks to the concrete underneath and I would want to do it in one go. So SBR or the slurry waterproofing screed already suggested.

I have used a self levelling screed which was designed to have sand added and it worked well and was much cheaper, would not go below about 8mm because of the larger particles in the sharp sand. It was about 10 years ago and I cannot remember the name, but it was from a well known brand.
 
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John Hall

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With projects like this it knowing what products are available…as mentioned earlier Sika sell all sorts of building products for all sorts of purposes…usually pretty expensive…It may be worth contacting them for advice…
I had exactly the same problem with an old garage years ago and raised the base by 25mm, using a fine concrete mix with a waterproof additve…thoroughly cleaned and pva coated the existing surface first….worked a treat..
 

SamG340

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What about a timber floor ? DPM, treated timber , OSB floor boards.

For me I think it would be simpler than messing with concrete and leveling compounds.

Maybe it wouldn't last forever if the timbers are getting wet but would do a good few years easily
 
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