extending concrete slab base for log cabin

[sak83]

I have a concrete slab base in my garden that has had a metal shed housed on it since i bought my house, The slab is 10cm deep and 2m x 2.4m. The current slab is right up against my rear boundary. I would like to replace the shed with a tuin log cabin mainly to be used as a workshop/storage.

I'm not sure what to do with my current base, do i break it up and re lay a new concrete slab with a damp proof membrane? The existing slab from what i can see does not have a damp proof membrane. Of do i extend the base, if that's even possible? Im a complete novice here so looking for some advise on the best coarse of action Ideally i would like to sort the base within £350 with me doing the work

[sak83]

I have had a search online and its suggested that the existing slab needs to be tied in to the new slab and there still may be some issues. This all sounds a bit much for a log cabin so I'm not really sure. Any ideas?

[anonymous-user]

If you are going to use it as a workshop and store tools, then you should relay with a base that has a dpm.

Its not that much hassle to break up and remove - just hire a kango - eg https://www.hss.com/hire/p/hydraulic-breaker-petro...

[sak83]

my neighbour actually has a breaker that i could borrow if i wanted and it was something i was thinking about

[sparkythecat]

Extend the slab and stand your cabin on some bearers on top of it. Put some strips of DPC membrane between the bearers and the slab and on top of the bearers. Make sue there is some fall on your slab, so water cant pool under the cabin .
The air gap under the cabin will keep everything healthy and it should kep dry and last for years

[Collectingbrass]

How much do you want to extend it by?

To be honest, I wouldn't even try and extend it. You'd need something like 16mm dowel bars every 300 - 450 mm or so drilled and epxoy'd in to 640mm deep, then you'd need to key the interface surface and cast a new slab. Then you've got the whole DPM issue as well.

I wouldn't even touch the old slab, I'd put a new pair of pads in, then decking joists on that, then shed on top.

[sak83]

Collectingbrass said:

How much do you want to extend it by?


I wouldn't even touch the old slab, I'd put a new pair of pads in, then decking joists on that, then shed on top.

After reading up a bit more i also think extending the slab is not worth pursuing. I need to keep the height of the cabin below 2.5m for permitted development and the log cabin im looking at has a height of 2.43cm. the current slab is just above ground level say 50mm so joists on top would take it too high.

I also looked at breaking the slab and using timber framed base using treated 6x2 siting on pads. But i don't think this will work either as to keep the height below 2.5m i would need to excavate and the frame would be below ground level

[magooagain]

Does the new cabin come with its own floor? If not then get rid of the old slab and cast a new one.

Membrane it if you want as it will be below outside ground level from what you are saying.

How are you going to stop water creeping in under the cabin if it's planted flat onto the slab?

If you can shamfer the concrete at the edges so surface water/rain can run away that would help,but you would need to be able to mark the cabin square onto the wet concrete to work out the shamfer.

Alternatively you could put the cabin on a course of semi engineering bricks to raise it. But that may affect the permitted height. Easy enough to work out befor you cast the slab though.

[TA14]

sak83 said:

the log cabin im looking at has a height of 2.43cm.

That's only just over an inch so should be no problem

[TA14]

You're going to need to do something like: excavate the area of the new cabin plus 200mm all round in plan view to a depth of 450mm (say goodbye to the old slab) cast a 6" slab on 6" insulation and visqueen and then build 6" high drawf walls out of class B eng Bk. Make sure that you have good drainage to stop a moat forming.

[sparkythecat]

TA14 said:

You're going to need to do something like: excavate the area of the new cabin plus 200mm all round in plan view to a depth of 450mm (say goodbye to the old slab) cast a 6" slab on 6" insulation and visqueen and then build 6" high drawf walls out of class B eng Bk. Make sure that you have good drainage to stop a moat forming.

Wow ! And here's me thinking the OP was putting up an garden shed.

[sak83]

TA14 said:

You're going to need to do something like: excavate the area of the new cabin plus 200mm all round in plan view to a depth of 450mm (say goodbye to the old slab) cast a 6" slab on 6" insulation and visqueen and then build 6" high drawf walls out of class B eng Bk. Make sure that you have good drainage to stop a moat forming.

So having thought about it, I'm going to remove the old slab and lay an new one. When I said the cabin is to be used as a workshop/storage. i meant that its likely to have a workbench one one side with a vice and some tools, nothing heavy. The other side will be rack shelving with some space in the middle for a few bikes. So is a base to this spec still necessary, The cabin manufacturer can't advise too much on the base but said a concrete slab of 100mmm depth is usually adequate.

So what does everyone think to the following:
-Break and remove existing slab
-Excavate to 100mm
-lay a dpm.
-Pour a concrete base the same dimensions as the cabin so water drips off the side. The cabin manufacturer says the base needs to be level with no fall
-Lay a foundation beam 50mm thick along the perimeter of the slab and build the cabin on to that to keep the cabin off the concrete. The cabin manufacturer can supply a profiled foundation beam that drips water away from the base.
-Lay 50mm battens on the slab and infill with 50mm insulation.
-Lay a floor on top
-Put in a french drain alongside the base

[TA14]

sak83 said:

So having thought about it, I'm going to remove the old slab and lay an new one. When I said the cabin is to be used as a workshop/storage. i meant that its likely to have a workbench one one side with a vice and some tools, nothing heavy. The other side will be rack shelving with some space in the middle for a few bikes. So is a base to this spec still necessary, The cabin manufacturer can't advise too much on the base but said a concrete slab of 100mmm depth is usually adequate.

So what does everyone think to the following:
-Break and remove existing slab
-Excavate to 100mm
-lay a dpm.
-Pour a concrete base the same dimensions as the cabin so water drips off the side. The cabin manufacturer says the base needs to be level with no fall
-Lay a foundation beam 50mm thick along the perimeter of the slab and build the cabin on to that to keep the cabin off the concrete. The cabin manufacturer can supply a profiled foundation beam that drips water away from the base.
-Lay 50mm battens on the slab and infill with 50mm insulation.
-Lay a floor on top
-Put in a french drain alongside the base

So your log cabin is closer to a normal timber shed than a garden room. Personally I'd just scrape of the topsoil where the existing slab is short and place some standard flags; then place (no mortar) quite a few class B bricks with a dpc on top and put the shed onto that. OK the shed will be about 2" above the permitted height but is that really going to be an issue for you/your neighbours?

Failing that:
Break and remove existing slab
-Excavate to 100mm
-lay a dpm.
-Pour a concrete base the same dimensions as the cabin so water drips off the side. The cabin manufacturer says the base needs to be level with no fall
-Lay a foundation bricks 65mm thick (or path edging 50mm thick) along the perimeter of the slab with dpc on top and build the cabin on to that to keep the cabin off the concrete.
-Lay a floor on top - does the cabin not come with its own floor?
-Put in a french drain alongside the base. Can't do any harm. Do you have sandy or clayey soil? If the latter avoid creating a moat - either a soakaway or connect to drainage system.

Note my list above always needed a pick and mix approach, e.g. the insulation is only any use if the slab is to be the shed floor.

[Collectingbrass]

2.43m high is 8 feet. You only need 2.1m / 6 1/2 feet for what you want so I would get a lower height cabin. It's not worth running the risk of pissing your neighbours off, if they take against you 10mm over height would be enough to force you taking it down.

[sak83]

TA14 said:

So your log cabin is closer to a normal timber shed than a garden room. Personally I'd just scrape of the topsoil where the existing slab is short and place some standard flags; then place (no mortar) quite a few class B bricks with a dpc on top and put the shed onto that. OK the shed will be about 2" above the permitted height but is that really going to be an issue for you/your neighbours?

Failing that:
Break and remove existing slab
-Excavate to 100mm
-lay a dpm.
-Pour a concrete base the same dimensions as the cabin so water drips off the side. The cabin manufacturer says the base needs to be level with no fall
-Lay a foundation bricks 65mm thick (or path edging 50mm thick) along the perimeter of the slab with dpc on top and build the cabin on to that to keep the cabin off the concrete.
-Lay a floor on top - does the cabin not come with its own floor?
-Put in a french drain alongside the base. Can't do any harm. Do you have sandy or clayey soil? If the latter avoid creating a moat - either a soakaway or connect to drainage system.

Note my list above always needed a pick and mix approach, e.g. the insulation is only any use if the slab is to be the shed floor.

thanks for the info, just the information i need.

I've decided to break the existing slab and lay a new one. I was apprehensive at first because i costed buying the ballast in 25kg bags(need to be carried through the house), cement and hiring a mixer. It seemed pricey for the effort and would be messy. But i spoke to a local concrete place and they can deliver 1.5 m3 of concrete for £177. They reckon 10-15 wheel barrow loads will get it to the back. Which will still have to go through the house but i can protect my floors in advance so its not too bad, i'll ensure its done on a dry day. This also means I have the flexibility of positioning and levelling the new slab where i want rather than being restricted by the old slab

The cabin doesn't come with a floor, the cabin is supported by the walls so the manufacturer leaves the floor up to the customer to work out according the the intended application.

My soil is clay and i hadn't considered that the french drain may not drain so thanks for raising that.

i think i have enough info now, so just need to plan the work. Im hoping to have the cabin up by the end of the month

[hyphen]

How long is the distance through the house to the site? As some concrete companies attach pipework to the mixer lorry which then extends to through the house, and so pump it straight out rather than wheel barrows.

Also I see you are in London, so once you have perfected your technique, can you come and do mine as well

[sak83]

its about 11m through the house and there is a straightish route through. Pumping is an option but its more worthwhile on larger quantities of concrete as there is a cost for arranging the pump. On a small amount like mine its not worth the cost

[C Lee Farquar]

Just as a heads up there is an anomaly with your concrete figures. 1.5 cubic metres of concrete weighs around 3.5 tonnes, so ten barrows would be over 1/3 tonne per barrow which is impossible. Going through the house you would want 50-60kg per barrow.

Strength wise, most timber building manufacturers recommend a 30 newton concrete (RC30 or P260).

Whilst your supplier may be cheaper than I am, I would be charging you around £280 inc. VAT. Combined with the 10 - 15 barrow comment I wonder if they are pricing for a lower quantity?

[sak83]

C Lee Farquar said:

Just as a heads up there is an anomaly with your concrete figures. 1.5 cubic metres of concrete weighs around 3.5 tonnes, so ten barrows would be over 1/3 tonne per barrow which is impossible. Going through the house you would want 50-60kg per barrow.

Strength wise, most timber building manufacturers recommend a 30 newton concrete (RC30 or P260).

Whilst your supplier may be cheaper than I am, I would be charging you around £280 inc. VAT. Combined with the 10 - 15 barrow comment I wonder if they are pricing for a lower quantity?

Thanks for bringing this to my attention. The price i got was from a guy i ran in to who worked for a local concrete place. He did a rough calculation while i spoke to him. So i will definitely go back and recheck pricing. I hadn't considered the weight of the concrete and you're right 10-15 barrows will most definitely not be enough. Which means i either have to arrange a few extra pair of hands or get a company that can help with the barrowing.

[Stig]

The most important consideration here (I have two Tuin cabins) is that the base is 100% level. You can get away with a fair bit of tolerance or a shed, but not a cabin.

One the plus side, whereas a shed base generally covers the entire footprint the only important bit for a cabin is the walls. As such, you can level up with slithers of timber (make sure they're tanalised) for the foundation beams.

If you're not level at ground level, you'll have a hell of a job piecing the beams together as they are machined to pretty tight tolerances. Out of whack and you'll have gaping holes at best and lots of problems down the road.

No issue with extending the base - but bear in mind the above.

Personally, I made a timber base for mine as they have 100mm of celotext insuation (and 50mm in the roof)