Can I build a shed on this (partial) concrete slab

[Arron]

Hi. I'm in the early stages of planning a new garden shed. Heres a few details:
1. build it myself, so I can make something that compliments our house aesthetically.
2. we are in Nth West Sydney
3. probably about 10 sq metres, maybe a little more
4. very keen that it be an exempt development (avoiding need for council approval)
5. not going to be used for woodwork - or at least not using woodworking machinery, perhaps some handtool stuff
6. mainly to be used for storage, perhaps wife's occasional sewing, and perhaps some other hobby stuff by me
7. nothing heavy, robust or dirty to be stored inside
8. will need 240 volt power.
9. nice well drained, undisturbed, gently sloping site, with shade trees.

Now one option for siting it is in the corner of the yard that is currently occupied by a little old leaky colourbond shed on a concrete slab. I'm thinking I could remove the shed and retain the slab and use it as partial foundations for the new shed - and I'm keen not to have to break up the slab and carry it out to be disposed of (long way to kerbside and only walking access to back yard). The problem is the old shed is smaller then I want the new shed to be, so the slab will only partially cover the floor area of the new shed. So my question is how do I build a shed on a partial slab?

Currently I'm thinking of building a treated pine subfloor, sitting directly on the slab where possible and supported on concrete block piers where it overlaps the slab. Advisable ???

cheers
Arron

attached is s photo of the site, and a diagram with the floor areas of the old shed in blue, and the new shed in red. The new shed cant be lined up with the slab on the longest side because the old shed was built 700mm from the rear boundary not the required 900mm.
oldshed.jpgshedsizes.jpg

[Damienol]

I would suggesting extending the current slab by laying more cement or build a slightly raised timber floor above the existing slab and not relying on it at all

[BobL]

I'm keen not to have to break up the slab and carry it out to be disposed of (long way to kerbside and only walking access to back yard).

Chicken ! I broke up a 5.3 x 3.6 x 100 mm slab and wheelbarrowed it 60 m up a slope to get rid of mine.

Your options are
1 extend the slab - >50% chance that it will crack and move at the join.
2 Lay slabs or paving alongside the slab - same problem as 1 i.e. ridges and dips.
3 lay a full slab over the top of the old slab.
4 lay a wooded floor over 1 - expensive

If it will be primarily a garden shed maybe options 1 and 2 ore still OK.

[Ed..]

Hi, if it was my yard I would be inclined to put a cheap timber frame around it and add probably 0.4-0.5 cubic meters of concrete which should do it. Key in the old edges to grab the new concrete and some shallow 200mm holes to stop movement and you should be good to go! To key the edges just drill some holes on overlapping angles and put in some cut rebar. Paint the edge of the old concrete with bondcrete (ie: PVA adhesive) when pouring which will also help to grip to the older concrete. I would also slightly oversize the pad so that you have at least 50mm extra than you need, it helps to hold in the bolts better without the risk of them cracking the edge under tension. If you are accurate you can set them in whilst the concrete is pliable. Put some builders plastic under the new area to help stop water rising through the concrete.

You could probably get a small mini mix truck to deliver it and would probably be about 6 large wheel barrows full, beats mixing by hand though! The driver should be able to pour into the wheel barrow till it was full and then take it out to the back to just dump it and repeat. Shouldn't take too long to do. I would also set up a small pad frame for a doorstep as you may have some excess, better to over order slightly than be a tad to short and if you don't have enough excess then no biggie! Once it is dumped than you have about an hour or so to work it in to level it. Piece of cake to do.

I don't think that you will save any money or time by trying to raise the floor level with timber and then put some sort of floor over it. Shouldn't take more than a couple of hours all up and then you don't have to worry about it any more.

Cheers

Ed.

[Arron]

I would suggesting extending the current slab by laying more cement or build a slightly raised timber floor above the existing slab and not relying on it at all

Ok, I'm more oriented to the second option then the first. Can you tell me why you would suspend the timber subfloor above the slab rather then having it sitting on the slab where it overlaps.

Another question. Which would cost more for a 10 sq mtr shed
1. a concrete slab (poured by professionals, but me levelling and prepping the area) or
2. a timber floor (treated pine bearers, yellow tongue flooring, sitting on concrete blocks) made by me and hence materials only?

Thanks
Arron

[Arron]

Thanks Ed. I understand everything you have written except the bit about ' shallow 200 mm holes to stop movement'. Whats that about?

Also, what do you think is the likelihood of the slab/extension cracking along the join if the rebar keys are used?

Cheers
Arron

[Ed..]

Thanks Ed. I understand everything you have written except the bit about ' shallow 200 mm holes to stop movement'. Whats that about?

Also, what do you think is the likelihood of the slab/extension cracking along the join if the rebar keys are used?

Cheers
Arron

Hi Arron, once the concrete sets the rebar locks the two slabs together, so it doesn't permit a large crack as such. However having said that once concrete is cured and another pour is added to it, there will always be a join line between the two, I don't think that you can solve it but it will be minimal. You can put sealers in between the two such as the PVA I mentioned before but the join line will always be there and with temperature changes and a little bit of ground movement it may open a little or close a little. But we are only talking about a minimal movement normally.

Let face it, large building floors are always built in sections to allow for temperature expansion, if there isn't any gaps to take up temperature expansion and contraction the forces will just crack the concrete anyway. To put it into perspective, about 8 years ago I build a large water feature (10 cubic meters of concrete) with water fall, a 2.2m wall at the back and two water holding ponds in it (7400L of water), there was no way I could build it in a single concrete pour, so built it in sections over a period of a month adding sections to it daily or every couple of days, rendered it with a mortar mix with cement sand and Bondcrete (PVA), and it held together quite nicely for about 4 years till we had the drought, I drained it as it was using two much water and with the extremely dry conditions and hot temperatures the ground started cracking which also forced a couple of thin minor cracks to appear along the join lines, about 0.5-1.0mm but enough to let water seep out slowly. I could have fixed them but decided against it as running the water feature was costing too much as electricity prices were rising dramatically.

So if you put the rebar in to join the two slabs you should also end up with only a minimal line. Get decent rebars such as 12mm or so, not the same as the mesh you will use.
As to the hole I mentioned just dig a small square or circular hole about 150mm deep and about 150-200mm wide about every 1m in the new area, just need a few. This small column at the bottom of the slab sort of anchors the slab from being moved otherwise the slab will sort of slightly slide on the top of the ground and plastic, sort of acts like footings, you can do that at the corners as well, helps with sandy soils.

As for the cost when I did my last pour of 10 square meter it cost me about $250 including the reo, I ordered 1.3 cubic meters but that was about 10 years ago, however since you only need about .5 cubic meter I would imagine about the same now but you would need to check with a mini mix company. I would not use a professional for such a small pour, they charge like wounded bulls. When I did mine I got a couple of quotes for about $300, so we did it ourselves, from the time of the pour to finishing took my wife and I about 40 minutes and that was for a full 10 square meters. This did not include the prep time though. For large pours certainly use them, but not for something your sie size. For my large shed (72 square meters) I got a professional and his off sider plus one of my friends and my self, took about 5-6 hours, definetly worth it.

Hope this helps.

Cheers

Ed.

[Arron]

WEll, thinking about this, all the logic points to just extending the slab, getting it over and done with cheap and quick. I can put up with a bit of movement around the joint - no big deal really and likely hidden below a cheap laminate floating floor anyway. The problem is, because of pre-existing injuries to my hands I cant really participate in barrowing concrete about - and I'm not comfortable asking other people to help if I'm going to be just sitting there watching them work. The core of the problem is that a truck turns up with your concrete and you have only a short while to get it up the 60 metres and 6 steps and 2 ramps to the rear of the yard. If I can do things like this incrementally, slowly, and stop when it starts to hurt then I'm OK, but not when it has to be all done at once. So unless I'm missing something I'd have to leave it to a professional concreting outfit which kind of destroys the DIY ambience and will likely end up expensive cos its a small job that no-one will be rushing for.

So i guess I'm swinging back to the other alternative of a timber subfloor. I can work with timber because I can do it at my own speed, bit by bit. Any thoughts on best way to incorporate a partial slab and a timber subfloor?

Thanks everyone for your responses.
Arron

[Ed..]

No problem, I can see where you are coming from, we all have to work within our capabilities. As for an alternative I can't offer you any as I have no experience in doing sub floors, but I am sure there are plenty of members here who can give you their advise. However it wouldn't hurt to get some quotes from some tradies, both for a concrete extension and also a wooden sub floor sort of thing. While they are there, pump them for info on how they are going to do it!

Cheers

Ed.

[Damienol]

Ok, I'm more oriented to the second option then the first. Can you tell me why you would suspend the timber subfloor above the slab rather then having it sitting on the slab where it overlaps.

Another question. Which would cost more for a 10 sq mtr shed
1. a concrete slab (poured by professionals, but me levelling and prepping the area) or
2. a timber floor (treated pine bearers, yellow tongue flooring, sitting on concrete blocks) made by me and hence materials only?

Thanks
Arron

The timber frame will last longer if it is not touching the ground. Use some copper logs for posts and then build your frame on top of this using some 90x45 framing (treated) timber.

This is pretty much how I built the kids cubby.

This method will allow you to build it as gradually as your hands allow


image.jpg

[BobL]

WEll, thinking about this, all the logic points to just extending the slab, getting it over and done with cheap and quick. I can put up with a bit of movement around the joint - no big deal really and likely hidden below a cheap laminate floating floor anyway. The problem is, because of pre-existing injuries to my hands I cant really participate in barrowing concrete about - and I'm not comfortable asking other people to help if I'm going to be just sitting there watching them work. The core of the problem is that a truck turns up with your concrete and you have only a short while to get it up the 60 metres and 6 steps and 2 ramps to the rear of the yard. If I can do things like this incrementally, slowly, and stop when it starts to hurt then I'm OK, but not when it has to be all done at once. So unless I'm missing something I'd have to leave it to a professional concreting outfit which kind of destroys the DIY ambience and will likely end up expensive cos its a small job that no-one will be rushing for. . . .

Sorry to hear about your hands, my current handicap is a weak left leg so there is no way I'd be borrowing at this stage either. I was supposed to be laying a new back law after our dogs demolished our previous one but that has had to go on hold.

Based on your situation it sounds like a wooden floor might be the go. In terms of the actual floor boards, what about a floor pack from a salvage yard? I don't know if they have these in your area but it can work out a bit cheaper that way.

One problem with a floor is building height. Seeing as you want to be at least 100 mm off the ground for the bearers and then you'd have something like floor joists you would end up being at least 300 mm at the top of the floor. This means a taller building otherwise you'll be ducking your head inside all the time - this probably means the council will want to know about it.