What came first, the shed or the floor

[RicB]

Prolly been discussed before but here goes.

I have noticed that a lot of shed builds starts with the slab. Is there a reason for this or is just easier to work on concrete when building the shed.

Years ago I did some labouring for a guy doing shed erections. We always erected the shed prior to the slab being poured. The main uprights were concreted in post holes and this certainly seems a stronger way to anchor the shed.

Is dyna bolting into newish concrete really that strong?

[Dave50]

Yep I've noticed a few going up that way as well, the one's I put up always had the shed first then the slab, the rails give us somewhere to put the tinny's while waiting for the slab to go off!

[chambezio]

I much prefer to set the posts onto piers with descent holding down bolts cast into the pier tops then erect the shed, then pour the slab up to the external cladding. Doing it this way you alleviate the future hassle with sealing the walls to the floor. It does make pouring the floor take a little longer but its nice to finish a slab in the shade.
My take on dyna bolts as holding down bolts, ask yourself....what happens in a BIG wind
(This is only my preference)

[Manuka Jock]

We have been doing the slab first for yonks over this side of the ditch .
Long before dynabolts , we cast cranked bolts , rods , or even no.8 wire into the concrete .
Doing the pour is far easier out in the open , a walled in box is no fun in comparison :)

[Harry72]

One advantage to shed first is you can control the curing better... but in hot weather its a bastard to do physically(sweat yer globes off)

[Barry_White]

Coming from the Shed Industry we always supplied "U" bolts to be cast into piers or the slab but also supplied a hold down bolt layout plan. The major problem is that most owner builders and even some shed erectors can't read plans or use a tape measure properly and get the hold down bolts in the wrong position which is critical.

The dynabolts are usually supplied for cheaper type sheds and garages which are ok but not the preferred method. We always supplied better quality sheds.

We always supplied engineering for the piers but not the slab and if you build the shed on engineered piers the slab doesn't need to be engineered.

As to which way to do it first, by pouring the slab first you can form up the slab and form a recess in the edge for the wall sheets to sit in to vermin proof the shed.

[Woodwould]

Barry, can I ask, what does the shed industry recommend to prevent the ingress of water between walls and slab (in a slab first, shed second errection), is it a case of a box full of silicone and a gun?

Presumably if the shed is errected first, then the slab is poured within the walls?

[Enfield Guy]

When we do sheds, and by sheds I'm talking about structures that are 1000 sq M or bigger, the slab is poured and the wall cladding projects 30 mm beyond the top face of the slab. An angle flashing at the bottom edge of the cladding provides for vermin proofing. My own shed is constructed in this fashion. The only variation being that I used expanding foam on the inside to better control temperature and humidity.

I find that the shed first method can pose problems if and when an extension to the shed is required. It is much easier and cleaner to pour up to a flat edge of concrete rather than an edge that is shaped like corrogated iron or trimdeck!!

Cheers

[Woodwould]

When we do sheds, and by sheds I'm talking about structures that are 1000 sq M or bigger, the slab is poured and the wall cladding projects 30 mm beyond the top face of the slab.
That makes the most sense, but would require accurate formwork for the slab, which I suppose isn't that difficult if someone brings a tape measure.

[artme]

As you can see there are advantages to both methods.

I did my shed slab first and very carefully set the anchorage bolts into the slab. The formwork ws dead square - 2mm out on the diagonals!
I was lucky wit the curing, perfect weather and a bit of care on my part by watering with the hose at regular intervals, Not even a hairline crack.

The house I am working on at the moment Was brought in and put on stumps - actually 75x75 steel posts - slab for bottom floor was then poured. It has cured beautifully, 3 weeks with not a sign of even a hairline crack. The frames are anchored with chemweld. Much better than Dynabolts when it comes to being near the edge.

[Barry_White]

Barry, can I ask, what does the shed industry recommend to prevent the ingress of water between walls and slab (in a slab first, shed second errection), is it a case of a box full of silicone and a gun?

Presumably if the shed is errected first, then the slab is poured within the walls?

Hi WW

When ever we did a slab we allowed to form up a 50mm x 50mm recess around the edge (except across doorways) of the slab. This was done by fixing a 50mm x 50mm timber to the top inside edge of the form-work. By making the wall sheets 50mm longer than the height of the shed the sheeting would sit down in the recess thus preventing water getting under the wall sheeting as well as vermin proofing the shed.

By pouring the slab after the shed is erected and allowing the concrete to to be formed by the wall sheeting and then if you you were to hose out the shed water would lay between the wall sheeting and the concrete and eventually create corrosion to the wall sheeting.

Attached is a sketch of what I mean.

[Woodwould]

Hi WW

When ever we did a slab we allowed to form up a 50mm x 50mm recess around the edge (except across doorways) of the slab. This was done by fixing a 50mm x 50mm timber to the top inside edge of the form-work. By making the wall sheets 50mm longer than the height of the shed the sheeting would sit down in the recess thus preventing water getting under the wall sheeting as well as vermin proofing the shed.

By pouring the slab after the shed is erected and allowing the concrete to to be formed by the wall sheeting and then if you you were to hose out the shed water would lay between the wall sheeting and the concrete and eventually create corrosion to the wall sheeting.

Attached is a sketch of what I mean.
Thanks Barry, that makes perfect sense. I may be in the market for a shed in the not too distant future and it's always good to know the best practices for a bit of leverage.

[coffenup]

i was lucky my carport came before both the shed or the slab. As i needed the carport to work out the height of the shed walls
Regards Michael

[banjoping]

Standing the shed on footings and then pouring the slab used to be quite common, but is a dissapearing practice in the shed game.

It used to be done for a few reasons:
1) perception that the shed is "stronger"
2) perception that it is cheaper

It is not done much these days as the concrete causes the accelerated oxidisation and rusting of the metal in the concrete. Indeed, you void your BHP warranty when you do this.

Secondly, it is much easier and quicker to stand a shed on a finished, level slab.

Setting up square and level Footings and insitu brackets is tricky and if it is stuffed up at this point the whole thing can quickly become a disaster.

Stronger? How long is a piece of string? I personally have been in the shed game 10 years and we have never had a failure at ground level using dynabolts. I have seen columns sheared at the base plate when the column has been struck by a vehicle and dyna's still holding down the brackets.

2) May be true to an extent if you are doing your own concrete, as maybe you can cut some corners. But setting up footings professionally costs you labour, and pouring footings then a slab later builds 2 delivery charges into your concrete as opposed to one pour. Also, by the letter of the law, the naked footings you pour to stand the shed up should be suffucient to hold down the shed without the slab on top. Hence, they are bigger then they actually need to be after the slab is poured on top, and hence increase cost.

Barry has already touched on ways to waterproof the slab, there are many methods out there including some simple shed products as well (google Campbell Shed Products).

[bennylaird]

I made the mistake of having a much larger slab poured before we built the shed. Unfortunately the run off hasn't worked as planned and I have large pools of water forming on the slab, (shed is nearly complete, just needs flashing and guttering). One of these pools is around the side door which has no bottom seal. I already have rust forming at the bottom of the cladding.

The C channel is very close to the floor so not much room to do much. I had thought of filling up to the C channel with cement (which type?) on the inside and silicone sealant around the outside. For the doorway I may have to install a drain grid etc. Inside I could try some floor levelling cement?

Any ideas would be appreciated as I'd like to get it sealed before I move everything in. :U

[banjoping]

I made the mistake of having a much larger slab poured before we built the shed. Unfortunately the run off hasn't worked as planned and I have large pools of water forming on the slab, (shed is nearly complete, just needs flashing and guttering). One of these pools is around the side door which has no bottom seal. I already have rust forming at the bottom of the cladding.

The C channel is very close to the floor so not much room to do much. I had thought of filling up to the C channel with cement (which type?) on the inside and silicone sealant around the outside. For the doorway I may have to install a drain grid etc. Inside I could try some floor levelling cement?

Any ideas would be appreciated as I'd like to get it sealed before I move everything in. :U

I'm no chemist benny, but filling up a channel with cement that is close to where the water holds to me sets you up for early, and accelerated oxidisation and rusting.

It is such a shame that BHP do not have their once excellent BHP Technical Services Hotline. You used to be able to ring them up and get some good advice.

[BobL]

My shed has the slab poured after construction up to the bottom of the ground level galvanised girt. The cladding on the outside then over hangs the top of the slab by about 100 mm. This of course leaves gaps between the girt and the cladding ridges but the reason I did this is I always reckon it's easier to breeze proof these gap than than water proof a ground level edge. I then got some expanding foam filler and filled the gaps between the cladding and the slab.

[bennylaird]

Hmmm I may just have to go with silicone around the exterior and let the inside breath so as not to retain moisture. Looks like I will need to cut some drainage slots into the slab outside to drain the pooling water away from the walls.

[bennylaird]

I have waterproofed the shed by removing the cladding and cutting a 30mm channel into the slab. I used sheet galv dropped into this channel and rivited it to the C channel. All joins are sealed with non corrosive silicon and then the cladding reinstalled outside of the galv. Everything is now sealed however the water still pools outside the shed on the slab. I have also used Clark rubbers garage door seals under the roller doors. I still have a major leak with the swinging door but will use a door seal there to stop the rain running down the door and inside.

I am tossing up how to cut channels in the slab to drain the places where the water pools. I don't want to put in the big plastic drains as that would be hard work cutting the slab. I am thinking of just cutting small channels in the slab and maybe putting grates over the concrete channel?? Any ideas?

[Handyjack]

Could you grind the concrete in the high spots to create some run off? Water will always go to the lowest point.
Good luck.