[Closed] Talk to me about how to make a concrete shed base please

you can use any wood for shuttering but stuff with minimum texture and a good slathering of mould release liquid is a good idea for a nicer finish.

If the hardcore level is clear of the surrounding ground you might consider blinding with sand and putting a damp proof membrane down to prevent moisture coming up through the concrete but if you're not bothered (or the h/c is below ground level) it will save you hassle and money without it.

The rubble is below the ground level and when the garden is finished the ground will come upto the edge of the base. Would you say a damp membrane was non essential as I'm trying to keep things simple?

Also what does "blinding with sand" mean, is that putting sand around the edge of the rubble/concrete base?

The wood needs to be level or the concrete will not form the shape you are after!

Get a spirit level on it!

The wood needs to be level or the concrete will not form the shape you are after!

pffft. Make a wet mix and let god be your spirit level 😉

DPM isnt essential on a shed base IMO but it might be considered good practice - you cant very well add one later 😉

blinding is imply putting a few inches of builders sand on the hardcore to get rid of the edges and voids that might puncture the DPM and give the concrete/DPM sandwich a smoother surface to sit on.

If the DPM is below ground level though it's not going to do a lot as moiisture can simply get into the concrete slab from the sides or through the blinding layer.

I've already been out and brought one in a rare moment of forethought lol

With the size of the base is it best to divide it into say three sections and concrete each one seperately or would you just do the whole thing at once?

The reason I ask is I'll be mixing the concrete by hand as we have no way of getting a mixer out the back.

So in this case its probably not worth putting a DPM down. Would there still be any point in putting a lyer of sand over the rubble in that case?

For a shed base any wood will do. As Stoner says smoother wood will result in a better finis but if it will be below ground that isn't important.

What you do want to make sure of is that the base is square (measure the diagnals as weel as sides) and level (form the wooden box to the level you want - not higher - and get a spirit level to check everything is level, then simply level off the concrete with a plank).

to save hassle Id get one of those "Trumix"* co's in. You can barrow it through from the front and you only buy what you need (no balance loads lying around being messy). But its quite a bit more expensive (IIRC about £80-90/cubic metre compared to £60-70 for just a mixer load)

* ask for recommendations in your LBuilderShop

Much faster.

dont bother with blinding if no DPM

Ummm it's nice to have a good tidy box to start with.

then simply level off the concrete with a plank.

easiest if it's >6' obviously - then you and a mate can tamp it back along the box top from one end to the other. For added poshness you can use a trowel to smooth a permiter band all the way round.

The idea of not having to mix it myself is nice but the only way I could get it out the back would be in buckets.

I had thought about doing it in sections to make the tamping down easier as two of the walls will be close to two existing walls in the garden.

Also should I make it smaller as previously advised (not here) or just go with the size of then shed?

Also should I make it smaller as previously advised (not here) or just go with the size of then shed?

I think theyre referring to rain water/shed wall run off landing on the slab and pooling under the shed - probably rotting the floor joists. If you are putting a kit shed with a wooden floor on the base then Id aim to overhang the base so that water can run down the shed walls and soak away through the ground/gravel around the base.

If you're building on top of the base with a "custom" made shed with no floor then Id think about how you get run off away from the wall bottoms.

Stoner suggested making the mix very wet so that it'll flow and find its own level. Nice idea, but too much water will make any concrete mix lose much of its hardened strength and you'll also find it tricky to get a good and impermeable (durable) top surface on the slab because all of the excess water will segregate to the surface. Much better to use a proper plasticising admixture if you can get hold of it - something like [url= http://www.buildingchemicalsuk.co.uk/FEBflow_Super_5L/p227683_661136.aspx ]this[/url] would do the trick nicely. You don't need much - 1% by weight of cement in the mix is typical.

Most ready mix concrete suppliers would include a plasticiser like this in their mix recipe if you told them that you were using the concrete to pour a slab.

Don't bother with the blinding. Having the base slightly smaller isn't a bad idea,
It'll help keep the water away from under the shed.
Something i would think about is keeping the wood of the shed
that is in contact with the base dry. Something like 4" Dpc

Whats a DPC?

DPC is the roll of black damp proof course (thick rubbery plastic sheet, 4" wide to you). Not expensive, you roll it ou over the finished slab and sit the floor joists on it. Prevents the joists absorbing moisture/sitting in water and so rotting.

Stoner suggested making the mix very wet so that it'll flow and find its own level

It doesnt need to be that wet to level with a bit of help with a rake and a board. But tamping along a shutter is the most reliable method.

Before putting concrete in think what security you want to put in the shed. The quickest and easiest solution for security that I could think of was a D Lock sunk in to the concrete so that the ends of the U shape would come up about 2" inside the shed floor to which chains/cables can be connected to.

Planning to handmix it? Have you ever tried mixing by hand? Even a couple of barrow loads is a good workout.

Can you really not get one of these through the house?

[img] ?8[/img]

I'd rather spend two hours getting this into the garden than handmixing. A quick online calculator reckons you'd need close to 1.9 tons of concrete. Edit...based that on 6" but you say you've half filled the hole with hardcore, so call it 1 ton.

You could also consider pumping it...there is a one man band sort of outfit near me that has a van and a concrete pump on a trailer...sling the hose over the fence and just be ready with the rake as the concrete pours in 🙂

mixers like that can be taken apart and though doorways easily.

if you're in Worcs, you can borrow mine.

EDIT - how are you getting the ballast through to the garden? by bucket?

I pity you 😉

[i]"The Youth gets together the materials to build a bridge to the moon, or perchance a palace or temple on earth, but alas, at length, the middle aged man concludes to build a wood shed with the materials."[/i]

- Henry David Thoreau

[i]"If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations* under them. "[/i]

- Henry David Thoreau

* or concrete slabs.

😀

We had the bathroom roof replaced last year and had about 200 concrete tiles leftover from that. I've used them for the rubble along with another concrete slab that was in the garden that I had to break up for Mrs Stayhighs garden plans.

Our back garden is walled on 2 sides and fenced on another, behind our house is train line on a bridge so no rear access or any way of getting it out the back.

I guess I could maybe hire a mixer and then bucket it through the house?

How much concrete I'll need was going to be one of my next questions after hunting through my trusty Collins Complete DIY 😉

PS nice quotes 🙂

let god be your spirit level

That is now my maxim for life. Top work.

If you have to get everything through the house, how the hell are you going to get the shed through the house???

We had the bathroom roof replaced last year and had about 200 concrete tiles ...

that's your hardcore.

Ballast is the pebble/sand mix that goes into your concrete mix with cement and water.

100mm x 3050mm x 1850mm = 0.56m3

Which is 8x 25Kg cement bags and a tonne of ballast.
http://www.source4me.co.uk/calculate_concrete_mix.php

I guess I could maybe hire a mixer and then bucket it through the house?

Given that the easiest thing to get to your back garden is water (assumin gyou have a hose) it would probably be marginally easier to carry the dry materials through the house along with a mixer and mixing it in the back - that way you save carrying the weight of the water, and aren't carrying buckets of wet concrete through the house (in my case that would be a recipe for certain disaster)...

Might be worth seeing if you can track down a wheelbarrow (or even a sack trolley if you're using bagged materials) that fits through a doorway to make the whole thing easier.

=0.56m3

= about 50 buckets

Hmmm, thats a lot of buckets 😯

I'll have to do some measuring of doorframes and go and see what local hire places have to offer. Failing that theres gonna be a whole lotta humping going on 😉

let god be your spirit level

Don't be daft. I've seem drawings and models, he couldn't even get a simple wooden cross level.

Would this be ok for the ballast, it says minimun content 850kg.

[img] [/img]

Is ballast all much of a muchness?

Thanks for your patience with all my questions 🙂

BTW I measured my door frames etc and could get something no more then 70 cm through.

Why don't you just lay some concrete paving slabs on some sharp sand on top of you hardcore. Got to be easier than buckets of ballast.

You dont even have to cover the whole area, just sufficiently support the floor joists.

I must admit I hadnt even thought of paving slabs. Would it be a case of fill frame work with sand and then lay slabs on top? The when the garden was finished off the ground would come upto the framework and hold everything in place?

+1 for flagging it. Not as perminent as concrete, should you ever decide to move the shed it'll be real easy to put the area back to garden.

Even easier, gravel it. Shutter it up with tanalised timbers fill it in with gravel, whacker it down firm, staple DPC to the bottom of some 3x2 tanalised and sit the shed base on the tanalised timbers.

2nd the slab option (especially if you cant get a mixer in the garden. I did a 4' x 6' base and there is no way i'd consider mixing by hand!

On the face of it that does seem like a much more straight forward option and much less faff then concreting.

How much sharp sand would I need, would that still be around a tonne?

nah - you only want an <inch under your slabs for bedding. Whacker your hardcore down first and then whacker the sand down too.

so @ 25mm = 0.14 m3 @ 1.6t/m3 = 220Kg or 10 bags.

PSA sharp sand at B&Q 20kg bag £1.34, I just got some for a job yesterday

Excellent stuff, its starting to look a lot more feasible now and will hopefully be able to make a start on it over the weekend.

Just a quick thought on paving slabs, do they need to be a particular thickness/load bearing or will any paving slab do?

My next shed will be on stilts.

I'll be pinching a bit of woodland at the back of the garden and building a shed approx 2.5mtrs x 4mtrs. The ground at the back slopes away, so rather than build it up, I'm going to dig 6 holes - one at each corner and one in the middle of each of the longest edges. I'll set some 4x4, or 6x6 timbers into these holes of varying lengths (with tops all level) and concrete in. Onto these I'll build a frame & floor and go upwards from there.

This will have 2 advantages for me - 1) not having to build the ground up and 2) I can keep the corner on the highest point of the ground, just above ground level, so the whole frame will be floating, thus helping keep damp away.

[img] [/img]
I prefer to use class 1 scalping's than sand it binds better and is less lightly to shift also once its bound by the concrete its very very strong (Its the stuff they use under roads) meaning you can use less concrete! See the pic (child labour is optional :wink:)
Note i used wood shuttering at the rear against the (crooked) fence as that's OK to leave or cut out at a later date, I put down about 4-5inches of concrete on this base,and a tip is see the wooden stakes in the scalping's they are level with the sides of the shuttering they act as a guide to help you judge by eye and with a level if you have any low spots in the middle when leveling the scalping and pouring in the concrete! If you do use scalping's just make sure they are well whacked down and if using shuttering like I've used at the front here remove it when the concrete is still green as it will separate easier and less lightly to chip the base edges ! Good luck dont panic and dont rush 🙂

oooohh we're doing pics are we? 😉

mine was made froma balance load from the house build so couldnt fill to level and tamp, had to let it fill till the mixer was empty. It's a little deeper than I was expecting, but at least it didnt break the shuttering 🙂
[img] [/img]
[img] [/img]
[img] [/img]

A pump for a shed base??? 😯
Oh and looking at that "shuttering" stoner you were lucky she didn't pop!!!

it was touch and go.

"oooh, shouldnt be more than a cube and a half left in there....."

about 3 and a half cubes later its at my welly tops and Im wonderin' how to clean it all up if it blows 🙂

Yeah looking at the shutters I was guessing at a good 300mm in there, can't beat a massively over engineered shed base tho, that ain't ever going to let you down. Oh and please don't tell me that there's two layers of a393 mesh in there 😆

I sank 5 of those concrete fence posts into the ground, smooth side up. Sat them onto small piles of concrete so I could get them all level. Lets me slide the shed out if needed, to get at the fence behind. It was also remarkably easy to do. Certainly quicker than putting up shuttering, mixing, pouring, tamping & all that. Having done plenty of concreting over the years, this approach was a doddle, but does lack the STW Extreme Engineering approach. 🙂

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