Concrete Base spec for Garden Office?

[Cliffoshay]

I've prepared / cleared the site for a 4m x 3.5m garden office. Just got a few questions, any help & advice would be very well received!

I was planning to lay a concrete base for durability. Will I be best off laying hardcore / shingle first? I see wickes sell 'Granular Sub Base MOT 1 Jumbo bag' for £38, I guess that would give me about 2 1\2", would that be deep enough? In regards to the concrete base, would 1:5 cement to ballast be a good mix? Would a similar depth be sufficient?

Thanks in advance.

 

[jdpjamesp]

Before you commit to concrete have a look at the Hawklok product. I've not used it myself yet, but I know folks who have and they say it's very good, and much cheaper than concrete. Perfectly adequate for a timber structure.

 

[Cliffoshay]

Thanks for the info. I've just had a look but unless I'm way out on my sums - which isn't outside the realms of possibility! - It looks like it would be a more expensive option. 3m x 2.4m is £230 and I'd need something more than half as big again.

The other issue is that my structure won't be square due to fitting into an irregular shaped corner in my garden. As a result, I don't think I have many options. I hope to sort the base for about £150 based on the depths I've noted in my OP. If they are sufficient?

 

[r896neo]

The top spec way to do it would be 100mm of MOT sub-base compacted and then a concrete slab.

If your pouring a solid slab then you could probably get away with the 2 1/2'' of hardcore which must be compacted with a vibrating plate then a 100mm slab. Any less than 100mm thick and the slab will probably crack.

So its up to you really, The top option is probably overkill but is very marginally extra cost and work wise.

 

[Inspire]

If you are going for a 100mm thick slab you are going to need about 1.5 cube to cover that area at that thickness, it would be alot easier getting it delivered ready mixed rather than mixing it yourseld

 

[r896neo]

Oh and your concrete ratio is fine, 6:1 would do even.

You will be surprised at the economics of mixing more than a cube of concrete by hand.

You would need about 1.4 cubic metres for a 4m x 3.5m x 100mm slab. Even if your very fit a solid 8-6 back-breaking day would probably not see it all mixed and put in. Then the material cost is also very high.

You'd need

2 ton gravel - 50 quid
1 ton sand - 25 quid
18 bags of cement 65 quid

Delivery 15 quid

Total - 155

Ring and get a price for 1.4 cube delivered it will probably be about the same maybe a fraction dearer depending on local prices.

 

[Cliffoshay]

Thanks for that, very useful. I was hoping to get away with not hiring a vibrating plate, perhaps that would be a false economy... shame.

Excuse my ignorance here... would it matter that much if the slab did crack? I will be installing a floating floor on top so any slight variation wont matter that much will it?

I'm basically on a very tight budget where every £ counts.

I was thinking that a concrete base would be preferable for my requirements because:
a) The office is a odd shape
b) It will last longer than treated timber on blocks etc

I did consider enquiring about delivery of ready mixed concrete but access is tricky. I'll make some calls though.

Thanks again for any tips!

 

[r896neo]

All you need is a barrow run, how were you proposing to move the 3 tons of stone and sand?

Concrete less than 100mm outdoors is prone to cracking, this will make it less stable if the two halfs settle at different rates, also once cracked freeze thaw will make it deteriorate much quicker in the medium - long term.

you could omit the hardcore if you throw a sheet of steel mesh in the concrete. Probably about 30 quid.

 

[Cliffoshay]

All you need is a barrow run, how were you proposing to move the 3 tons of stone and sand?

Concrete less than 100mm outdoors is prone to cracking, this will make it less stable if the two halfs settle at different rates, also once cracked freeze thaw will make it deteriorate much quicker in the medium - long term.

you could omit the hardcore if you throw a sheet of steel mesh in the concrete. Probably about 30 quid.

That's interesting and helpful, thanks.

100mm concrete it is then. I like the idea of using steel mesh in lieu of MOT T1 base which will save time, money and hassle. Will that definitely be suitable? Presumably it doesn't need to be in one single section?

 

[Cliffoshay]

Okay, I've done my calculations now and the slab size eqates to 1.1 cubic meters. I think due to access and pricing local ready-mix I'm going to have to do it all myself in a mixer.

What's the best way to work out the sand & cement quantities based on this?I'm happy with a 1:5 mix.

 

[r896neo]

Good luck! the calculators below are all pretty good as is the rest of the paving expert site

http://www.pavingexpert.com/calcall.htm#focus10

5:1 is about a C20 mix

 

[ABCwarrior]

I used an online calculator (http://www.clean-water-for-laymen.com/making-concrete.html) for an estimated 1.5 cubed at a 1:2:3 mix and I needed roughly 23 bags of cement, and 2 tonnes of sharp sand and 2 tonnes of 20mm chippings- total cost about £160(aggregate from the quarry not jumbo bag ripoffs)....The local readymix(Breedon) quoted £348 inc vat....big difference

 

[r896neo]

flip me, i can get 1.5 cube of readymix for about 170. Cement must be cheap in northern ireland.

 

[Cliffoshay]

Well, I managed to get a quote for £110 +vat per m3. As i need 1.1 it'll be £122 +vat So I'll be opting for that!

Loads cheaper than the others I tried.

 

[r896neo]

Good on you for perservering your body will thanks you for it. Mixing a cube by hand is not much fun and readymix is much better concrete than hand done.