Concrete blocks as base? - Wary of laying a large concrete pad...

[IanMelb]

Hi there, I've had a quick flick through the forum and cannot see an obvious answer to my query.

Anyway, here's the outline:

(1) I'm planning on building a wooden frame workshop in section of land at the bottom of the garden
(2) The workshop will be roughly 4800x6000mm area (with pitched roof, 3400mm at apex, 2400mm at wall height)
(3) I have some underfloor heating pipe and polystyrene insulation left over from a previous extension build so I would like to incorporate that in to the project. The integrity of the UF heating is not that vital in the long run, it's just a use for the stuff that I have left over.

Here's the rough groundwork thinking:
(1) Dig out to about 300mm depth and put 100mm of dtp1 sub base
(2) Put a 100mm concrete base on this
(3) Put the underfloor heating stuff on this (use kingspan or similar where there is not enough pipe)
(4) Screed on top of piping
(5) Chipboard on top of screed (to protect it and provide a bit of weight distribution)

Here's my main query:

I am hesitant about pouring a 2.7 cubic metre concrete slab on to the hard core. I don't know how permanent I want the structure to be.

Could I use concrete blocks (440x215x100mm breeze blocks or similar) laid on their faces) with some kind of mortar (dry mix) as the bedding and pointing/grouting/

The cost of the blocks is on a par with that of the poured concrete slab and I figure that the weight distribution from above would be quite even (once it has worked its way down through the other layers)

(as an extra - where would the best layer for the DPC be?)

[lutonlagerlout]

are you stark raving mad??<joking>
blocks laid on hardcore are definately not suitable for a floor
blind the hardcore with 50mm or so of sand ,use 1200 gauge damp proof membrane ,shutter the sides and pour the concrete
if you grow tired of it just break the crete out,
what i would be more concerned about is planning
although you are under 4 m in height (good) best just to check with your local council that you can build a structure of 70m3 of flammable material at the bottom of your garden,i think the limit might be 30m2 of floor area for non flammable construction but its always best to check,as it hurts taking stuff down
cheers LLL

[IanMelb]

Thanks for the response (yes there is some lunacy involved)

The block idea came about when somebody at work suggested 'beam and block' but without the beam. No lay person could say why it wasn't a bad idea, especially with the weight distribution coming down through 25mm of flooring, 50-100mm of screed and 50-100mm of insulation.

We are still waiting on the Council to get back to us as to whether we need planning - it should have been 10 working days and it's been 20 :-(

I didn't mention that we won't be building off the floor - we'd planned on using concrete footings and a couple of courses of blockwork for the foundations...

[IanMelb]

With Tony's advice it looks like we'll go down the concrete pad route (assuming we can get a 6m3 lorry down our lane and back again).

Given the stuff that's going on top, does the pad need reinforcement?

I also note that the website suggests that we would need an expansion gap - is this vital when the pad is so far down below the floor level?

Thanks again

[lutonlagerlout]

that size pad i would nt worry about an expansion joint,reinforcement is always helpful but only an engineer can tell you if you definately need it
if you have tight access you can get 3 m3 smaller readymix lorries there are also many companies that knock it up for ya on site then you barrow it in (hard graft mind) normally about a oner a metre for barrowmix
hope this helps
LLL

[IanMelb]

Cheers mate

I'll measure up the lane access (it's a private road at the back of the property) and talk to the road owner about whether he's had anyone that size for his build

[IanMelb]

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