Detached Garden Outbuilding, Specification help with base.

[wheelerssjk]

Hi all, Just looking for some advice / suggestions. I want to build a garden outbuilding for use as a games room / gym / tv room, for the children!!. Have spent quite a while looking at kit form garden offices and log cabins but would like to self build. I am going to keep at least 1m from boundary's, under 30m2 floor, less than 2.5m high, less than 50% of total garden etc..
So the building I am wanting to build would be 7.9m x 3.9m x 2.5m high.
I am intending to build a timber frame with flat roof with well insulated floor, walls and roof. At this stage I am concentrating on the base, obviously I want to have as much headroom as possible inside so was looking to build the timber frame onto an insulated concrete slab, would this spec work? from the top down, 150mm concrete slab with A142 mesh 40mm from top, 50mm thick jab floor insulation, 1200 gauge dpm, 30mm blinding sand compacted, 100mm crushed concrete compacted. I was hoping to make the top of the concrete slab flat enough to tile on to, and the top of the slab would be approx 30-50mm above ground level. I realise this would be a lot more digging out than laying a concrete slab and putting bearers on top with insulation between, but am looking for maximum headroom.
Thanks in advance. P.S. once I get the base sorted I will be back with more questions on the wall and roof construction!

 

[wavetrain]

Interesting ideas.

You should have the rebar mesh top and bottom of the 150mm slab.

But as for the rest of your ideas, since the top of the slab is only just above ground level, the DPM will not keep the slab dry since the slab will be wet from the ground. I know DPMs are often placed under concrete to prevent the wet concrete drying out too much into the hardcore, but you are putting it there becasue you think it will keep the slab dry aren't you?

And because of the above, what is the point of the insulation since it cannot do any good with the slab being damp.

 

[mointainwalker]

what is the point of the insulation since it cannot do any good with the slab being damp.

Can you explain that please ?

 

[RedHerring2]

I'd personally go for 100mm insulation but reduce the concrete to 100mm.
I'd consider increasing hardore to 150mm. Sand blinding to 50mm.

But as Wavetrain says, all a bit pointless with the dpm edges open to ground. You'd need a dwarf wall that will allow the edges of dpm to turn up at edges. Ideally this would be 150mm above external ground level.

You could increase headroom by using a pitched roof and raised-tie trusses. The 2.5 m limit refers to the eaves. I think it's 3m to ridge.

For finishing off the concrete use a long handled float like this:
http://www.hss.com/g/44119/Easy-Float-Trowel.html

You might still need to use a floor leveller compound prior to tiling.

 

[wheelerssjk]

Hi all, Thanks for taking the time to reply to my post, and spotting a very obvious mistake on my part!
My plan is to have my timber framework finish flush with the edge of the concrete slab and then clad vertically with siberian larch that would sit just outside the slab.
In hindsight would it be feasible to bring the dpm up the side of my concrete slab and then on to the top of the slab and fix down my timber frame on to it?
Also I was thinking of putting 25mm thick jab floor insulation on its side around the perimeter edge of the slab/insulation up to ground level?
Would doing both of these things make the slab both waterproof and properly insulated?
RedHerring2, would you suggest a particular brand of 100mm insulation?
Would you also agree with Wavetrain and put the Rebar mesh top and bottom of the slab?
and would therefore the slab have to stay at 150mm deep?

 

[wavetrain]

I'd personally go for 100mm insulation but reduce the concrete to 100mm.

RedHerring, with concrete slabs, they should not be too thin relative to their area. I cannot at this moment recall where I last saw a table for this, but from memory even 150mm is pushing it for this size slab although if the ground is stable it may not crack over time if made from strong concrete - not the stuff you could mix yourself.

To the OP: the purpose of rebar mesh in top and bottom is to give tensile strength to the slab when subjected to ground heave and ground subsidence. Your proposal to have it in the top only, would only provide tensile strength under ground heave. So under ground subsidence conditions, the slab would be more likely to crack from the bottom up.

To everyone: I believe you can consider slabs to be like a beams so the strength needs to be at the top and bottom with just the buckling and shear forces to contend with in the middle. And as such the thicker the slab, the stronger it is to resist bending by virtue of the increased separation of the top from bottom, not from the added material in the centre..

Slabs can and are built using rebar and hollow pots in the middle to keep them lighter for upper floors, but has anyone known of a slab say 250mm+ thick being built using scrap bricks as the middle filler material (with some concrete surrounding them to bond them together) so save on the otherwise considerable concrete material cost?