Concrete slab garden room....help?

[Stanley36]

Hello all,happy new year

 

I'm in process of building a 4mx3.5m garden room to house a few aquariums. Im wanting to do a concrete slab that finishes just above the surrounding land and have the concrete as the floor inside the room this will help with the 2.5m height ive got to work with. 

 

  I've dug the area deep enough to allow 150mm of compacted type 1 mot......25mm sand 100mm insulation and 150mm of concrete. However ive joined a group on Facebook for home builds/offices etc and had a few people say that a slab built in this way won't be strong enough. The walls will be 2x4 then osb and cedar cladding and roof 2x6 with osb then rubber so not exactly light but far from weight of brick building. Ive been advised to do trench foundation and blocks to height of floor then concrete in the middle, is that really necessary for wooden building? 

 

 Id also like to insulate underneath the slab, ive seen videos of people lining the bottom and sides of the form with insulation boards and then pouring the concrete onto this. Would this be good way to do it? As you can probably tell im no builder ?(car trade) and im quite confused as the best way to do it. If someone could help me out or point me in the right direction it would be much appreciated.

 

Thanks ? 

 

 

 

[PeterW]

No idea who’s told you a 150mm slab is too thin but they don’t know what they are talking about ..!! There are houses on here built on less ..!

 

If you want an insulated slab then you need to work out how to finish the sides of the insulation - with a 50mm insulation upstand you could put the walls over this and run the cladding down lower but you will need to find a way of finishing below ground (cement board may work) as timber will rot. 
 

Have you sketched any plans ..? And what’s the insulation going in the walls ..??

[daiking]

There’s a few of us interested in this at the moment. I need to build a base for a log cabin in the next few weeks

[TonyT]

I would have no problem with 150mm concrete, done a few buildings, built on sand, with 150mm concrete slab , timber walls and slate roof so more than happy to say it  would be fine. We had reinforcing mesh in the concrete slab, with a c35 mix

[Stanley36]

Thanks Tony

 

Please excuse the poor drawings but these were 2 of my ideas. I was told that building walls on the edge of the slab would cause the edges to crack.

 

open to any criticism or advice ?

[Stanley36]

Insulation in the walls will be celotex or similar 

[Stanley36]

Also I was planning on 2 course of engineering bricks around the bottom edge to raise the timber wall away from the ground a little 

 

[Temp]

9 minutes ago, Stanley36 said:

Also I was planning on 2 course of engineering bricks around the bottom edge to raise the timber wall away from the ground a little 

 

 

+1 and put some DPC on top. 

 

If its a windy location you can put some bolts (head down) between the bricks or in the slab to bolt the frame down to later. Just be careful not to fall on the bolts during construction.

 

My outbuilding is clad with boards on vertical battens. I ran the battens down the outside of the bricks stopping 1" above the slab so you can't really see the bricks after cladding. 

 

[TonyT]

A252 mesh was the reinforcing and will help the slab.

don’t see a problem with either drawing for structural but insulation in a slab may affect and I’ve not done insulated slabs.

[daiking]

23 minutes ago, Stanley36 said:

Also I was planning on 2 course of engineering bricks around the bottom edge to raise the timber wall away from the ground a little 

 

The suggestion made to me was to lay 3 courses of engineering bricks on a small trench footing (300mm wide by 150mm deep) as a perimeter. Then insulation and cast a concrete slab inside.

 

I am trying to avoid this as I have never laid bricks/poured concrete etc.

[Temp]

Laying a few courses of engineering bricks isnt too difficult. Just set up some strings on posts as guides for the top edge of the brick. I'd never done it before I did mine.

 

 

 

[Stanley36]

1 hour ago, daiking said:

The suggestion made to me was to lay 3 courses of engineering bricks on a small trench footing (300mm wide by 150mm deep) as a perimeter. Then insulation and cast a concrete slab inside.

 

I am trying to avoid this as I have never laid bricks/poured concrete etc.

I've been told the same but was hoping to do it as a slab in one pour as it will be quicker and easier for me to do....ive never layed bricks either.

 

[Stanley36]

This is the way I've been advised to do it? Can I ask your  professional opinion on this is this overkill or the best way to do it 

[PeterW]

@Stanley36 that’s overkill. You could lose the inner bricks and just line with insulation and a DPM and fill with 100mm concrete. 

[Stanley36]

Thanks, would concrete blocks work instead of bricks? 

[Mr Punter]

If you cast the slab unreinforced onto insulation and build up to the edges it will probably break.  The insulation under slab designs here have quite complex insulation and concrete specs plus reinforcing.

 

I would go with 100-150mm concrete cast on the type 1, 2 or courses single skin brick perimeter or 7.3n blocks if you prefer.  112mm dpc.  Treated timber sole plate held down via straps.  Timber framed walls.  The bottom of the sole plate must be at least 150mm above finished ground level, 225mm preferred.

 

DPM in middle, lapped to DPC.  EPS floor insulation (or whatever is cheapest) in middle.  Polythene sheet, 22mm T & G chipboard flooring.  You could screed but probably overkill and not easy to DIY.

[daiking]

8 hours ago, Stanley36 said:

Thanks, would concrete blocks work instead of bricks? 

Previously I’ve been told yes. But they look bad.

 

it’s something I’m considering though

Edited January 4, 2021 by daiking