Garage conversion floor

[Ian H]

I’m making a garage into a room.

There’s a rough concrete floor in there at the minute and I need to come up 200mm.

Do I need to break out the old floor or could I level it with sand, DPM it, insulate it then concrete it?


Thanks

 

[mrgobby]

Few ways possible but which is best suited may depend on whether this is a building regs applied for garage conversion or a man cave without any planning related application. If the walls have damproof in them you could treat it like it was a basic floor slab, put a DPM sheet down and up the walls, fit 100mm of celotex or similar taped up and pour 100mm of concrete. If you were looking to keep the floor level the same height then you'd need to take up the slab.

 

[^woody^]

You can leave it in

 

[catlad]

Get yourself some 150mm 8x4' sheets of celotex and glue it to the floor with expanding foam, from there you can put 25 mm flooring down and you will be about the right level.

 

[Ian H]

Thanks for the replies, I’ve got a load of 100mm Celotex so will go with that.

If I put a 25mm Celotex upstanding around it does that go inside the DPM or between the DPM and wall?

And the concrete, is there anything I can ask for that will be easier to level than regular ready mix?

 

[mrgobby]

When we did our new workshop/garage extension room I put 25mm inside the dpm to limit any cold bridging. Concrete, if you tell the supplier what it is for and how much is needed they will advise but usually it is ST3 for garage floors. Find a local volumetric supplier as no waste because they mix onsite from the lorry, supplying down to the last barrow. If access is good a few wheel barrows and helpers will see it in there quickly. Price depends on volume I found it anywhere from £90-£120+vat a cube depending on volume. They don't tend to use retardants so it sets faster than premix but you will be fine if you are prepared with wellies and a helper to methodically start one end and screed it off as you go to your exit. 100mm lengths of timber will give you a useful gauge to come off, depends how level your base is, concrete flows but doesnt exactly self level. You just pull the timbers back then out as you go. You don't want a very watery mix either but they will add a little to help if you ask and often a bit runnier if it's being pumped. Tamp the finish or you can trowel it as you go too if your keen. Use a big float if you do. If you want, a few hours after you can get on it using a stiff board or plank and trowel it to a really flat finish

 

[pilsbury]

I would do exactly as you say.
Leave old slab in situ
Sand blinding
DPM
100mm insulation
25mm upstands inside dpm
Cheapo plastic on top of insulation to stop the individual sheets floating up
Set battens around the wall nice and level to screed off
100mm exact-a-mix dumped on top
If you’ve still got a few mm spare height, a few bags of SLC to rectify the finish.

 

[Ian H]

I put a level on the floor and it was a bit all over so I broke it out and found parts were really thin.

There’s an 1800x800mm brick thing in the middle, hopefully an entrance to an underground lair.

Now it’s broken out i’m wondering if a suspended floor might be better/easier than the concrete. Only thing it is not ventilated at all.

I’ve still got loads of 100mm Celotex and rolls of DPM so not sure which way to go.

 

[pilsbury]

I would think you’d need air vents to lay wooden joists. By the time you’ve fitted them, bought hangers or built supporting brickwork, bought floor boards or chipboard, the cost will be the same. Concrete floor is so much more substantial to walk on.

Anyhow, enough of this nonsense, what’s under the slab in the middle? My money is on an Egyptian mummy and untold treasure.

 

[John D v2.0]

Probably the bodies under the slab.
There's no reason not to use celotex on a suspended floor, we did no problem. The joists were old 4 inch ones and we have little clearance to the ground. Suspended floor would be less pressure to DIY. But i agree on the ventilation thing.

 

[mrgobby]

Tomb of the last builder who mentioned needing an air brick with a timber suspended floor to the previous owner perhaps? DPM insulate and concrete it

 

[Ian H]

Just wackered it now, i’m going to give it a go over with a float and level before I put the plastic down.

I have a ton of decent fabric/membrane stuff, is there any point in a layer of that on the sand to protect my plastic even more?

 

[Ian H]

Double post

 

[Ian H]

Didn’t bother with the membrane.

Should I be doing the 25mm upstanding on all 4 walls or just the outside walls?

And the insulation is a bit bent, should I put weight on it for a week before I pour the concrete or will the weight of the concrete push them down?

 

[mrgobby]

I'd put upstand it all the way around it, minimise cold bridges and holds the membrane up too if its a tight fit. Weight of concrete should flatten the celotex but you can put concrete blocks on while you wait for the concrete to arrive if you wanted or just cut it across into more pieces. I silver taped joints on mine (or you can thin plastic sheet it instead) as what happens in the liquid of the pour can get through the gaps between the celotex (probably wont make it float though) I think some got past my tape as there were a couple little air holes which appeared as the concrete settled, I troweled them out.