Garden studio foundation

[Kirk]

Hi I need some advice with my garden studio build.  It’s Roughly 5 metres x 3metres 

I’m building on a concrete base and I’m all about ready to pour the concrete. 

I dug the foundation’s 12 inches deep around the outside where the walls will sit and around 10 inches in the middle.  My garden is on a slope so there was not much to dig at the front. 

I purchased 8 inch boards in order to combat the difference in hight at the front. 

With the current situation I was struggling to get supplies so I used the time to smash down  a brick wall at the front of my house as it’s going to be removed anyway, I smashed it up in to very small pieces mostly , took me a week ??‍♂️ I used this to for a layer of hardcore. Followed by 10 bags of mot sprinkling over the top. 4 inches in total around the edge and 2 inches in the middle. I Wacker plated.  this down , I then added 2 inches of sand over the whole lot and Wacker plated it again hoping  to fill any voids . This was then covered in a 1200 gauge dpm, on top of this is 2 inch insulation boards. Leaving a gap around the wall edge. On top of this will be re bar. All in all it will work  out at just over 4 inch slab over the insulation and 6 inch around the edge. 

I’m now worried that I didn’t do enough or the correct hardcore  type and I’m concerned it may move or am I just going over the top. 

My soil is pretty hard but there was a bit of clay here and there. 

Don’t fancy pulling it all up again. 

[PeterW]

Depends what the building structure is.?

 

How wide are the edges ..?

[Roundtuit]

You'll get a more informed reply from any number of other members, but that doesn't sound like much insulation.  If you don't want to disturb your sub base, maybe raise the slab a bit to accommodate some more?

[Kirk]

It’s going to be  timber frame , standard 2.5 metre high flat roof. Most likely 38mm x89 mm studs.  Want to double up on stud walls. 
the gap is around 6”

 

with regards to the insulation I just went with the recommended , cant really go higher as I’m trying to stay within the 2.5 m

[Russell griffiths]

I would remove the insulation, without it forming a fully insulated slab it will do nothing to help heat loss

 

i would get a concrete slab down and then build your building and then insulation on top of your concrete and put a floating timber floor on top of that. 

 

Pictures would help, don’t worry we won’t bite, much. 

[ProDave]

My concern is with the description of the depths and the sloping site, at the low side, you have barely scraped the turf off. Whether that is enough or not depends on what the soil is and how long since it was last disturbed,

 

I would certainly be putting steel mesh in and pouring the concrete without insulation to get a decent thickness reinforced slab.  Then build on top of that laying an insulated floor inside the building,

[Archer]

Could you use shuttering to raise the slab depth? Agree with others, you probably want at least another 2 inches concrete and depending on what it will be used for possibly more insulation

[Russell griffiths]

4 minutes ago, ProDave said:

My concern is with the description of the depths and the sloping site, at the low side, you have barely scraped the turf off. Whether that is enough or not depends on what the soil is and how long since it was last disturbed,

 

I would certainly be putting steel mesh in and pouring the concrete without insulation to get a decent thickness reinforced slab.  Then build on top of that laying an insulated floor inside the building,

 

I was also concerned with the slope and thought exactly the same

[PeterW]

So for a small building, your 100mm concrete is fine if it’s timber framed. The issue is the insulation and as what @Russell griffiths says, it is not fully insulating the floor. 
 

I would keep the insulation for now, cast the slab 50mm lower (ie without the insulation) and then double up the sole plate with a second 50mm tanalised beam and drop the insulation inside this.  It won’t be “warm” floor but it will at least insulate the whole internal floor. 

[Kirk]

Here is a picture as I was finishing off the hardcore but before I Wackered it down and added the sand. And the second pic is as it stands now. 
2 reasons I didn’t go to the edge with insulation was to get a thicker concrete around the edge and also as it’s a music room I will have 2  studs very well insulated which will go past the gap and over the insulation. Also I didn’t want to lose headroom inside with insulation on the inside of the floor. 
I plan to build a small wall of some kind to cover the concrete at the front. 
It’s a fairly new build so ground is pretty hard and full of rubble. Is it to big a job to go in and take another 2 inches out the middle or is it better to do as described above 

thanks everyone ??

[Kirk]

What about this? 

I reckon I have 2 inches I can come up with my boards as I’m currently sitting below the bottom of my gravel boards at the back. 
I could take boards and dpm off. Sweep sand or remove it temporarily. Ad another 2 inch of mot 1  across whole lot( would a tonne bag  do it ? ) put sand blinding back Dpm insulation  re bars etc. 
or just ad 2 inch to my boards as is s as and fill with concrete. That would be 6 inch in middle and 8 inch around the edge. 
1st option I believe is cheaper but more labour. (Don’t have much else to do currently) 

2 nd option more expensive as a fair bit more concrete. 
I really wanna maximise head room inside

[Russell griffiths]

TBF that looks very good, I was expecting to see a right old mess from your description 

i don’t think there is anything wrong with that at all. 

 

I would leave the edges as is and increase the insulation in the centre

add insulation until you end up with a 100mm thick slab in the centre, leave a nice thick edge. 

What you spend on insulation you will save on concrete

get a nice thick sheet of mesh in there and just carry on as you are. 

 

Carry on bud ??

[Kirk]

11 minutes ago, Russell griffiths said:

TBF that looks very good, I was expecting to see a right old mess from your description 

i don’t think there is anything wrong with that at all. 

 

I would leave the edges as is and increase the insulation in the centre

add insulation until you end up with a 100mm thick slab in the centre, leave a nice thick edge. 

What you spend on insulation you will save on concrete

get a nice thick sheet of mesh in there and just carry on as you are. 

 

Carry on bud ??

Hi Russell, I’m a bit confused with that As if I add to the insulation  I would then only have a 2 inch slab across all the middle. Do you mean go up with the boards 2 inch then add more insulation. And not bother with re digging out and adding more hardcore 
thank you 

[Russell griffiths]

43 minutes ago, Kirk said:

Hi Russell, I’m a bit confused with that As if I add to the insulation  I would then only have a 2 inch slab across all the middle. Do you mean go up with the boards 2 inch then add more insulation. And not bother with re digging out and adding more hardcore 
thank you 

Sorry picture looks deceiving 

it looks like the centre is very thick

100mm in centre

150-200-250 whatever you want around the outside 

tie it all together with some good mesh over the whole lot, support mesh on chairs 50mm off of insulation. 

 

Try to get a mix on site service instead of mixing yourself, better mix and more consistent. 

 

Regarding your walls, would you not be better off with 125- 140 studs fully filled with sound slab, then double board with plasterboard 

will this save you some internal space. 

 

Your biggest sound/ heat loss will be the windows and doors. 

[Kirk]

41 minutes ago, Russell griffiths said:

Sorry picture looks deceiving 

it looks like the centre is very thick

100mm in centre

150-200-250 whatever you want around the outside 

tie it all together with some good mesh over the whole lot, support mesh on chairs 50mm off of insulation. 

 

Try to get a mix on site service instead of mixing yourself, better mix and more consistent. 

 

Regarding your walls, would you not be better off with 125- 140 studs fully filled with sound slab, then double board with plasterboard 

will this save you some internal space. 

 

Your biggest sound/ heat loss will be the windows and doors. 

ok thank you for your input, really appreciate it. 
so what I think I will do is lift off the boarding insulation and membrane. Remove some  sand Ad another 2 inch of hardcore  , which should raise the boarding up.  Re blind with sand, leaving myself 8 inches this time. Double up on insulation and hey presto leaving me a 4 inch slab across the middle  ?

 

with regard to the walls I was just working from a book by rob gervais called build it like the pros. He talks about leaving a 1 inch gap between 2 studs creating an air gap Which is most effective for sound insulation . But I could do as you say and build with thicker studs and maybe use resilient channels to create the gap

[Simplysimon]

is there a reason for soundproofing, are you having a @pocster type room/dungeon? ⛓️ ?

[Kirk]

2 hours ago, Simplysimon said:

is there a reason for soundproofing, are you having a @pocster type room/dungeon? ⛓️ ?

 

2 hours ago, Simplysimon said:

is there a reason for soundproofing, are you having a @pocster type room/dungeon? ⛓️ ?

 

2 hours ago, Simplysimon said:

is there a reason for soundproofing, are you having a @pocster type room/dungeon? ⛓️ ?

Music production/DJ room