Single skin block-work workshop with added retaining walls

[i_am_fubar]

I have the below horrible north facing side of my garden adjacent to a carriageway. It's dim and gets about 2 hours of sun a day.

Elsewhere I have a brick built workshop. The elsewhere is literally the best sun trap you can imagine and we would like to make it a more family oriented space rather than a 'man cave'... So I would like to build a new one down the side of the house, demolish the old one and presto, better space.

Current specification
Roughly 5.5x2.6m, leaving the path as it (minus some drainage) to make 14.3sqm.
The ground has been (as can be seen in the other picture) excavated about 600mm from natural gradient since we bought it. This should allow, in theory, 3m maximum height from the patio, or 2.4m from natural gradient under permitted development.
Construction would be rendered blockwork, single skin (I would love to go cavity for insulation, but that'll eat another 300mm from my internal width and put the cost way up. I know I will need pillars, but again, they take up precious space, so any alternatives would be appreciated.
I'll probably lay to unreinforced slab foundations, but put mesh in anyway.
Oh, and I want it to be devoid of a step up ideally.
Flat roof with EPDM covering. uPVC door and windows. and a 60A feed with local consumer unit.

Question
My problem comes with the need to retain the fence-side wall and far-end wall up to grade before the 100mm blockwork wall can start. While I have no issue with doing that with 215mm blocks, I can't get a comfortable feel for how I would damp proof it to keep the insides dry. If anyone had any bright ideas on how this could be achieved, I would love to hear it - I should also point out I know the need to add drainage behind the retaining wall

Cheers
Fubar

 

[23vc]

Could do away with the raft and do trench fill, Lay the 215mm as 2 skins and have a heavy gauge dpm go up the middle of them and continuous under the floor slab if it’s not too high to do without wall ties getting in the way, and sit the single skin above on the outer skin not inner.

Can’t see how you can make it work with a raft easily tbh but others may

 

[freddiemercurystwin]

 

[23vc]

FMT can you not sit the 100mm leaf on the outer 140 skin, and lap the dpm out the outer skin rather than back over the inner?

 

[freddiemercurystwin]

You could do but you don't want any DPM exposed to the outside so it's got to step in at some point, this kind of retaining wall is a mass retaining wall so it's the weight that's doing the work. If that's actually what you were asking?

Or I guess you could use a protection board (eg cement board or other suitable bituminous protection board) to protect the outside.

 

[23vc]

I meant same as your first pic but the single skin is built on the outer edge of the 140mm so the step in is on the inside of the building not the outside and you don’t need that chamfered slope bit on the outside

 

[mrrusty]

Alternatively, once out the ground, you could timber frame, exterior clad in profiled steel cladding (inexpensive), insulate and internally line. Or even build the walls out of SIPs and then externally clad. https://www.simplysips.co.uk/products/.

You could use SIPs above the DPC line in @freddiemercurystwin illustrations above. The external cladding run-off would then be on to the chamferred area in the ilustration

An advantage of profiled steel cladding is it only needs 9-12 screw fastenings/sheet so fast and easy to install.

 

[i_am_fubar]

Thanks all for your ideas & advice.

MrRusty, I like the idea of the timber and steel, but as 1) I'm close to the boundary so I need to be 'substantially non-combustible' and 2) I will be wanting to bolt a lot to the walls and possibly put in some form of gantry crane. so the added mass of the blocks is an asset. When it comes to the steel, a good cosmetic, but takes up an extra few inches of width.

23vc, as I can't get a digger in to line up a trench without destroying the relatively new fence makes a raft (plus a lot of elbow grease) a preferable idea. Also, a lot less soil to dispose of and only a single concrete pour needed.

I'm starting to wonder if I've got this round the wrong way and my easiest approach is to waterproof the INSIDE of the structure rather than try and keep the below grade masonry dry. Essentially, pour a raft, build the gravity wall out of 215 blocks where retaining is needed and single course of 100 blocks elsewhere, put a conventional DPC down, build block work up to roof height. Then just paint the inside with epoxy DPM and then epoxy floor paint to protect it. Worked well in my garage-come-gym. Providing draining is solid, I should be able to keep hydro-static pressures down to almost nothing. Then paint up a bit of sand cement mud on the outside and paint with weather proof masonry paint above DPC.

Optionally, I could put a thin self leveling screed down between the DPM and final epoxy paint to protect the DPM if I drop anything on it.

For reference, the paint I've used in the past is this stuff for the damp proofing and then a coat or two of The floor paint.

 

[mrrusty]

If its under 15sqm internal, the substantially non combustible rule doesn't apply. https://www.labc.co.uk/news/does-my-new-outbuilding-need-building-regulations-approval

 

[i_am_fubar]

Ohhhhhh, thanks! I'm border line on this. maximum external dimensions I can manage are 5.88 x 2.72, so 15.99 (and I do want this as big as I can get) - but will be well within 15m internal floor area.

That said, I'm going to be welding and grinding in there, so block work offers a desirable nature.

Above the DPC though I'm not too fussed and have options. This thread is very much on the below grade waterproofing requirements (not that I mind the extra information )

 

[i_am_fubar]

Following excavations, thinking, research, re-reading comments from y'all and more thinking... I'm leaning towards this as a solution:

There are three main things that have added to the design...
1) The whole slab and retaining wall will be insulated with 50mm of EPS. This will act as heave mitigation, protect the DPM and while it will saturate, it will still offer reasonable insulation beyond direct earth contact.
2) The DPM will tank the whole slab and the back of the retaining wall.
3) To deal with the exposed DPM above grade, I will get some custom stainless steel flashing made (I have a local sheet metal supplier that can do them at £24 per 2m length and I can customize the height to fully cover the DPM down to below grade. I may modify from the above slightly to encompass the insulation as well. This will then be sealed to the DPC with bitumen paint on the lowest brick course.

This isn't including any details on external wall render at this time.

My biggest outstanding worry is that the slab will be pressed down by the weight of the walls around the perimeter and then place the surface under tension. With a 350 deep, 300mm wide perimeter and 100mm thick C30 main slab with steel, I can't tell if I'm massively overthinking the potential problem, or if this is obviously doomed to failure.

I figure the EPS will act to take up some irregular movement and the whole thing will end up just being a huge reinforced concrete slab that will last longer than I will.

Any criticism or support welcome. Every other element of this build I'm happy with. But i've only got one shot to get the foundations right.

Minor comment on the retaining wall, the 215 reinforced blocks look overkill for only 2/3 courses. However they have to deal with an adjacent pavement and roadway as well as cantilever forced from the fence posts in high speed winds. Again, rather go over-kill once and know it'll last than risk failure half way through build.

Cheers,
Fubar