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Matt's Workshop - planning phase

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Matt's Workshop - planning phase

Postby canoemoose » 31 Mar 2021, 02:34

Well, then. As I said in my introduction thread, it's workshop time. :eusa-dance:

A quick overview:
Garden is ~8.5x10m. It rises away from the house - the far end is about 320mm higher than the house end.
I'd like a workshop, and need a small shed for my + other half's bikes, the lawnmower, buckets, etc.
Original idea was to build something 5x3m internal floor area in the bottom left corner, with an internal wall separating shed from workshop and separate external doors - a semi-detached shed. However, I think it'd be too imposing and actually give too much space over to shed stuff, so there's a new working idea:
The workshop will be 4x3m in the bottom corner of the garden. The shed will be adjoining, and be 1.5x2m on the front corner of the workshop. This will allow for a compressor and/or dust extractor to be put in the shed in the future and piped in through the adjoining wall, if my interests grow beyond shop-vac extraction or into air tools.

So the constraints: This will be about 400 from the boundary on two sides - that's the smallest gap I reckon I can squeeze in for maintenance, and I can store ladders there locked to the fence structure. Therefore, total finished floor area under 15 sq m to keep out of Building Control's interest.
I hope to be able to dig out and essentially terrace the garden, with the left side dug back to the level of the patio at the house, reatinaed close to the boundaries and by sleepers to the edge of the new lawn down the centre of the garden. This will gain me an additional ~320mm height to play with, taking me to a max building height of 2820 but still under permitted development as everything I can find says that height is measured from the highest natural ground level adjacent to the building.

From here on I'm less certain - I know I'd prefer a duopitched roof - I think they look so much better. I assume EPDM is the way forward, I'd love (eternit) slates but I know I haven't got the height for the necessary pitch. I'm hoping with the guidance of Mike G I can use a structural ridge beam and end up with no joists and therefore gain as much headroom as possible, with a roof that still looks sensible at about 12 or 15 degrees pitch or so.
I'd want to do the same on the shed bit - a duopitched roof of the same pitch, with no joists - and I'm unsure of how to avoid a messy detail where the shed ridge meets the workshop verge.

The general construction will be "Mike's way" and as I said before, I plan to get a local builder to do the groundworks and to build a plinth wall, and I will do all the carpentry and fit out myself. I can get electrics finalised and signed off by an electrician at work, so no problems there.
This whole small estate is built within the footprint of a former factory, and under a thin layer of topsoil the ground is very poor - probably a result of putting the old factory floor slab through the concrete crusher! Luckily, nowhere near the clay and former brickworks.
I'm sure there will be more questions as I think of them, but this post is enough of an essay already!

Finally, here's a drawing of the proposed shed/workshop, additional slabs and garden works etc overlayed on the existing garden and levels:
Image
The back of the house is across the 5295 dimension at the bottom, the dog-leg with the gate in it is at the end of the drive adjacent to the house - access is easy if nothing else!
I'll take a couple of photos in the morning and add them in.
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Re: Matt's Workshop - planning phase

Postby Mike G » 31 Mar 2021, 07:48

I like your design, Matt. Two sheds each of appropriate design is usually better than one large one divided, and it certainly has less visual impact. In fact, my own workshop is built to look like 2 adjoining buildings rather than one big one, and Mark (TrimTheKing) has done the same.

I don't know when you are going to start work, but in the next few weeks I'll be building a 4m+ x 3m+ bike shed very much along the lines you describe, with a vaulted ceiling (no ties). It's really easy to achieve that with these dimensions.

Just a thought. Given the size, you could build it using 3x2s instead of 4x2s, and save having to put a line of 50mm blocks inside the brick plinth. I think that's what I'll be doing, although with a bike shed there is no need for insulation. Using 70 x 45s will reduce the available depth for insulation.

Edit....
Surface water drainage is going to be an issue. You will effectively be creating a sump in the ground by lowering the ground level, and that will need draining. You need to have a good look at how you'll do that, because the invert levels of the drain you connect to will possibly be the limiting facter as to how deep you can go in digging your hole.
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Re: Matt's Workshop - planning phase

Postby canoemoose » 31 Mar 2021, 13:35

Thanks, Mike.

I'm still very much at the drawing phase here. I like to get my ducks all in a row before starting, and I want to come up with a scheme for the rest of the garden at the same time so it all works together.

With that said, I think i could have a good go at drawing the wall frames and a couple of sections through the two buildings - what size ridge beams would I need? The unsupported ridge spans will be 4m for the workshop and 2m for the shed.

What is the best way of avoiding an awkward junction between the rooves? I had assumed the left-hand pitch of the shed and workshop rooves would be continuous, but maybe this isn't the best way? With a 150mm overhnag at the verge as planned, this makes a silly tapering gap as the verge comes down and the right-hand pitch of the shed roof comes up, and the verge has to neatly finish against the pitch of the lower roof.

I had assumed nominally 4x2 material for walls and 6x2 for rafters for insulation space. (what size is it really these days? 45x97 and 45x145?)

Finally, drainage - to be honest, I hadn't really thought about this at all. I had planned for the 400 strip around the sides of the building to be gravel or maybe slate chippings, and naively hoped that this along with the tapering level would be enough, as it's not dug in on all sides. Perhaps this was optimistic?

Here's a photo of the site, just for additional context. (We like pictures!)

Image
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Re: Matt's Workshop - planning phase

Postby Mike G » 31 Mar 2021, 14:09

Your rafters need only be 95x45 for strength, but you'll want 145 depth in which to get your insulation. 600 centres is fine. The ridge purlin .....not sure, but I think I'll be using a 200 x 45 or a 220 x 45, and my roof has the additional wight of plain tiles. Don't forget, I'm an architect not a structural engineer, so if in doubt, ask an engineer.

Personally, I would not have the back wall line of your 2 buildings running in line, thus making the rooves discontinuous as well.
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Re: Matt's Workshop - planning phase

Postby canoemoose » 19 Apr 2021, 22:31

So things might not be quite going as planned.

I was able to speak off-the-record with a planning officer last week, and he felt that the common interpretation of the Town and Country Planning (General Permitted Development) (England) Order 2015 was different to my own in the case of buildings dug in to a slope. Long story short, as he would interpret in event of a dispute, if there is a flat area created with a low retaining wall like I was planning, then the height is measured from the new level created. If however the ground is retained by the wall of the building, height is measured from exising ground level - what I wanted to achieve.
Seems like I have three options, then:
(a) Accept I can't gain the additional 380mm and go for a flat roof instead, with associated loss of headroom inside
(b) use the building walls as retaining walls on two or three sides
(c) Apply for planning permission

(Or (d) do what I was planning anyway, and be outside of permitted development by a technicality)

I'm not interested in (d), and wouldn't shout about it online if I was!
I'd prefer not to go through the planning game and associated time delay, and I really don't like the flat roof approach - both from not liking a flat roof, and not liking the loss of headroom - by a back-of-envelope adding up, internal headroom could be as low as 2.1m, which I feel I'd be bashing stuff into all the time. While not a giant, I'm also fairly tall myself and worry it'd feel claustrophobic.

So my question is - how hard would it be, and how complicated the detail, to have a taller plinth wall on the two sides of the building against the boundary and have this wall retain the ground? The height (depth?) to be retained is only 380mm maximum. I understand the concept of tanking but not how to apply the theory (although I could have a guess), and what difference would it make to a foundation, seeing as Mike's slab method relies on the sides of the slab being above ground level to remain dry? Is this a whole lot of agro more than I really need?
Thoughts on a postcard gladly accepted!
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Re: Matt's Workshop - planning phase

Postby Mike G » 20 Apr 2021, 07:47

Having an outside wall of a building act as a retaining wall is easy, but it requires some thought, and it should be built off an orthodox strip footing rather than off a slab. This is the only way to have a continuous DPM/ DPC, and thus keep the slab and the building dry.

So, you are going to have to dig a little deeper than planned, in a strip 400 to 450 wide. This doesn't need to be a metre deep like a house, but it should be at least a foot deep and down to some solid ground (ie through your topsoil).

It's easy enough to have a step in the top of the plinth, but you might consider running it all around at the one level, which will be 150mm above ground level at the highest point of the ground level around the building.

If you want a drawing of the plinth/ foundation/ slab detail, drop me a PM.
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Re: Matt's Workshop - planning phase

Postby Malc2098 » 20 Apr 2021, 09:20

I had a garden slope in two directions and neither BC nor Planning would give me answer to where the datum would be, without an application for planning permission.

So, I applied, and in the design raised the ridge to have a workable height inside. They approved. The process took only 2 months and I got the design I wanted.
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Re: Matt's Workshop - planning phase

Postby canoemoose » 20 Apr 2021, 20:50

Malcolm - Planning here have been as explicit as to where they would measure from, it's just not where I was hoping!
If I were to apply, I wouldn't be at all surprised if they were to reject - I am admittedly attempting to put a reasonably large building in a fairly small garden. If I were to apply and get rejected I'd be back here anyway...

Keeping it within permitted development is a little cheeky and removes the opportunity for them to reject my plans... :eusa-whistle:

Mike - PM'd. Much appreciated.
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