Daves Workshop build

[pharcycle]

Hi everyone

About to make a start on my workshop at the bottom of my garden, nothing too extravagent at 5.16 x 4.35 m footprint and ~ 4.5m high at the apex. I went beyond the 4m height as I had to go for planning anyway due to the proximity to the boundary (garden is around 7.5 m wide so 1.2m ish to the boundary). Fortunately don't need building regs involvement so that's good but it may add additional hurdles to any changes to the overall appearance.. such as more windows that I realise now might have been worthwhile! General construciton will be 3-courses of bricks with insulated timber frame on top.

Anyway here's the 3D CAD model I made a year or so ago of the external design. I've taken most of what Mike G detailed for (one of?) his workshops since that covered in great details many of my early questions.




I have now started excavating the ground for my slab and doing more reading online about how to construct this I have a couple of concerns that may or may not be warranted. However before we get to that here's what I've been able to do in the last week...

Overall site


slightly closer detail on the levels


I've currently gone down around 200 mm across the whole area and down to 400 mm (all +/- a lot, still to get it level). I had previously dug up that entire area and rotavated to probably 300 mm depth (was going to put a lawn but decided a workshop would be more fun) so digging it out wasn't too arduous.

My plan was as follows:

1) Whack ground down (hire vibrating plate compactor)
2) 150mm of MOT type 1 whacked down
3) 50 mm sand over the hardcore
4) Construct shuttering around perimeter
5) Lay DPM
6) Install mesh (not sure if required but figured it can't hurt, spaced off accordingly)
7) Ideally employ a company to deliver, pour and float the concrete (was going to mix it but think that's unwise now).

I have a few uncertanties though it would be great if someone has some experience and has any advice..

a) I have remembered that there is a lot of clay in the soil. I found some of this along one of the deeper parts when I was digging it out. I've heard this can swell with rain etc.. should I adjust my foundation plans knowing this? There have been trees on the ground before (but taken out several years ago).
b) There's an acer (we think) on the edge of the property which is quite well established. Plannign to removing it but wonder if this plus the clay might be setting myself up for issues down the line?
c) Can anyone recommend a concreting company in the Oxfordshire area that could also do the floating? My garden is quite long so it's maybe 150+ feet from the road to the site, also down a bit of a slope. Semi-detached house so at least can get down the side but it would have to be barrowed quite far.

Last issue I'm considering at this stage:
I like the idea of 50 mm kingspan on top of the slab with 22 mm T&G chipboard on top of that. How do I deal with the door threshold? Do I run the first course of bricks as a continuous perimeter to cover the end and have a step up into the building? Should I run 50x50 battens to fix the T&G to or just leave it floating on the kingspan? Do things not forever creak as you walk on it doing this?

Anyway, already written far too much, thanks to anyone who reads this and any advice you can offer.
Cheers,
Dave

 

[Mike G]

I'm heading out, so brief answers.....

Rather thsan work to set depths, work to soil types. You should ideally remove all the top soil (ie anything with organic matter in it), and build back up with the minimum amount of hardcore/ Type 1 you can. If you don't need to build back up (ie you can get the required depth of concrete without packing up), and your sub-soil is even in nature, then omit the hardcore altogether. If you get to clay, and it's all clean and even, then you can, if the depths are right, just pour your concrete on that (on the DPM).

DON'T do 50mm of sand. The sand is only a blinding to protect the plastic from any sharp stuff in the hardcore or subsoil. Just put down the amount you need to be certain of protecting the plastic.

The trees are an issue, when in conjunction with clay. I'm an architect not a structural engineer, so I'm not qualified to do those calculations for you. However, if you have tree roots close to the site then you should err on the side of caution. For your dimensions of slab your depth of concrete should be 150mm at least with clay and nearby trees, and with mesh towards the bottom and top of the slab thickness (to allow for both heave and ground shrinkage). Don't put mesh at the mid-line!! That's a common mistake. Don't forget that you need the top of the slab to finish 50mm or more above ground level (there is an alternative, but it's more complex).

As for people to do it.....I don't live anywhere near Oxford, but you don't want a concrete specialist. You want a decent jobbing builder. This is standard every-day stuff for them. Ask around family and friends to see if anyone knows a decent one-man band or small builder.

 

[Mike G]

I'm back, so to the rest of your questions.

If you are going to have a floating floor then it is best to have it properly floating rather than screwed down. It must be glued together at all the joints, which should be T&G, and trapped by a skirting board all round. The threshold is done by running the outer skin of brickwork across the doorway, with the door cill/ threshold covering the join. If you want to use 50mm of insulation then 50 + 22 (for the flooring) comes to approximately the same as a course of bricks on its mortar bed.