Building a car repair shed from scratch

[Zebra]

Bit off topic, this, hope you all don't mind. My friend Giraffe has recently moved, and wants to build a shed - a large one, 4m x 6m, or 12ft x 20ft or thereabouts, for all his car and welding stuff. He also wants to be able to roll a small car inside (he currently drives a Smart Roadster) and do long term projects in there, like engine swaps, but this would probably not be very frequent.
We are considering options for the base. While concrete would seem the obvious choice, we are also aware that it's his girlfriend's house, we very much hope they will stay together forever but it would be an advantage if the thing were not too very permanent. And a concrete base that big, to the recommended depth with all the compacted sub-base underneath, adds up to quite a cost, plus it's not great environmentally. But it does make for a solid shed base.
An alternative as I see it would be brick piers set on concrete pads, then timber beams - plenty big ones, and a thick ply floor. You might have problems with heavy hammering as the floor would have some bounce in it, but I reckon if you knew where the piers were you could jack a car up on such a construction. You'd have to ventilate underneath, clearly, but the whole thing just seems a bit more manageable and DIY-able than concrete.
Are there any other options I've not thought of? Cost is a consideration, as is environmental impact.

 

[Cubist]

A mate of mine built a workshop/shed/garage in which he could restore old cars - at the time he'd got an MGB-GT, a Land Rover and his wife old Mini in bits around the place. His design for the structure was a pair of parallel piers, each about .5M wide with a ramp in front. Around these he constructed a timber frame shed to provide the work-space and his design for that allowed for it to be de-constructed and moved should it be needed. Same could be done with a solid block structure to form the piers.

A few years later he found himself doing just that but took an easier route to the move, theres a video somewhere of the event in which 8 blokes can be seen lifting the shed like a sedan chair and walking it to its new location.

 

[malcolm]

I'm doing my car work in a large tent. But that's not ideal. Much nicer to have walls as well as a roof. I'm guessing the build is on bare earth.

For a temporary ish structure concrete flags would be cheap for a floor especially when bought used on Facebook - just set them down on sand like a patio (maybe use a plastic sheet). The flags give you a much wider foundation than most of our houses. A plinth wall can be built in lime directly on the flags (not right on the edge) - two brick height is enough, then a structure built with 3 by 2 and feather boarded is surprisingly resilient. Do it nicely and it'll be good for 100 years.

Disclaimer - I'm not a real architect.

 

[Zebra]

@malcolm, that was actually one of Giraffe's other ideas, paving slabs. I just feared that it would become uneven, that it wouldn't adequately prevent moisture from rising up through the floor (though the plastic sheet would mitigate that), and that rats would tunnel under it. But it's certainly cheap and low impact.
@Cubist, tell me more... you mean like dwarf walls like you'd have under a house but wider? Spanned with timber? What was the floor made of?

This is really encouraging that you both think there are options that would work and you're not all saying no, you need concrete.

Edit: I think the reason I have such a dim view of how well paving slabs would perform is because at my mum's stable, and at Giraffe's mum's greenhouse, they've gone wonky...but they're laid directly onto earth there. So with the right sub-base, and maybe with a splodge of mortar under each corner....hmmm, yes...

 

[Cubist]

Personally I would favour Malcolms approach of using paving slabs as perhaps the easiest and cheapest strategy to create a patio like base upon which to construct the shed/workshop. However, I would shy away from sand unless a rubble infill was used first - its not rats you need to worry about its the perishing moles (I have a slab path to the front of the house laid on sand that the moles just love to live under and every year I have a battle with the subsidence they cause - you ever seen Jasper Carrots 'I've got this mole' monologue? That me.)

My mates solution consisted of parallel runs of concrete that sat inside the workshop and provided no support for the building itself, slabs/pavers could be used in the same way. He left the rest of floor as bare earth and used ground spikes to anchor the posts of the timber frame of the shed. I'm not a fan of the spikes myself and have used a slightly different approach here to construct the timber frames for fruit cages and potting sheds. The question is whether your mate wants a thoroughly dry, wind-tight and secure storage capability? If so I would be erring on the side of a patio with a large enough footprint for the workshop to sit down on.

 

[Cubist]

An important afterthought. The big issue for your friend Zebra is the 'point loads' that may need to be handled by the floor. Car jacks and tripod engine hoists will exert considerable downward pressure in very small areas and, without a solid base/platform, theres a possibility of potentially catastrophic and dangerous failure. If say, a jack were to be positioned on the corner of a paving slab there is every possibility of the slab tilting or even breaking - particularly if it is bedded on sand alone. 'Dabs' of mortar in the corners may be equally risky as any cavities left below may become points of failure under heavy loads. In my view it's vital that the floor is constructed to be as strong as possible over all of its usable surface area.

 

[Zebra]

Thank you for your further thoughts, Cubist... I'm starting to feel a bit more confident. Yes I think we want thoroughly dry and wind-tight. Giraffe did say that he'd be happy to have to consider internal load-spreading - like using a lump of 6x2 as a load-spreader to jack off of, so the paving slabs are a possibility... My only other thought was that if you wanted the finished slab to sit slightly above ground for water run-off (and the ground is sloping, so one end may have to end up above ground), with concrete you can shutter, but with slabs, what's to stop all the sub-base and sand sliding out the side. Perhaps a brick surround where the ground falls away...

 

[Cubist]

It's always wise to enclose the material any slabs are to sit on to prevent just that. Concrete edging stones, 50x150x900 from any hardware outlet for about a £5 each, can be sunk to an appropriate depth and infilled with a gravel and sand mix that can be compacted (may be an idea to use a compacted aggregate as a base layer then a bed of finishing sand/gravel) to create the final bed for the paving slabs.
These can also be used to help establish the water run-off slopes before the slabs are laid.

 

[philpjuk100]

When I needed extra garaging I built a "lean-to" on my mower shed, The chap next doors builders had a few too many wheelbarrows full of concrete delivered so some shuttering was quickly put in and a thin floor was laid. They also gave me their wood offcuts and used shuttering and pallets. The chicken farm down the road was having new bird flu-resistant sheds built so I cadged some roofing sheets, timber, and hinges from the old sheds, my son miss measured his decking so I got half a van full of his surplus. Concrete blocks were obtained by dismantling my compost heap but I had to BUY a length of guttering but managed to cadge a poly barrel which the riding stables got molasses in for a water butt and bought some fence life. The size was kept small to economise on materials and I can just squeeze out after backing in ( the car has no doors) It is now known as " Grumpies Garage as the grandkids made me a sign for it. Total cost has to be under £50.

 

[philpjuk100]

A Giraffe in a smart car?

 

[Zebra]

When I needed extra garaging I built a "lean-to" on my mower shed, The chap next doors builders had a few too many wheelbarrows full of concrete delivered so some shuttering was quickly put in and a thin floor was laid.

PhilipJuk100, may I ask, how thick was this "thin floor" of leftover concrete, approximately? And was it directly onto the soil?
Your shed looks awesome and I love that it was cobbled together from so many free leftovers.
I am wondering what we could "get away with" just as you have done. I am sure that the stables at the house I grew up in had thin floors like that, a bit of concrete directly onto earth, and it's still there 50 years later. Do horses count as "point loading?" Four points of load.....

 

[philpjuk100]

The floor was built in a rush, just two of us with wheelbarrows. The surplus concrete was in one of those "mixer lorries". The driver chucked a bucket of "stuff" in it and said "if you make me a coffee you have an hour"I gave him coffee AND cake and we laid a strip of concrete blocks and two lengths of floorboards, did a quick bit of digging and pounded in some broken bricks, slabs and plant pots and started barrowing in the concrete the first five or six barrows were ok then it started to get a bit "thin" so we dumped the rest, Some of the hardcore stood proud so we hammered it down below the concrete. There was no membrane and the floor ended up about 3 to 4 inches thick and of poor quality but I just park the car on it and it has been there about eight years. If I need to do any work on the car I do it on the driveway

 

[Cubist]

Do horses count as "point loading?" Four points of load.....

I need to be very careful how I say this but......yes, the horses hooves each count as point loads as do the heels of the First Ladies Le Boutons. The latter can punch a 5mm deep divot in an oak floorboard at each step and, he hastens to add, the First Lady is only slightly built..

 

[Zebra]

What about the concept of a suspended timber floor? Giraffe thinks that jacking up heavy loads would be rare, and his current car only weighs 800kg.
My own shed has small concrete pads, small brick piers (2 bricks x 2 courses), a bit of dpc on each pier, then timber joists and a ply floor. It's much smaller than what we're talking about here but I'm wondering if we could scale it up.
I'd have to look at timber sizes and spans, and calculate the number of piers needed. We could make sure any potential points of load had a pier underneath.
Adequately ventilated from beneath, the timber should survive OK. It would be warmer than concrete.
Mad idea?
What sort of gap between the earth and the bottom of the joists should we consider to be a minimum?

 

[Cubist]

Theres nothing wrong with the idea of using a suspended timber floor, and for the job I would recommend a heavy 18mm marine ply sitting on a 100x50mm timber frame and that supporteded by carefully positioned brick piers. Best for the bottom edges of the timber to be at least 150mm off the ground to avoid rats taking up residence. One thing to consider is the length of the vehicles wheelbase and the lowest height of the vehicles underside. A sturdy ramp of an appropriate length will be needed to avoid the vehicle grounding.

 

[philpjuk100]

How about making a suspended floor using plenty of joists covered with scaffold boards? I have never done it but have used scaffold boards for other heavy-duty purposes. My engine bench is covered with them.

 

[Zebra]

Progress pictures!

 

[Zebra]

We did a made-up, unusual solution for the base. The requirement, as I mentioned, was for not-too-permanent, low-impact environmentally (i.e. not poured concrete), able to jack a car up on, and no damp to permeate up inside.

The solution was some type 1 sub-base, sand, a DPM, more sand, and then block paving. Unusual, to use block paving with a DPM..... My friend bought a whacker and will sell it again afterwards, so we were able to compact everything. We then dry jointed the block paving with sand and cement.

The block pavers and all the timber were second-hand, reused materials. Only the sub-base, sand, DPM and screws were new.

We're still cladding the sides, and it will have a roller shutter door.

 

[Cubist]

Much more sophisticated than I anticipated - looking good.

 

[a twig]

Very nice! Love the two tone effect, whether intentional or just from available materials, looks really good! Might be too late now, but did you go back over the pavers with the whacker plate after putting them down? - will help prevent the trough you might otherwise end up with from having a vehicle going forward and backward over the same route.

Only other thing to be careful of (if it bothers you, might well not do as its a shed rather than a driveway) is used engine oil spills. As I found it, once they soak into your expensively block paved driveway, they are an absolute pain to get out