Advice on installation of floating garage floor / trade-steps

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Hi everyone.

Well I wasn't sure where to start with this - whether to go straight to trades / architects or structural engineers. So though I'd start with the forum to get a starter for 10.

I've just bought a new house - we're completing in a few weeks.

The main reason I've bought it is for the garage. I restore classic cars, do a lot of machining as well as joinery so need lots of space.

The garage is odd, but amazing.

From the front its a normal, detatched oversized double garage build face-on into a bank. From the back (down the bank), you realise its actually two story with a personnel access door into the undercroft. Through the door and under the garage are four small spaces, separated by substantial soldier walls (supporting the floor joists above) with plenty of head-height that are perfect for separate hobby areas (small machine shop / electronics area / computer area / compressor area). Effectively this gives me the floorspace of a good 3 to 4 single garages in the footprint of a double.

But obviously the garage has a floating floor - and a wooden one at that: board over joists sitting on the soldier walls.

Now the garage is otherwise very well build, breeze block and render with a slate roof. Its just the wooden floor that I need to address - its almost like they cheaped out at the last second. While the current owners have had two range rovers in there, I wouldn't feel comfortable for one second with the floor as-is: not with engine cranes and axle stands being used (point load) - not least the flammability. So I've budgeted a good amount to have this addressed and I've learned that floating garage floors in the form of block and beam topped with screed and mesh are actually relatively common.

So, I'm going to get prices to have block and beam retrofitted into the garage. No easy feat as the roof can't come off - so a usual crane-in job is going to be tricky. I'm guessing they'll break through the existing walls at floor level (accessible from the outside by digging a small amount of turf off at the top of the bank) and will bring the beams in with a telehandler through the vehicle doors. They'll obviously have to check/modify the soldier walls/columns under the floor too to get them ready to receive the beams.

So - with that in mind would it be best to just start calling builders and ask them to scope up the work and do it by their best judgement, or consult an architect or go straight to a structural engineer for them to calculate the point loads/beam spec/block orientation and screed/reinforcement thickness?

I believe the regs just say 100mm slab, ideally with mesh. I can find a few references to suspended garage floors, but not detailed regs on the spec/system.

If there's a better system to get a stronger floor (steels/etc) then I'd welcome advice on this. Its a bit of a catch-22 as I need a particularly strong floor as this will be more than just a parking area, but the thicker it is, the less head height I have at both levels.

Any advice would be very welcome. Thank you.
 
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I think at the very least you will need someone to assess if adding the beam and block, screed and then the weight capacity you want will be supported by whatever is below.

Can get spendy quick as the surveys of the existing garage may reveal other issues, with drainage and suchlike.
 
Could you solve your issues by overlaying the floor with steel plate, painted nonslip? That could mitigate point loads and flammability from spilt liquids?
 
Thanks guys.

the space below is up for grabs, we can re-use or double up/improve on the soldier walls to support concrete beams or I beam.

unfortunately the floor needs to be replaced wholesale: it effectively has domestic floor joists with two layers of plywood on top. It’s all coming out: the structure itself is usual breeze block - the steel is a good shout on the point load but would have to go down over a new floor.

Having been back to have another look, my basic plan is for the whole floor and joists to be removed. Thereafter I’ll have the brickies in to build the supporting leaves (where it can’t be tied into the existing blockwork) and then either:

1) block and beam. In this case I’m told I can take photos and drawings with anticipated weights/ point loads to the beam suppliers and they can do some basic calcs to give me a design (or warn me against it)

2) alternatively I’m told that steel I beam is often used in this case with special corrugated steel stet over the top to receive the slab pour which is reinforced.

Feel a little more informed now, so guess the next step is either a structural engineer or straight to a block and beam supplier. Once I have some form of plan I can start getting quotes.

been crapping myself over this for a while now as I’d never ever seen a garage with a suspended floor - couldn’t imagine it. Until it dawned on me that every multi-storey car park is like this: massively more heavily engineered but correspondingly more massive spans, so must be a way to scale it down.
 
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I've done a few suspended garage floors, they've all been beam/block so you're on the right track, you'll need a structural engineer to asses existing supports/floor, they may just be able to beef up what's there. If beam/block they'd design appropriate supports and specify the beams/block/centre's/sizes/screed thickness and then ultimately the beam/block supplier will do some design work as part of their appointment.
 
That’s reassuring to know! Thank you.

I’ll line-up the structural engineer for the supporting structure and then the beam company to do the layout.
 
I always regret never having a pit in my garage - might it be worth considering including one in the design of the suspended floor?
 

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