Garage floor slab & pit

[Scorpio]

Hi guys
I am about to build a garage 20' x 30' and want to incorporate a concrete pit approx 12'L x 3'6"W x 4'D, so I can sit on a stool.
I intend to lay a sub base of approx 4" MOT then DPC the 4" of concrete with a re-inforcing mesh at half depth.
My question is how do I do construct the pit and when in the construction phase?
I am a reasonably good DIYer but don't necessarily know 'building terms' so please explain fully.
Thanks
Scorpio

[Scorpio]

Hello
Someone out there must have had to do this before - I am getting closer to getting the sub base down but dont want to do something stupid that is a lot of effort/cost to change.
Please help.
Thanks
Scorpio

[Stuarty]

sounds like your best option would be constructing the pit with blockwork. Lay the blocks on their side, because there is fair bit of concrete needed for that floor. Basically dig out the pit area, get all founds etc in for it, and build your blockwork. Build it to the finished height of the concrete. Once the blockwork is done, leave it for a couple of days, and get the surrounding area prepared for the concrete. Once your happy the wall can take the weight, its time to call in the jigger

Alternatively, you can construct the pit with shuttering, dig your hole, but to be honest, i prefer blockwork, mainly becuase i have done all too much shuttering, someone else may have a better idea of what you would need to do

[seanandruby]

dont forget to rebate arond the top so you can cover pit when not in use. also allow for a sump hole in the base.also ducting for cable etc.

[Tony McC]

You build the pit first, then cast the slab around it. As Stuarty said, used dense CMUs or 225mm Engineering brickwork for the pit. It is possible to cast the pit as part of the slab, but it's not a job I'd recommend for a DIYer, unless you were completely at home with formwork.