Hi all.

I'm currently getting the plans drawn up for an attached garage in the side of my house. For varying reasons I can't build a traditional block structure so Its going to be a timber framed structure adjoined to the house sitting on a concrete slab. My architect has come back and specced a 230mm reinforced slab base which seems a little overkill to me? I was thinking 150mm would be plenty? Or has he beefed it up because the wall structures are being built on the slab too?

I'm not averse to overengineering if it's required. It's just the deeper the slab the more I have to dig out!!

For my 6m x4m timber frame I dug 600mm trenches around the outside and loaded with concrete for the dwarf walls, then infilled with 200mm reinforced slab on 150mm wacked down type one.

I moved I would gues 10 yards of soil, bloody hard work even with a little digger.

Previous house had a 3m x3m shed when purchased, after a couple of years the slab cracked, this was around 200mm thick without any mesh.... Scared me so did not want to repeat the same problem.

Cost wise it's not that much more for peace of mind.

As with many things the answer is depends, what is the ground like? , any history of subsidence?, is it clay?, is it rocky? will it be single story or in the future will you want to build up? what in the future are you likely to put in it? heavy lathes/machinery or just a car?, obviously the architect has to an extent to plan for possible future use not necessarily by you. Assuming good ground I would put 150mm of well compacted hardcore and then 150mm of concrete with added fibres as a minimum for a garage floor so that would be just as much dig out as your situation. hth

OK I probably should have given some background. It's going to be a 3.2 x 6 m single storey built onto the side of our detached house. It has to be a slab Base due to the sewers below, or we'd have to dig down 3m to install lintels over the sewer at great expense. Hence the choice of timber structure. No intention to build upwards. No particular heavy equipment just my impending 7 build and maybe a bit of tintop tinkering.

The ground is quite decent according to the architect. No subsidance or movement history but then again the houses are only 18 years old.

[Edited on 19/8/15 by sdh2903]

Ah well, if it's going over the top of a sewer run that explains the thicker slab and reinforcement, in that situation I wouldn't think it's particularly over specced

The mesh is only an extra £100, silly not to throw in a couple of sheets

Yep the base includes 2 layers of a193 mesh as required by scottish water. It would have been going in anyway for piece of mind.