footings and slab

13 Apr 2007
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United Kingdom
Hi all.

I am after as much info, diagrams and step by step guides as possible before i embark on my garage extension.

I searched and searched but im yet to find one website which nails exactly what im after.

I need help with footings and how to prep and pour the slab. I have a general knowledge of DIY but having never seen a slab prepared its very hard to get my head round especially with regards to DPC etc.

One thing that is confusing, the weak concrete in the cavity is to be poured level with the ground surface yet my slab in the orginal house is level with the outside ground surface. Does that mean the weak concrete is ok to be poured so its level with the ground surface and the slab, doesnt DPC come into play here?? My new garage slab has to be level with the house slab due to the garage being turned into an extension 5 years down the line.

Hope you knowledge people can help and guide me.

Cheers all

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can the weak concrete infill (goes in the cavity) be level with the ground surface and the concrete slab??
As nose said, but the dpc has to be at least 2 brick courses above exterior ground level.

If you are planning on converting this garage in the future then you will need insulation under your concrete, you should have designed this into the slab.
Otherwise, the best way to proceed IMO is to have the concrete slab at least 150mm below existing house floor. That way you can simply add insulation and screed, or insulation and floating chipboard deck on top of the concrete slab, to bring the floor up to the same level.

You should take into account ceiling height in relation to existing, plus height of any windows etc once the floor gets raised up when you come to do the conversion.

Any doors, other than the main garage door? These will need to be altered when you convert, so sit any lintels 2 courses higher than the door frame and infill the gap with brick/block.
Thanks for your reply.

Im planning on insulating under the slab rather than on top.

I was just trying to get my head round the weak concrete part. Spoke to a builder today and he looked at me as if to say 'what you on about'. He simply said 1m trench, 850mm concrete fill, brick up for dpc making sure two bricks above ground level JOB DONE!.
850MM concrete sounds way over the top, i believe they call this trenchfill. I was hoping for a strip footing mainly to save some money. BCO will determine which method is best, hopefully strip is his reply.

Whilst im at, concrete: which concrete is best for footings? im hoping a leveling concrete is available to reduce the chance of error. Which is your best option guys??

Thanks all your help

BCO will not determine which is best, they are simply alternative methods, trenchfill being used for speed as opposed to block footings. If you are laying the blocks yourself block footings will be much cheaper but if your paying someone to do it then it may work out cheaper to mass fill.

Simply ask for a foundation mix and they will give you the right stuff. there are many different types of concrete and so ready mix suppliers will be able to help you. For example you'd want a different mix for the floor slab etc.

All concrete is levelling to a certain degree. Just gets your pegs set up firm and accurate and rope in as many people as possible to help and you can't go to far wrong. The worst thing in the world is to be 'chasing' conrete as it goes off becuase some mis-hap has delayed you due to poor preparation.
You could probably do strip footings and use trench blocks to come up to just below ground level.

Trench fill is quicker but slightly more expensive, though maybe not a lot in it.

You might use trench fill when you're on a slope and don't want the bother of stepped footings. (Trench fill to lower part of site, blocking up on higher part of site.)
you lot are great, thanks a lot.

Im liking the idea of trench blocks because im doing the brick work myself.

Preparation is a massive part of doing my footings, slab etc. All i have done this weekend is read, read and read as well searching this forum for as much advice as possible. Its a cracking forum especially for those who want to have a crack at doing the work themselfs.

Got to get block drawings and sketch to BCO this week, pay my fee and hopefully start digging a section of footings to see what im up against. Its gonna be back breaking but rewarding at the same time.

Im trying to get my step son in the trade so hopefully this project might give him some idea of whats required, he might even enjoy!!

Chaps thanks for eveything. If you have any more advice etc let me have it, nothing wrong with being over loaded, lol


PS, anyone know of a free online planning programs which give drawings etc??

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