Building Foundations???

toshy321

Building an extension and just wondering what is the correct width and depth for a foundation. Its only a bungalow so its not storey and a half or two storey. The wall will be made up of two 4 inch solids and a 4 inch cavity between, reading up on other forums they say minimum width of foundation is 3 times width of the wall your building so my my wall would be 300mm x 3 so 900mm wide. Not sure of depth but i'd imagine 300mm to 600mm would be norm thanks in advance for any replies.

Carlow52

900 is a good starting point, depends on soil conditions: one of the buckets on a jcb is a metre I think so....
the depth of the founds below the frost line is important so 300 or so with a A142 mesh or rebar below the frost line will work, assuming good soil and a low water table

feckincrazy

Yer bang on with three times the width x the depth. As yer man said use mesh. where ya can. sit the mesh up on stock bricks or waist blocks while poaring the foundation. If ya want an automatic laser they can be rented from trinity hire i think. Leave the foundations for a week before ya lay the first coarse of blocks.

toshy321

Thanks for replies lads still not sure whether to go for 3foot or 2 foot someone was telling me 2 foots is adequate enough for two 4 inch blocks and 4 inch cavity so Im back too square one.

gman2k

toshy321 wrote: »

Thanks for replies lads still not sure whether to go for 3foot or 2 foot someone was telling me 2 foots is adequate enough for two 4 inch blocks and 4 inch cavity so Im back too square one.

Don't take anymore advice from the person who told you that the 2 foot wide foundation would suffice, so, you are not back at square one,
A 3 foot wide by 1 foot deep foundation is needed at a minimum.
Bottom of foundation is generally 3 foot below ground level.
The rising will will be centered on the foundation, ensure when you dig out the trench that there are no spot spots, and no sitting water - if so, pump out.

If you are in any more doubt, hire an engineer for a couple of hours to advise you. It won't cost much these days.

toshy321

Cheers gman im going 3 foot wide now neway thats settled. Wondering about steel in the foundation putting 3 lentghts of rebar in it these will be sat on blocks half way in foundation and tied and overlapped in corners, because its an extension the foundation will be butting up to old foundation does the reinforced bar have to be tied into the old foundation as in drilling holes in and putting bars in.

gman2k

toshy321 wrote: »

Cheers gman im going 3 foot wide now neway thats settled. Wondering about steel in the foundation putting 3 lentghts of rebar in it these will be sat on blocks half way in foundation and tied and overlapped in corners, because its an extension the foundation will be butting up to old foundation does the reinforced bar have to be tied into the old foundation as in drilling holes in and putting bars in.

Personally I would never tie one extension onto another existing building, as there will always be some form of settlement.
It's better to use an expansion joint between the two structures, and to 'Hide' this joint with a RWP or similar.
There is a system by Ancon that is suitable for what you are doing, it ties the two structures together, but allows some vertical movement.

Seriously though, I'd get an engineer for a couple of hours, will cost less than the steel you are proposing in your founds (which may not even be necessary).
If you need a name, I'll PM you a couple of numbers.

toshy321

No boder, was thinking along the lines of settlement myself it would be wrong to tie into old foundation.

rayjdav

As said above, hire an engineer for a site visit. Worth every penny in the long run.
Stop listening to Billy down the bloody pub about building advise. He'll tell you all sorts of wonderful ways to build YOUR extension.
Re the mesh noted, I would have a tendancy to upgrade mesh to A343 in foundations and A142 in floor slabs. Mesh not to be placed on blocks, half way up. Lay them with max 75mm cover from base of foundations.
Once again, get an engineer to validate it though.

Merch

gman2k wrote: »

Don't take anymore advice from the person who told you that the 2 foot wide foundation would suffice, so, you are not back at square one,
A 3 foot wide by 1 foot deep foundation is needed at a minimum.
Bottom of foundation is generally 3 foot below ground level.
The rising will will be centered on the foundation, ensure when you dig out the trench that there are no spot spots, and no sitting water - if so, pump out.

If you are in any more doubt, hire an engineer for a couple of hours to advise you. It won't cost much these days.

1 foot deep is sufficient? I'm doing a shed and was planning on going more than that, Im not sure though.

You say the bottom of the foundation trench is 3 foot deep, then pour 1 foot of concrete, I presume you mean raise the rest up with solid blocks on their sides in a a number of (2 or 3) courses. I'll be doing a shed in the summer, so this is info I'm looking for myself, not hijacking.

Carlow52

Merch wrote: »

1 foot deep is sufficient? I'm doing a shed and was planning on going more than that, Im not sure though.

You say the bottom of the foundation trench is 3 foot deep, then pour 1 foot of concrete, I presume you mean raise the rest up with solid blocks on their sides in a a number of (2 or 3) courses. I'll be doing a shed in the summer, so this is info I'm looking for myself, not hijacking.

that's the normal approach for the blocks.

As noted the width depends on having good ground at the 3 ft level

toshy321

Thanks for replie rayjay just wondering what is so wrong with placing the rebar on the blocks is it not as effective or what??

gman2k

toshy321 wrote: »

Thanks for replie rayjay just wondering what is so wrong with placing the rebar on the blocks is it not as effective or what??

Minimum 'cover' for rebar is 25mm, any more than 50-75 and the steel is useless, as it's not correcting the tension forces in the base of the foundation.

Merch

gman2k wrote: »

Minimum 'cover' for rebar is 25mm, any more than 50-75 and the steel is useless, as it's not correcting the tension forces in the base of the foundation.

What height above the base of the foundation trench should the steel (rebar) be? and what diameter should it be for a shed?

Do you mean between 25 - 50/75 mm from the base

rayjdav

Merch wrote: »

What height above the base of the foundation trench should the steel (rebar) be? and what diameter should it be for a shed?

Do you mean between 25 - 50/75 mm from the base

As Gman2k noted, from 25 to 75mm. If you have any old patio slabs lying around, break them up to use as "chairs" for the steel, let it rest on these and you'll be grand.

Something in the line of A343 as I said above should be grand.

If you have any concerns about foundations, seriously, ask an engineer to look at it. Grand getting advice off the record, but if things went pear shaped down the line, for the sake of a few quid, I know what would be good money and well spent.

Merch

rayjdav wrote: »

As Gman2k noted, from 25 to 75mm. If you have any old patio slabs lying around, break them up to use as "chairs" for the steel, let it rest on these and you'll be grand.

Something in the line of A343 as I said above should be grand.

If you have any concerns about foundations, seriously, ask an engineer to look at it. Grand getting advice off the record, but if things went pear shaped down the line, for the sake of a few quid, I know what would be good money and well spent.

Whats A343? I'm only doing it for a shed for myself, not worth my while paying for an engineer really, I'm reasonably handy but just want to get some pointers. I know you said Gmak noted 25-75, I just wanted to clarify if that meant 25-75mm from the base of the found trench, as "cover" & "chairs" doesn't mean anything to me as I've not come across these terms regarding foundations.
I've also been told previously not to break stuff up and put under the poured foundation cement? Initially thats how I planned to get rid of my few existing slabs, as years ago, always seen people pour driveways over broken blocks/paving slabs etc but here recently been told not to do that(put in broken blocks/slabs) under foundations, I'm assuming the odd block to support the rebar is ok then?

harry21

To the OP can I just suggest maybe increasing the cavity size to allow more insulation (even if he/she plans to insulate internally). As mentioned already 3 times the foundation is the standard. If usings rebar use 5no. 12mm bars or mesh if you wish. How long is the wall?

Ensure adequate cover to prevents rusting of rebar (the minimum depends of the maximum size of aggregate and nothing else) and use proper rebar chairs (thier cheap anyway). Having a excavator operator who is good will make a huge difference too regading waste concrete and ensureing adequte cover all round.

Regards tieing into the house, if the extension is small I would'nt worry about settlement unless the groud conditions indicate that there might be a problem. It is very rare to tie the foundations together, but ensure you tie the walls together well.

Hope all works out for you...

toshy321

Thanks lads for all the help so far was looking for some advice on the insulation for the ground floor before I pour my concrete screed as far as I know its 75mm insulation just would like to be certain though cheers.

toshy321

Bump, anyone any experience in this area (insulating under concrete floor)

harry21

Part L recommends 100mm insulation under 150mm concrete and nominal (say 25-50mm) at the edge of the slab to prevent thermal bridging.

heinbloed

There is an " Eurocode " to which one should stick when working with reinforced concrete. What Harry21 said concerning the size of the aggregate determining the coverage

Ensure adequate cover to prevents rusting of rebar (the minimum depends of the maximum size of aggregate and nothing else) and ....

is not correct on it's own. The environment plays the vital role when building in damp conditions i.e. underground.

If the reinfoced concrete is burried in non-agressive soil the min. coverage should be 30mm with concrete strength class C45/55, that would be exposure class 2a and 2b. And this minimum coverage has to be the absolute minimum incl. the wires or any other metal.

Where the concrete is exposed to frost and/or de-icing salts (exposure class 3) the minimum coverage must be 35mm, again for the minimum strength class of the concrete of C45/55.

These are the min.coverages, if the concrete is weaker or the environment even more agressive (as with salt water or salt air for example) the coverage has to be better.

Contact a civil engineer, the foundation is usually the only part of the structure which will never be seen and inspected again.

toshy321

Cheers for replies lads in relation to the 25mm-50mm around edge of concrete slab for thermal bridging could anyone explain this a bit better what purpose is this insulation around the slab.

gman2k

toshy321 wrote: »

Cheers for replies lads in relation to the 25mm-50mm around edge of concrete slab for thermal bridging could anyone explain this a bit better what purpose is this insulation around the slab.

it's to prevent thermal bridging......at the edge of the concrete slab....

harry21

gman2k wrote: »

it's to prevent thermal bridging......at the edge of the concrete slab....

Yes:D