Starting foundations, what Final height?

[Mr Chibs]

Morning all.

I’ll be helping a mate who has decided to build his own extension.

Mate’s intention is to fill the trench (dug already) to cut down on block work.
He’s mentioned laying trench blocks to start with, and continuing up with block work.

How do you work out What will be the correct height for the foundations to finish at?


Tia

 

[noseall]

How do you work out What will be the correct height for the foundations to finish at?

You look at the existing DPC level and work down from there. If you are re-building in metric and the existing is imperial, ignore their coursing and set yours accordingly.

An Ideal measurement down from DPC to top of concrete would be say 525mm. This allows bottle gullies to sit without having to mess about with the concrete and also means that you can be left with a single course of bricks on the inner skin (525mm works two courses of block and one of brick on the internal skin). This single course of bricks becomes a boon when dealing with the floor screed later on.

If you want to be really tight and the site is nice and level, you can get away with having the concrete higher i.e. 450mm down from DPC, but its slightly more risky.

 

[Mr Chibs]

Much appreciated for the detailed reply.

Mate, can be a bit slap dash, and it’s been worrying me, that we start wrong and it’ll be a battle the whole way up.

Any other nuggets shared are appreciated

 

[noseall]

How do you work out What will be the correct height for the foundations to finish at?

Just to add: Some existing brickwork/floor levels are far from being level themselves. Always look at where you are going to be doing the knock through etc and where you will be coursing up against the house, when deciding where to measure from.

 

[thomp1983]

As noseall has said, you need to carefully check the existing coursing, our 1970's house lost a whole brick over 12 courses and as such the new extension doesn't line up, fortunately we've managed to lose the obvious joins behind drain pipes

 

[Mr Chibs]

Another question if I may.

Inspector has been out, and asked for rods/bars to be drilled into existing foundations and left protruding, to be encased in the new pour.

Is this standard practice, as it was a new one on me?

He said it was ok to resin in... was thinking drilling 200mm deep (400mm total bar length)

Thanks

 

[^woody^]

Has the inspector just left Uni?

 

[Mr Chibs]

Has the inspector just left Uni?

Take it that’s a no then

 

[noseall]

Another question if I may.

Inspector has been out, and asked for rods/bars to be drilled into existing foundations and left protruding, to be encased in the new pour.

Is this standard practice, as it was a new one on me?

He said it was ok to resin in... was thinking drilling 200mm deep (400mm total bar length)

Thanks

I've been asked to underpin existing (shallower foundations), in particular if there is a point load (steel beam etc) being fitted above, later on.

 

[Mobo_14]

Another question if I may.

Inspector has been out, and asked for rods/bars to be drilled into existing foundations and left protruding, to be encased in the new pour.

Is this standard practice, as it was a new one on me?

He said it was ok to resin in... was thinking drilling 200mm deep (400mm total bar length)

Thanks

Been asked to do this alot lately even when the foundation was 400mm below ground level and a slab of 150 year old sandstone.

just take a photo of it to pacify them if they ever ask about it…..which I doubt they will.