Guidance on best approach for excavation / foundations

[flanagaj]

Our proposed dwelling is as below (the steps down will be where the man is).  The garage floor level is 400-450mm higher than the floor level in the house and I am trying to understand how to do the required excavation / concrete pour. I was not planning on using beam and block due to the spans.  As a result, is it simply the case that the outer wall of the house that adjoins the garage will have to have some sort of tanking  to stop damp or should I go beam and block in the garage so that the oversite is below the dpc level of the house? 

 

[Nickfromwales]

I would have assumed that 'good' groundworkers would be able to pour that split-level slab with a 4-500mm step in it, in one hit, by managing slump and shuttering it to suit. Have you spoken to any preferred contractors about this yet?

 

Has your architect dealt with the sloping floor of the garage, to cope with fuel spills, or another method such as a bund (reduced floor level) to meet compliance at the wall party to the residence?

 

Any part that is subterranean will need to be waterproof concrete and tanked externally, but 4-500mm is not a huge issue. Are you using ICF? That would make this a lot easier to do tbh.

[Conor]

Make life simpler and better, have it all the one level. Drop one by 200mm, raise other side by 200mm. Keep ridge heights the same. Would it make a big change to the appearance of the house from the road? (i.e. would anybody notice/care?)

[Russell griffiths]

  On 04/04/2025 at 09:56, Conor said:

Make life simpler and better, have it all the one level. Drop one by 200mm, raise other side by 200mm. Keep ridge heights the same. Would it make a big change to the appearance of the house from the road? (i.e. would anybody notice/care?)

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This. 
looking at lots of localised flooding i would lift the house up, i would not want any surface run off from surrounding land coming down onto my house without a good way of the water running around the house and out the front onto the road. 

[saveasteading]

You are right to minimise excavation. I'm struggling to see what the problem is....standard building construction should be sufficient. 

 

Except "where the man is". If that area is  against the house wall then it needs tanking and an unusually high dpc.

But keep that raised area away from the wall and it is simpler.

[flanagaj]

  On 04/04/2025 at 09:09, Nickfromwales said:

I would have assumed that 'good' groundworkers would be able to pour that split-level slab with a 4-500mm step in it, in one hit, by managing slump and shuttering it to suit. Have you spoken to any preferred contractors about this yet?

 

Has your architect dealt with the sloping floor of the garage, to cope with fuel spills, or another method such as a bund (reduced floor level) to meet compliance at the wall party to the residence?

 

Any part that is subterranean will need to be waterproof concrete and tanked externally, but 4-500mm is not a huge issue. Are you using ICF? That would make this a lot easier to do tbh.

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"Has your architect dealt with the sloping floor of the garage, to cope with fuel spills, or another method such as a bund (reduced floor level) to meet compliance at the wall party to the residence?" Haha, I doubt that very much!!  I wasn't aware that there did need to be a sloping floor away from the house.

I was planning on doing the footings myself.  It's not rocket science and I have a good laser level and would simplify matters by hiring a pump truck when they are poured.  But do i actually need to have a step in them?  Can they not be all one level, and it just means that there are more blocks used to come out of the ground for the garage section?

[saveasteading]

  On 04/04/2025 at 12:06, flanagaj said:

sloping floor of the garage, to cope with fuel spills,

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You don't need a slope. Cars rarely leak. Commercial garages don't have slopes except at wash areas. So it's your choice.

But you do need to keep the rain from getting under the garage doors. My standard detail has a ramp coming a good 100mm inside of the door, to let wind blown water run back. You can't rely on the bottom seals. It's best to shutter that off when pouring the slab, and deal with fiddly door details later.

[Nickfromwales]

  On 04/04/2025 at 12:06, flanagaj said:

  On 04/04/2025 at 12:13, saveasteading said:

You don't need a slope. Cars rarely leak.

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  On 04/04/2025 at 12:13, saveasteading said:

So it's your choice.

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Methinks not! Garages usually have a sloped floor or a step down of 100mm, for the few that do or did leak, to create that rule.

 

"Has your architect dealt with the sloping floor of the garage, to cope with fuel spills, or another method such as a bund (reduced floor level) to meet compliance at the wall party to the residence?" Haha, I doubt that very much!!  I wasn't aware that there did need to be a sloping floor away from the house.

I was planning on doing the footings myself.  It's not rocket science and I have a good laser level and would simplify matters by hiring a pump truck when they are poured.  But do i actually need to have a step in them?  Can they not be all one level, and it just means that there are more blocks used to come out of the ground for the garage section?

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Just click on and highlight text in a post and then you'll see the option to "quote selection"  

 

  On 04/04/2025 at 12:06, flanagaj said:

Haha, I doubt that very much!!  I wasn't aware that there did need to be a sloping floor away from the house.

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Well, you'd defo need that if there's a doorway, but maybe not if there's just a wall and no shared roof space for fire to breach through to the house within. BCO and your architect should be approached now to address / identify this requirement. Door way to garage or not, assuming not due to levels, but there's nowt as queer as folk!

 

  On 04/04/2025 at 12:06, flanagaj said:

Can they not be all one level, and it just means that there are more blocks used to come out of the ground for the garage section?

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Are you referring to strip foundations and then dwarf walls for back-filling, then the garage structure atop? Tanking would be 'complex' to say the least. Walls would be block on flat then, or otherwise suitably beefed up, to deal with the outward forces from the back-fill.

 

A lot more excavation, muck-away, and back-fill, so wouldn't be my first choice solution.

 

What are you building with? Standard masonry walls with a cavity?

[flanagaj]

  On 04/04/2025 at 12:36, Nickfromwales said:

Just click on and highlight text in a post and then you'll see the option to "quote selection"  

 

Well, you'd defo need that if there's a doorway, but maybe not if there's just a wall and no shared roof space for fire to breach through to the house within. BCO and your architect should be approached now to address / identify this requirement. Door way to garage or not, assuming not due to levels, but there's nowt as queer as folk!

 

Are you referring to strip foundations and then dwarf walls for back-filling, then the garage structure atop? Tanking would be 'complex' to say the least. Walls would be block on flat then, or otherwise suitably beefed up, to deal with the outward forces from the back-fill.

 

A lot more excavation, muck-away, and back-fill, so wouldn't be my first choice solution.

 

What are you building with? Standard masonry walls with a cavity?

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The garage is a flat roof and I want standard cavity walls for the garage as well.  Plan was to also to use strip foundations.

[Mr Punter]

Bear in mind that Building Control may (and should) ask for step free access from the car to the front door.  Part M.  Failing that you may end up with a very long ramp.

[Nickfromwales]

  On 04/04/2025 at 15:09, Mr Punter said:

Bear in mind that Building Control may (and should) ask for step free access from the car to the front door.  Part M.  Failing that you may end up with a very long ramp.

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Good point!