Foundation height issue

[iwanttodiy]

Guys, sorry for creating a new post about foundation hight. However I am feeling uneasy about the height of the foundations which will be poured on Monday.

I am looking to get a suspended timber floor installed so that the floor is the same throughout the house and thus allows passage of air in the void. I understand there are building regs relating to minimum void height below joists, and wall plates etc. Thus to get my designed floor height I need the top of the foundations to be at least 300mm below ground level, meaning the trench should be 1.4m deep. Allowing 1m for the trench fill foundation. (As seen in drawing and as specified by SE as 600x1000 C35 concrete)

However, the contractors have only dug to a max of 1.1m below ground level (one side is 1m below ground level), though I paid extra to get it dug to 1.4m. I’ve been told “don’t worry, it won’t be a problem for your floor”. Unfortunately they have booked the Concrete to pour the foundations in 2 days.

Given the state of things, the foundations will be approx ground level. This effectively means once the joists go in, the joists closest to the foundations would effectively be sitting on top of the foundations with no void.

How big of a problem is this? And is there a way to install the joists without raising the floor level once the foundations are in?

 

[Notch7]

Do you need 1 metre depth of foundation?

Did the building inspector approve depth?

 

[^woody^]

How big of a problem is this?

Very big.

Don't pour so much concrete in!

I don't think you quite grasp foundation design. You don't need 1m deep of concrete but you may need the foundation to be 1m deep. The two things are different.

One of the things you don't want to mess up is the foundation height so you need to think about things a little before filling the trench.

 

[iwanttodiy]

I thought that, so long as it’s 1m deep, due to it being clay, and obviously building regs requirements, the foundation itself doesn’t have to be fully filled to 1m. But when I asked the builder if he can reduce the thickness of the trench fill so that it is not 1m deep of concrete, however his feedback was it must be 1m deep as that is building regs.

Funnily it would save him money as he doesn’t need to purchase so much concrete. I’m surprised he hasn’t jumped at the opportunity. Unless of course he’s thinking he needs to pay someone labour to lay more bricks.

 

[iwanttodiy]

Do you need 1 metre depth of foundation?

Did the building inspector approve depth?

Yes, had BCO visit, they are fine with the depth of trench, next visit DPM ...

 

[Notch7]

In general, foundation depth means depth from ground level down to bottom of trench.

The depth of conrete doesnt need to be very deep.

Trench fill is a modern construction method, especially since ready mix. It used to be the case that a footing was poured, then blocks or bricks laid to get out of the ground.

Im not saying you dont need 1metre depth, it may be needed for a reason

 

[iwanttodiy]

I’ll check with my structural engineer who specified 600x1000, but I can already anticipate his answer. Build it to his specification ...

I’m half considering just asking the builder to order less concrete and let him pocket the difference...

 

[HERTS P&D]

The height of the foundations should be '4 bricks below damp'

Andy

 

[^woody^]

But when I asked the builder if he can reduce the thickness of the trench fill so that it is not 1m deep of concrete, however his feedback was it must be 1m deep as that is building regs.

Your builder is a fool. And he won't get any better as things get built - and neither will the building.

Your concrete could be 200mm deep.

 

[Notch7]

however his feedback was it must be 1m deep as that is building regs

I cant think your builder meant 1 metre depth of concrete. Surely no builder would think that?

It is standard for building works quotations to say 1m deep x 600mm wide footing - its like a basic standard used for pricing, reality is determined by the BCO.

Depth could just be 800mm in hard ground, or 2.5m in shrinkable clay.

The depths though still refer to foundation depth. The concrete doesnt have to be that depth, its just cheaper these to mass fill trenches rather lay blocks below ground.

 

[iwanttodiy]

well my builder was the one who suggested to me to excavate deeper if I wanted the top of the trench lower. Which I was effectively charged more for.

 

[^woody^]

well my builder was the one who suggested to me to excavate deeper if I wanted the top of the trench lower. Which I was effectively charged more for.

LOL. What a knob. (the builder)

Any jobs going at you place, BTW?

 

[Notch7]

Yes, had BCO visit, they are fine with the depth of trench, next

If the depth is sufficent, the concrete pour needs to be done to a height that suits your construction. It isnt dictated by the depth of concrete.

Foundations are dug deep to reach stable ground with s high load capacity. They arent deep for a need to have x depth of concrete.

As Woody says, 200mm can be sufficient in some circumstances.

If you have a structural design requiring acmetre depth, then maybe thats needed......we dont have the full information.

 

[Footsoldier888]

On your plans there is a 21cm void under the joists but nothing showing how it is vented to the outside, you are like 50-60cm below ground level at this point.

I see your foundation concerns but how does it work exactly, airbricks on the patio or something???

 

[iwanttodiy]

On your plans there is a 21cm void under the joists but nothing showing how it is vented to the outside, you are like 50-60cm below ground level at this point.

I see your foundation concerns but how does it work exactly, airbricks on the patio or something???

The design I showed was more for measurements.

I have attached the same design noting the mechanisms.

Yes, there will be a 3x telescopic air vents about 150mm above raised ground level. At 2m centres. This has been checked and ok’d by BCO. As the rest of the house is also timber void, it will have cross ventilation from back of the house to front of the house.