Have i messed up my foundation levels?

[Krogers]

I have been browsing for a while and finally made an account 

 

So I have started my timber frame self build pictures to follow. 

 

The foundations are poured, and I'm project managing the brickies as best as I can but think I may have made an error and wanted to check on here before I embarrass my self and potentially have to take down their work or cut it back with a disc cutter  

 

300mm concrete trench blocks have been laid so far but I decided to alter the floor level slightly. 

 

I told them to build 900mm of trench block which would off took me to ground level,  I then decided to get them to add another 225mm of trench block as I felt the plot sat to low but this is where I think I may have gone wrong I should of got them to form the cavity @ 675mm then continue with 4inch block to take me to 900mm and not continue in trench??? 

 

As now to form a cavity ill be increasing the plot by another 225mm??

 

Or can I cast my 150mm concrete slab into the trench block they’ve already built and bring my dpm over the trench block and then form a cavity off that trench block going up? 

 

Ive got 150mm reinforced concrete slab followed by 120mm insulation & 80mm of screed so there will be another dpm/dpc to follow after the trench??

 

I know they say getting out the ground is the hardest!!! I see why now 

 

Keith

[Temp]

I think Building Control normally want 225mm of cavity below the dpc.

 

Google suggests this can be reduced to 150mm if weep holes are provided.  At least the nhbc say so here...

 

https://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part5/section1/sitework.htm

 

That means you can/should start the cavity at ground level or below as the dpc is typically at 150mm above ground level.

Edited March 15, 2022 by Temp

[Gus Potter]

41 minutes ago, Krogers said:

I have been browsing for a while and finally made an account

Keith..Welcome then.. lot's of interesting stuff here.

 

Don't panic! Draw what you have on a bit of A4 paper and what you had originally . External ground levels, the brick / block coursing, the kit, the floor, suspended or ground bearing slab etc. Take a photo or scan and post. Don't worry about scale as just draw the blocks and say their size. Lastly it's not a drawing test, just convey the info as best you can.

 

Gus

 

 

[Krogers]

Please see rough sketch attached its a ground bearing slab not block & beam

 

 

[Krogers]

PIcture of trench block

 

 

[Krogers]

18 hours ago, Temp said:

I think Building Control normally want 225mm of cavity below the dpc.

 

Google suggests this can be reduced to 150mm if weep holes are provided.  At least the nhbc say so here...

 

https://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part5/section1/sitework.htm

 

That means you can/should start the cavity at ground level or below as the dpc is typically at 150mm above ground level.

 

 

What are we classing as DPC as there is another dpc above which has got me confused

 

--------------------FFL top of screed

    ]

[---     if i bring my visqueen up and over?

-------------------GL @ top of trench currently

 

If i can form my cavity on top of the trench there will be another 225mm of block work above?

 

Have i over complicated all this

 

Keith.

Edited March 16, 2022 by Krogers

[PeterW]

12 hours ago, Krogers said:

Please see rough sketch attached its a ground bearing slab not block & beam

 

 

confused here - what is your issue ..?? Do you have a site height limitation ..?? And what is your outer wall..? Brick or render on block ..?

 

If you’re going for insulation and screed then get the brickies to do a double row of blocks / brick out block inner with your cavity and then they will back fill that. You’ll then need to make sure you have enough room for a 125mm of MOT1 and then compact / blind this and put a DPM and cast your concrete to the top of the inner block. 
 

From there, bring your inner block up another level, bring the DPM up and over that and that is now your FFL when you infil with insulation and screed.  

[Krogers]

39 minutes ago, PeterW said:

confused here - what is your issue ..?? Do you have a site height limitation ..?? And what is your outer wall..? Brick or render on block ..?

 

If you’re going for insulation and screed then get the brickies to do a double row of blocks / brick out block inner with your cavity and then they will back fill that. You’ll then need to make sure you have enough room for a 125mm of MOT1 and then compact / blind this and put a DPM and cast your concrete to the top of the inner block. 
 

From there, bring your inner block up another level, bring the DPM up and over that and that is now your FFL when you infil with insulation and screed.  

Hi Peter, 

 

 no height limitation, outer wall is render

 

I was hoping to use the concrete trench within the slab then form the cavity on top of the slab 

 

but what your saying is I should put a 4’ block around and use that to cast my slab into?? 

[PeterW]

1 hour ago, Krogers said:

was hoping to use the concrete trench within the slab then form the cavity on top of the slab 

 very non standard ..!!! Talk to your BCO about what they want, you need to use the blocks to keep level and also remember that a cast floor isn’t structural unless it’s been designed that way so building your entire internal load bearing structure off it is a bad idea !!

[Krogers]

150mm type 1 

50mm sand 

radon membrane 

a252 mesh

150mm c35

followed by 

120mm insulation 

80mm of screed 

 

is my floor make up 
 

My issue was forming the cavity @ GL hence the original query of forming the cavity within the trench block As I have built the trench block to high and didn’t want to cut them or knock them down!!! 
 

so the original gl for the slab was 60.00 

I’ve added an additional 60.225 as I asked them to so new GL 60.225

 

I should of formed the cavity @ 59.775 hence asking if I could form the cavity 

so now I have to form the cavity @ 60.225 ( above the trench block) 

 

which by the time I add screed and insulation takes me to 60.500 FFL ?? 

Which is fine 

I either have to take down the trench block and form the cavity or put a 4’ block around and lap the dpc over. 
 
But I wondered if I could lap up over the trench and start the cavity on the trench

 

hopefully that makes some sence 

 

pictures may be easier tomorrow when I’m back to figure this out