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I need to order some footing blocks, what types?


epsilonGreedy

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As a minimum I want to get a course of foundation blocks laid on my trenchfill just poured last Friday. If the heavens open and finish this summer drought, muddy water can then wash into the trenches without causing too much trouble because I expect the trenchfill blocks will keep their upper face above any puddling.

 

The trouble is I am confused by all the block types on the market.

 

Given the following diagram specific to my house what should I order to get up to the beam & block floor? My guess...

 

  1. 200 x Thermalite trenchfill blocks 440 x 215 x 300 (7N).
  2. A large delivery of regular 440 x 215 x 100 dense concrete blocks.
  3. Footing lintels for service entry points.
  4. Telescopic air vents for the inner wall (need some extra reach due to the elevated main dpc).
  5. 100mm DPC roll for the beams to sit on.
  6. Coursing blocks (65mm) for that awkward 150mm part of the inner block wall (to avoid block cutting).

 

Should I amend this order to substitute in thermal blocks to reduce cold bridging near the beam floor?

 

RossDavFooundation.png

Edited by epsilonGreedy
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7 hours ago, PeterW said:

Check your SE spec but everything below the beams should be 7N. What’s your cavity / fill..?

Good advice I was set to use trench block

SE said he didn’t like TB with B.B. floor  and to use 7n concrete 

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My personal opinion of course. 

I wouldn’t use a thermalite trench block if you gave them to me for free!!

i have just had my trench blocks delivered standard weight concrete  7kn 

 

i personally have brought a special block designed for footings that is 300mm wide so you don’t have a 440 block wasted by being laid on the flat, dearer than a standard block, but I think they make a nicer job. 

 

Can you not lay one flat and one stood upright to make up your width. 

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29 minutes ago, Russell griffiths said:

Can you not lay one flat and one stood upright to make up your width. 

That's what my chap wanted. I offered to get in 300s but he was happy to keep'em standard.

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1 hour ago, nod said:

Good advice I was set to use trench block

SE said he didn’t like TB with B.B. floor  and to use 7n concrete 

 

 

Are you referring to just the course of blocks the beams sit on? Or did your SE advise not to use 300mm trenchblocks for the lowest course?

 

My concern with any type of aerated block below ground is that they could suffer from free thaw cycles when damp.

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3 minutes ago, epsilonGreedy said:

 

Are you referring to just the course of blocks the beams sit on? Or did your SE advise not to use 300mm trenchblocks for the lowest course?

 

My concern with any type of aerated block below ground is that they could suffer from free thaw cycles when damp.

 

What is your wall cavity build up ..?? Is it 100/100/100 or have you got a wider cavity ..? 

 

As @Russell griffiths alluded to, trenchblocks are the devil and a pig to work with. As the first course they will need to muck up the corners etc anyway or split where the rounds are high so make it easy and use standard 7N concrete blocks underground. 

 

 

 

 

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8 hours ago, PeterW said:

Check your SE spec but everything below the beams should be 7N. What’s your cavity / fill..?

 

 

Not decided yet.

  1. If I raise an inner block wall to 10ft using Celcon thin-joint in the two months before my facing bricks arrive then blown beads would be the preferred option because the facing brickie would not then be fighting the expansive tendencies of cavity batts as he skins the structure with facing bricks.
  2. Or if my inner block wall is built with thermo blocks in step with the facing bricks then the inherently good U-value of these mean I can use the less dense version of cavity batts. My potential brickie advised that the uber efficient variety of cavity batts expand too much if taken out of the tight manufactured roll too early in the day, then the brickie has to fight the material's tendency to push apart the cavity.

Would I be right in thinking this decision can be delayed until the beam & block is installed but I will need to decide before the facing engineering bricks are taken up to my elevated ( 150mm + 150mm) DPC?

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11 minutes ago, PeterW said:

What is your wall cavity build up ..?? Is it 100/100/100 or have you got a wider cavity ..? 

 

 

Yup. Unless persuaded otherwise in this thread I will be following the diagram posted on the OP.

 

13 minutes ago, PeterW said:

As @Russell griffiths alluded to, trenchblocks are the devil and a pig to work with.

 

 

Ok. I thought they were an expensive labour saving solution.

 

14 minutes ago, PeterW said:

... or split where the rounds are high so make it easy and use standard 7N concrete blocks underground. 

 

 

You mean where my concrete pour is high? I sounded the finished foundations with a borrowed rotating laser and found 50% is +/- just 2mm and 90% is +/- 3mm.

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No because you need to start from the foundations at the correct cavity width. To meet regs from memory you will need 125mm of bead unless you line with PIR or thermaboard, 

 

The thermal difference of “thermal” blocks vs ordinary is negligible anyway, and they cause more problems than they solve when it comes to internal finishing and hanging things etc, not to mention they are about 60% more expensive than standard 4N blocks. 

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1 minute ago, epsilonGreedy said:

 

Yup. Unless persuaded otherwise in this thread I will be following the diagram posted on the OP.

 

 

Ok. I thought they were an expensive labour saving solution.

 

 

You mean where my concrete pour is high? I sounded the finished foundations with a borrowed rotating laser and found 50% is +/- just 2mm and 90% is +/- 3mm.

If the pour is that flat then well done as they can appear flat but aren’t level...!

 

So are you going with thermaline or PIR on the inside then ..? 100mm Rockwool on its own won’t meet building regs now. 

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1 hour ago, Russell griffiths said:

I personally have brought a special block designed for footings that is 300mm wide so you don’t have a 440 block wasted by being laid on the flat, dearer than a standard block, but I think they make a nicer job. 

 

 

Now I am confused, I can picture the layup with alternating flat/upright courses comprised of standard block. It sounds cheap in material costs and avoids pouring a concrete mix into the below ground cavity. What I do not get is:

 

300mm thermalite trenchblocks = bad

300mm special concrete blocks = good

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3 minutes ago, epsilonGreedy said:

It sounds cheap in material costs and avoids pouring a concrete mix into the below ground cavity.

 

You have to fill the below ground cavity for strength - either rigid insulation or weak concrete mix - so is saving you nothing really. 

 

 

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The blocks used here for founds are mainly 350*300*100mm. They are much easier to work with than them headstones you have listed above. If you want a 100/100/100 set up then you build them the 300mm wide way. If you want a 100/150/100 build then you go the 350mm wide route. They are just standard 7n concrete blocks and are no heavier than a 440*215*100mm block.

If you are going for lightweight block on the inside skin further up then you're last course is built in these. So it's just 1 course right round the building. That gives you your thermal break. 

https://stowellconcrete.co.uk/foundation-trench-blocks/

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2 hours ago, epsilonGreedy said:

 

Are you referring to just the course of blocks the beams sit on? Or did your SE advise not to use 300mm trenchblocks for the lowest course?

 

My concern with any type of aerated block below ground is that they could suffer from free thaw cycles when damp.

No he didn’t want them in the footings at all

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Ok I am getting a strong sense that aerated trench blocks are the invention of the devil. I still do not really understand what the problem is but the consensus is obvious. Some elements of the building trade and SE's must find trenchblock acceptable otherwise they would not be manufactured.

 

I now need to rework the block course maths to get to the right beam support height using the 350 x 300 x 100 examples mentioned above. The JCB actually removed an extra 70mm of oversite so I can comfortably maintain a 200mm void clearance height should the beams sit a bit lower (10mm to 20mm) relative to DPC.

 

Furthermore, should I opt for thin-joint and jumbo blocks for the inner wall then there is no need to maintain course height parity with the facing bricks (apart from ho hmm external door thresholds).

 

Or Or, final thought just start the cavity footing with standard 7N concrete blocks sitting on the poured concrete because the finished level has such a small variation?

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2 hours ago, epsilonGreedy said:

Ok I am getting a strong sense that aerated trench blocks are the invention of the devil. I still do not really understand what the problem is but the consensus is obvious. Some elements of the building trade and SE's must find trenchblock acceptable otherwise they would not be manufactured.

 

I now need to rework the block course maths to get to the right beam support height using the 350 x 300 x 100 examples mentioned above. The JCB actually removed an extra 70mm of oversite so I can comfortably maintain a 200mm void clearance height should the beams sit a bit lower (10mm to 20mm) relative to DPC.

 

Furthermore, should I opt for thin-joint and jumbo blocks for the inner wall then there is no need to maintain course height parity with the facing bricks (apart from ho hmm external door thresholds).

 

Or Or, final thought just start the cavity footing with standard 7N concrete blocks sitting on the poured concrete because the finished level has such a small variation?

Trench block are widely used

 it not houses have B.B. floors

It seems to be the B.B. that can cause the SE to spec an ulternative 

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2 hours ago, nod said:

Trench block are widely used

 it not houses have B.B. floors

It seems to be the B.B. that can cause the SE to spec an ulternative 

 

 

This seems to be a common grassroots perception but I cannot find any science to back up the claim. A 2-story masonry house is not that heavy per sq mm so I doubt the overall weight of a house is enough to stress the lowest course of aerated trenchfill foundation blocks to the point of fracture and crumble.

 

I can comprehend that once a beam & block floor is loaded with blocks and 80mm of screed it is heavy and there could be concerns about point loading at the seat points where a beam is supported by an aerated block. However I am not proposing such a layup of the courses, with my original plan the point loads of the b&b floor would be taken by two courses of dense blocks which will transmit tolerable loads onto the lowest course of aerated trench blocks.

 

Maybe the issue is differential thermal expansion where footing block courses are made from a mixture of dense and aerated blocks?

 

What irks me is that everyone is indicating that there is something dubious about lightweight trench fill blocks but the online BM sites are oozing with such examples whereas no one sells dense blocks of the same size probably because if such a beast was manufactured it would be as heavy as three suitcases on the verge of attracting a Ryan Air excess weight surcharge.

 

I think the greatest risk to my selfbuild right now is that I will put my footings brickie team in hospital and end up dealing with their injury solicitor.

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Lightweights are for standard builds - not B&B - so don’t have to be as strong as the load they are bearing isn’t concentrated on the edge of the block. 

 

I’d suggest a chat with an SE who should be advising on this sort of thing would pay dividends. 

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I did a lot of brickwork in the early 90s aerated blocks were the big thing I have seen whole houses built with them, but 20 years later we noticed lots of failures hairline cracking everywhere, 

i just don’t like them, go and buy one and take it home chuck it about for a bit, then decide if you want your house sitting on it for 100 years. 

 

I think the thing with beam and block is a higher loading on the footing wall as opposed to a slab on ground type floor. 

 

The blocks i have brought are the ones @Declan52 pointed out from Stowell Concrete. 

They are on site and I like what I see, but the price was a bit sharp. 

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1 hour ago, PeterW said:

Lightweights are for standard builds - not B&B - so don’t have to be as strong as the load they are bearing isn’t concentrated on the edge of the block.

 

 

Re. the "edge of block" bit, yes I agree which is why I am proposing just aerated foundation blocks with two courses of dense above. The upper dense blocks would carry the immediate point/edge loads of the beam & block floor.

 

Since starting this thread I found the following claim on a product page at H&H.

 

Quote

Available in Standard Grade (3.6N/mm2) and High Strength Grade (7.3N/mm2).  Foundation Blocks can be used to support 3 storey constructions. They are suitable for the support of solid or cavity walls, framed construction or suspended floors, including beam and block.

 

https://www.hhcelcon.co.uk/solutions/by-product/celcon-foundation-blocks

 

Surely one of the major manufacturers of blocks in the UK would not risk litigation over a foundation component with such an unsubstantiated statement.

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24 minutes ago, Russell griffiths said:

I did a lot of brickwork in the early 90s aerated blocks were the big thing I have seen whole houses built with them, but 20 years later we noticed lots of failures hairline cracking everywhere, 

 

 

From my reading up on Celcon thin-joint I understand that aerated blocks are known for settling and causing plastering problems. Are you saying the hairline cracks develop into true failures?

 

28 minutes ago, Russell griffiths said:

The blocks i have brought are the ones @Declan52 pointed out from Stowell Concrete. 

They are on site and I like what I see, but the price was a bit sharp.

 

 

I also checked out their web site earlier, a Britstol based family business. Are their blocks in the general BM supply chain?

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Have you actually priced up - or checked availability of Celcon HS Foundation Blocks ..??

 

H+H Celcon HI-7 Aerated Concrete Block 440mm x 215mm x 300mm 7.3N Pack 30

 

£6.17 EA

 

Availabilty - to order (2-4 weeks)

 

That’s straight off my trade pricing  page with one of the big BMs 

 

Solid Dense 7.3N Concrete Block 100mm Pack 100

 

£0.87 EA

 

Availabilty - in stock

 

So that’s available on site on Thursday. 

 

Pays your money, takes your choice. 

 

 

 

 

 

 

 

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