Advice on trench block spec' please

[Tony L]

I have a bit of a dispute going with my builder who’s dug footings, poured concrete, laid 3 courses of trench blocks & should be going on to top this off with B&B floor then dig drains & make a connection to the sewer.  We’re building a 1.5 storey house, block inner & block outer walls with render.

 

After a visit from BCO & SE, we needed to abandon our plan for 1m trenches with 600mm poured concrete, & dig deeper trenches (1.95 across about 25% of the trenches & 1.65 deep on the rest) & pour more concrete, to take the level up so there was no more than 650mm of “trench block/underbuild”, to quote the SE, below B&B. The builder said he’d not allowed for any trench blocks o

in his original quote, however, his invoice for extras resulting from the BCO visits includes £3215 for trench blocks.  That’s just for the blocks – he’s also charged £600 for extra sand+cement & £2400 (95m2 @ £25, yes rounded up by £25) to lay the 300 trench blocks & 500 regular size concrete blocks.

 

The builder says the trench blocks cost £10.71 each.  I said they should cost him less than £7 each (before VAT).  He said they’re expensive because he’s used 7N trench blocks.  They’re 440 long x 350 wide & I think they’re 220 high, but I may have the height wrong – some of these are 215 high.

 

My first question is: how can tell if what he’s used are 7N trench blocks? My understanding is, it’s unusual for this type of block to be used in this situation.  Are 7N & the usual 3.6N trench blocks different densities?  How much should one of these weigh?

 

He didn’t allow for any trench blocks in his original quote – he was going to build a cavity wall straight off the concrete at the bottom of the 1m trenches.  He hasn’t yet explained the benefit of the trench blocks over using regular blocks + cavity.

[nod]

He must have allowed for something to be built below dpc 

No need

 to use 7n blocks 

[Tony L]

He did.  The original quote, based on 1m trenches included:

 

Block solid concrete 7.3Kn 100mm cavity walls.

Fill cavity with concrete to within 150mm of DPC level.

[nod]

8 hours ago, Tony L said:

He did.  The original quote, based on 1m trenches included:

 

Block solid concrete 7.3Kn 100mm cavity walls.

Fill cavity with concrete to within 150mm of DPC level.

Who would specify 7n block on a house 

Certainly not your SE 

[Canski]

11 hours ago, Tony L said:

He did.  The original quote, based on 1m trenches included:

 

Block solid concrete 7.3Kn 100mm cavity walls.

Fill cavity with concrete to within 150mm of DPC level.

So he needs to add on the trench block quote (£25 m2 labour is pretty cheap) and then deduct what he originally had for 2 x 100mm skins and the cavity fill. I would charge around £22 m2 per skin of blockwork so there should be a decent saving on labour.
 

Trench blocks are usually 215 mm deep without a bed. the 140 mm ones above are used to alter coursing if needed. You should be looking at a slight increase due to the more expensive cost of the blocks. 

[Oz07]

Helpful if you refer to trench block cost in m2 terms. They come in a few different sizes

[Kelvin]

7 hours ago, nod said:

Who would specify 7n block on a house 

Certainly not your SE

Ours did for the foundation blocks. 

[Canski]

2 hours ago, Kelvin said:

Ours did for the foundation blocks. 

Nearly every house we build has 7kn  blocks below DPC. 

[nod]

2 hours ago, Kelvin said:

Ours did for the foundation blocks. 

Why ?

[Oz07]

Isnt it a case of standard concrete blocks need to be 7n below ground. Aircrete can be 3.5n as more resistant to freeze thaw cycle. Then if 3 storey you need 7n aircrete?

[Conor]

Here in NI, 7.3kn blocks are the standard stock, 3.6kn would be unusual.

[Tony L]

22 hours ago, Conor said:

Here in NI, 7.3kn blocks are the standard stock, 3.6kn would be unusual.

 

Yes, but what kind of blocks? 7.3N concrete blocks (100mm wide) are normal, but I couldn't even find 7.3N trench blocks (350mm wide) when I Googled - they're all 3.6N.  My partner called Travis Perkins & Jewson & they both told her, they can supply 7.3N trench blocks, but they're unusual, so they don't stock them; they have to make a special order.

 

What happened was, after the BCO had told us to dig an extra metre down (& still pour only 600mm of concrete in the trenches), he made a second visit, to see the 2m trenches, & he pointed out a tree, which meant we had to change our plan again & pour a lot more concrete.  I think the trench blocks were ordered when the builder thought he'd be building up from the bottom of a 1.4m trench (ie top of concrete 1.4 below GL), then when we were given the new spec' to deal with the tree, the trench blocks (over spec'ed TBs) had already been ordered, so he used them rather than send them back.

 

I'm annoyed because I've got 3 courses of TBs on top of the concrete & that's got us up to the level to put the beams down.  What's built doesn't look like the drawing the builder is supposed to be working from.  I was expecting the beams to be sitting on the inner leaf of a cavity wall made from 100mm wide concrete blocks, but he's going to put down 100mm wide DPC on the inner edge of the TBs & sit the beams on this.  He says he told the BCO what he intended to do & BCO was OK with it.  I'll just remind everyone, I'm new to this & there are vast gaps in my knowledge, but my thinking is: each of these £10.71 blocks (certainly the top 2 courses) could have been two £1.50 100mm wide concrete blocks, with a cavity between them, & the additional labour to lay the second £1.50 concrete block would still leave a good saving over each £10.71 TB.  Am I right?

[dpmiller]

4 hours ago, Tony L said:

Yes, but what kind of blocks?

I've never seen a trench block...