Trench block confusion

[Ben Weston]

Hi all,

 

We're about to pour our foundations and am looking ahead to getting to DPC. We need one course of trench blocks and two courses of brick.

 

However, despite it being a standard-ish two storey extension, Building Control and structural engineer are saying we need 7.3N trench blocks?! My bricklayers say they've only ever used 3.6N and 7.3N are for three storeys+.

 

3.6N 300m wide blocks (for a 100mm cavity) are readily available but 7.3N Thermalite or concrete blocks are seemingly out of stock with long lead times across the ccountry. I just can't get hold of them.

 

The only alternative I can think of is to use two 100mm 7.3N blocks and infill with concrete. Those blocks are available too.

 

Have I missed something? Is 7.3N normal here?

Edited May 26, 2022 by Ben Weston

[nod]

No 

You haven’t missed anything 

Your Brickie is correct 

But what does he know He’s only the guy that lays them

 

Anything other than standard trench block is a nightmare I’ve just laid 340 and had to put a 100 mil concrete block in front of every row for a 125 cavity 

No 355 trench blocks available I was told they would be double the price if I ordered some and a long wait 

 

SE will always over spec BC also 

Go back to BC and explain the situation

You would if you could But you can’t 

Our brick suppliers have said the 7n FB are being pre orders in there there thousands by Persimmon Barrets etc

In the same way that caused the face brick shortage 

 

 

 

[Ben Weston]

Thankfully ours is a fairly standard 100/100/100 build-up so I'm hoping SE/BC are happy with 100mm 7.3N block, concrete infill and another 100mm 7.3N block instead. More work (of course) but I can't afford any more delays if I can avoid them!

[George]

Probably a miscommunication or not aware of the product (trenchblocks aren't ubiquitous).  I would accept 3.6N trenchblocks but I would not accept 3.6N standard blocks below DPC.

 

It usually isn't a question of strength but durability. 

 

Is the SE clear they are proper trenchblocks? 

 

 

 

 

[Dave Jones]

use 2 standard 7n blocks flat one on top of the other.

 

 

[Ben Weston]

Thanks all.

 

After a lot of confusion, and speaking with Thermalite directly, we're using 3.6N Thermalite trench blocks. They pass the requirements for the load-bearing, frost protection, etc below DPC that NHBC/Building Control require. We will then use two courses of engineering bricks externally to DPC and a standard aerated block internally.

[Gus Potter]

On 26/05/2022 at 18:53, Ben Weston said:

Hi all,

 

We're about to pour our foundations and am looking ahead to getting to DPC. We need one course of trench blocks and two courses of brick.

 

However, despite it being a standard-ish two storey extension, Building Control and structural engineer are saying we need 7.3N trench blocks?! My bricklayers say they've only ever used 3.6N and 7.3N are for three storeys+.

 

3.6N 300m wide blocks (for a 100mm cavity) are readily available but 7.3N Thermalite or concrete blocks are seemingly out of stock with long lead times across the ccountry. I just can't get hold of them.

 

The only alternative I can think of is to use two 100mm 7.3N blocks and infill with concrete. Those blocks are available too.

 

Have I missed something? Is 7.3N normal here?

@Ben Weston

 

1/ Take everything the brickies say with a pinch of salt. If you have large bifold doors say then you could have point loads and need at least something with a bit of strength under areas of localised loads. Your average brickie is not going to spot this. A few may do but it's evidence you need not "blarney".  Question.. do you let the brickies advice take precidence over BC and the SE?

 

2/ To get the best economy you need a really level found to build off + /- 5.0mm if you only have a couple of leveling courses before the masonry becomes visible. In practice this does not often happen.  5.0mm is too tight a tolerance for practical day to day site work on concrete founds in the domestic market. It will be hard to achieve a level bed for the engineering brick with only one course of trench block and still comply with the maximum and minimum mortar bed thickness required by the codes.

 

3/ Practically and cost wise your founds are unlikely to be flat on the top. Expect in real life a variation of some  + / - 15mm often more .. if you get a hot day then the concrete won't flow so well, a really wet day and the folk laying it will want to just go home.

 

Solution. Use 7.0 N dense blocks laid flat and bonded to get the coursing right and a level bed. Then put your Engineering brick on top.You will get a bit more play in the mortar bed and this will make it easier to level the coursing before the engineering brick.

 

It may be that the engineering bricks are forming a damp resisting function particularly if matching into an existing building that has no DPC but blue / engineering brick as the DPC. If this is the case then they could be decorative and it needs to be spot on visually. To realise this you need to give yourself as many points of adjustment in the mortar beds under the ground as you can = more courses.

 

In summary forget mixing and matching, stick to dense concrete block and concrete bricks if need be if it turns out the top of your founds are not quite where you expect them to be... they probably will not!

 

Forget the one course of 3.6N trench blocks. It is false economy.

 

 

 

 

 

 

 

 

 

 

Edited June 2, 2022 by Gus Potter

[saveasteading]

Given that trench block is intentionally lightweight, 7N does seem excessive. But it depends on the construction and where the loads are coming from.

Hence ask the SE why, and there is likely to be  a good reason.   Anyway your situation is sorted.

 

However I prefer 7N blocks because the quality is usually much better, the finish is better, and they are denser and more durable,

[lstevensuk]

@Gus Potter @saveasteading Would anyone mind doing quick sketch of the blocks laid flat? I think understand the bonding but wanted to make sure.. also just about to order trench blocks.

 

 

Edited June 5, 2022 by lstevensuk

[saveasteading]

[Brickie]

Personally,I double them up (so two courses flat laid with the joints in line & then the next two laid half bond & so on.) The blocks compressive strength is standing upright-laid down it hardly takes any force with a hammer to crack them. 
That’s my reasoning,anyway. 

[lstevensuk]

Oh ok thanks @saveasteading I think I missed the point that these are 300mm wide blocks?