Block strength for single story.

[iwanttodiy]

Hi guys.

Anyone have any thoughts about the usage of the below blocks for a single story flat roof extension

The celcon solar blocks
https://www.travisperkins.co.uk/H+H-Celcon-Solar-Aerated-Concrete-Block-2-9N-100mm-Pack-100/p/701733

Vs

Celcon Hi strength Block
https://www.travisperkins.co.uk/H+H-Celcon-Hi-7-Aerated-Concrete-Block-7-3N-100mm-Pack-100/p/837344

I personally like the idea of the solar blocks as it’s a better insulator but weaker compressive strength. However builders are spec’ing hi 7 blocks.

Understandably the costing of these blocks is roughly the same.

But, considering the tech sheets say the solar blocks can handle loads of up to 2 stories (my house is 2 stories) it seems overkill for the hi 7s? Maybe it’s one of those let’s over spec just in case. But I know I’ll never get any more than 1 story in this spot.

Although if I’m considering both these blocks, it seems that the standard blocks (https://www.travisperkins.co.uk/H+H-Celcon-Standard-Aerated-Concrete-Block-3-6N-100mm/p/700055)
Have a best of both worlds in Terms of insulation value and strength. And it has an added benefit of being considerably cheaper!

Is there a reason my quotes are specing these blocks that are overkill for a single story? Am I missing a point somewhere? Is it a better be safe than sorry moment?

 

[^woody^]

You can hold up a roof with bits of cardboard. I'm sure that a solar block that will take over 500kg directly will be ok.

Shouldn't your designer be specifying the blocks, not the builders - based on their discount at the merchants no doubt.

 

[iwanttodiy]

My structural engineer just mentioned standard 300mm cavity wall in the paperwork.

 

[John D v2.0]

To be honest unless they are in training for the Olympic weight lifting I'm surprised the builders prefer the heavier ones.
The lighter ones would be fine, even trestin lintels should be ok. But it's down to the specifier as above.
Ours was all specified as lightweight blocks but the builder decided to use concrete heavy blocks for most of it too.

 

[Notch7]

Most builds use the standard 3.6N blocks.

Your wall construction needs to achieve the u value in part L1b. Thats inside skin, outside skin, cavity insulation, internal facing outside facing.

 

[iwanttodiy]

Yes, already looked at that.

With standard 100mm outer leaf Block -> 100mm full fill pir insulation -> 100mm celcon Block I get a u value of 0.17 if I use solar or 0.18 if I use the standard.

I believe this is way sufficient.

 

[John D v2.0]

You should check that, I'm not sure pir board is usual for full fill. Normally 50mm would be plenty.

 

[iwanttodiy]

Yeh, was planning to raise it with BCO if my area is ok with full fill. one of my neighbours showed me his build during construction and it used full fill pir.

Otherwise, I could use something like this, http://www.xtratherm.com/products/thin-r-plus/ct-pir-cavitytherm

Though it is quite a bit more expensive than the normal pir.

 

[tony1851]

The dense (7N) block is far less prone to shrinkage cracking than the lightweight aac blocks. Many builders prefer the 7N block for that reason.

 

[garyo]

I've just used heavier on my outer skin to make the rendering easier and to avoid cracking/shrinkage issues.

 

[^woody^]

100mm full fill pir insulation

Is that even permitted? Has that been approved by the council without a cavity?

 

[iwanttodiy]

I have asked them the question, just waiting for their response.

Seems weird how a neighbour would have a 2 story extension build with full fill, which I believe he would have had to check out with a BCO. And I don’t get approved.

I did ask one of the builders yesterday who quoted 100mm PIR insulation in a 100mm cavity about issues with a full fill cavity. I mentioned BCO may not like it as it doesn’t offer moisture protection of having an air gap. He looked confused, and mentioned all his extensions have always been built like that.

 

[John D v2.0]

looked confused, and mentioned all his extensions have always been built like that.

In my experience, when people say they always did something a certain way, that is a non reason and means they have no idea whether it's right or not. Just means they got away with it.

 

[iwanttodiy]

Checked with BCO, they mentioned that typically it’s 300mm airgap with PIR, fullfill if non PIR, but it comes down to the product.

 

[John D v2.0]

Well that's even worse, usually it's 50mm cavity which can be reduced to 25 if the wal is built carefully and accurately enough, but 300 could only be the whole wall!