Wall Construction Advice

[carlosdeanos]

Hi, starting my first self build, just wondered if someone could skim my attached plan and see if looks okay, plus a few questions please.

Project is a 8000 x 3400 Single storey rear extension to a 1930s Bungalow.
Foundations - 1000 depth x 600 width Trench Fill, with a concrete oversite.
Below DPC - 7N dense concrete blocks. 4 course of brick on the outer to match existing. 
Render finish to match existing so the rest of the cavity wall can be block. 
    Outer- concrete block 
    Inner- lightweight block
Part fill insulation

 

Can anyone tell me what blocks I should use on the inner/outer? I think I have the right idea but there seems to be a lot of options, for example is lightweight a Thermalite?

A quote I had from a bricklayer stated  'a lean mix cavity fill' below DPC, can you tell me what that is? Is this the concrete mix in the cavity up to ground?

Lastly I keep reading about weep holes, do I need to consider this?

Thank you.
 

[Carrerahill]

My concerns would be over those founds, that is a big foundation and a mass of concrete, what is the original house on? I doubt anything close, you run the risk of differential movement as the house will likely move more easily than the huge mass of concrete, result can be seasonal ground movement but the extension will stay stationary. Ideally foundation should always be similar in design and construction to the existing (assuming the existing is built well and does it's job obviously!). Another issue with big founds is that you may then find it settles significantly due to the huge weight, and the house stays put. Foundations are a serious area of contention particularly with BCO's who are not structural engineers or even if they are, are not specifically well versed in concrete foundations etc. 

 

Totally ignore me if the above has all been carefully designed taking into consideration the existing and the ground make-up etc.

 

I'd just use concrete block in and out. Lean mix is just lean on the cement concrete, and it is usually just sharp sand and no aggregate, it is to stop the walls before ground level from being pushed in essentially. I personally use a no fines concrete as that is permeable if the walls ever did flood. 

 

Looks OK to me, the flat roof is only 2400mm high? What height does that give you in the room?

 

 

 

Edited April 21, 2020 by Carrerahill

[carlosdeanos]

@Carrerahill Nope the foundations were purely my idea from a book I have and the net, I will run it by control, thanks.

 

I had to look up no fines, so If I used that to fill where would the water go after that if its sitting on top of a concrete trench? Added image of how I thought its was put together.

 

Sorry the internal ceiling height is 2.4m which is why that measurement is on there.

Edited April 21, 2020 by carlosdeanos
Added image

[Carrerahill]

50 minutes ago, carlosdeanos said:

@Carrerahill Nope the foundations were purely my idea from a book I have and the net, I will run it by control, thanks.

 

I had to look up no fines, so If I used that to fill where would the water go after that if its sitting on top of a concrete trench? Added image of how I thought its was put together.

 

Sorry the internal ceiling height is 2.4m which is why that measurement is on there.

A cavity wall will always have moisture migration and transfer at ground level - trapping it isn't a good idea. This is the beauty of engineering, there are always solutions and options that can be used in isolation or in conjunction with each other to create ideal situations. 

 

I just use no fines as it lets the later transfer to the lowest point and then it weeps through the block/mortar joints - a concrete block wall is about as waterproof as a sieve! You would be amazed at just how much water will pass through a block/mortar joint.

 

Lean mix is absolutely fine, no reason not to use it, I just personally used no fines on my personal builds as our civils guy always specs it. The thinking behind it is that is fills the cavity with a solid mass, but it allows water to drain particularly during wet season and helps to balance the moisture content out on both sides of a wall, very dry on one side and very wet on the other can cause its own set of issues.

Edited April 21, 2020 by Carrerahill

[carlosdeanos]

@Carrerahill ok great that saves me some complexity then. I will make some changes! Cheers.