Frost Level

[Nail_it]

Hi Everyone,

For the last few weeks I have been reading a lot of your posts and I would firstly like to thank all you experts who give up your valuable time to help others.

My first self-build project for next year is a single storey rear extension (approx. 2m out by 5m wide).

Untill I begin this project I will be asking for some advice, that is if I can not find the answer in a previous post.

So for now, my first question is regarding building in the frost-level (which I have learned is below DPC + 225mm below Ground Level).


Which of the following are suitable to be used for the outer leaf within the frost-level :-

1. Concrete foundation (ready-mix)

2. Dense Concrete Block 7N

3. Trench Block 3.6N (300mm)



I have read some posts which have touched on this subject, so I am already aware that Blues, Class B Eng are ok but I would be grateful for some more clarification on the above products.

Many thanks.

 

[jm2000]

Hi...by frost level i take it your talking ..up to damp proof course. (dpc or dpm). engineering brick are a very hardy brick and are used widely for below dpc. i dont know if you are rendering the outside of your self build or it is brickwork. either way red/blue engineering brick up to dpc are always a good start as it gives you that break that you can render too and there is also no need to paint them like you would to a common brick or block.

 

[Nail_it]

Thanks for replying jm2000.

The outer leaf will be brick finished. Yes I am aware that I can use engineering/blue bricks.

What I'm really after is clarification on which of the materials listed in #1 is suitable for use 225mm below the ground level.

 

[jm2000]

Thanks for replying jm2000.

The outer leaf will be brick finished. Yes I am aware that I can use engineering/blue bricks.

What I'm really after is clarification on which of the materials listed in #1 is suitable for use 225mm below the ground level.

Ok thats what i was talking about as well, from your concrete foundation up to dpc i would use engineering brick, unless of course you have a deep footing, i generally pour my footing to 3 course below ground level then build up the engineering brick there. if you have gone deeper than that the 7n common 100mm block will do in the substructure, just remember to cavity fill up to 65mm below external ground level, i say this as the 65mm below will allow for mortar droppings.

 

[denso13]

What I'm really after is clarification on which of the materials listed in #1 is suitable for use 225mm below the ground level.

Any of the materials you have listed are suitable

 

[Nail_it]

i generally pour my footing to 3 course below ground level then build up the engineering brick there.

Is there a reason why you pour concrete 3 courses below ground level, rather than, say 1 course below ground level and brickup from there ?

 

[r896neo]

just in case he has balls it up!

 

[jm2000]

just in case he has balls it up!

Ha ha..thanks...mainly due to the fact of undulation of the ground. And as stated above. I have worked on footings that start at ground level, i have also worked on a job not to long ago where the ground altered so much, we started with 2 block deep footing to the other end where it was 13 courses deep.

But Thats what we tend to use 3 courses down. then you have 3 courses to play with say if your concrete pour got a little high...or low.

 

[Nail_it]

Thanks guys. Can I get you's to have a quick look over plan and tell what you think.
Roughly to have a 1000mmx600mm trench (subject to BCO approval) and then continue with one of the following 3 options. Alll of which should bring me to DPC level.

Option 1
Fill trench with concrete 625mm thick, then :-
Inner leaf - 2 course of Concrete (7N) block + 1 course of engineering brick
Outer leaf - 1 course of concrete (7N) block + 4 courses of engineering brick
-> Laying heavy concrete blocks seems less favourable to trench blocks ??

Option 2
Fill trench with concrete 475mm thick, then :-
2 courses of trench blocks (300mm wide)
inner & outer leafs - 3 courses of engineering bricks
-> If trench blocks are definately not susceptible to frost damage ??

Option 3
Concrete fill to a depth of 925mm then :-
inner & outer leafs - 3 courses of engineering bricks
-> a lot of concrete to carry round to the back !!

 

[jm2000]

Option 1
Fill trench with concrete 625mm thick, then :-
Inner leaf - 2 course of Concrete (7N) block + 1 course of engineering brick
Outer leaf - 1 course of concrete (7N) block + 4 courses of engineering brick
-> Laying heavy concrete blocks seems less favourable to trench blocks ??



Option 2
Fill trench with concrete 475mm thick, then :-
2 courses of trench blocks (300mm wide)
inner & outer leafs - 3 courses of engineering bricks
-> If trench blocks are definately not susceptible to frost damage ??

Option 3
Concrete fill to a depth of 925mm then :-
inner & outer leafs - 3 courses of engineering bricks
-> a lot of concrete to carry round to the back !![/quote]

Ok either way youve got to be moving materials....myself i dont like trenchblock but thats my opinion.....475mm dont sound deep enough...dont know what building control will have to say. so to me its option 1....have you even dug this footing yet.

 

[Nail_it]

.....475mm dont sound deep enough...dont know what building control will have to say. so to me its option 1....have you even dug this footing yet.

No, I'm starting sometime early next year, when hopefully I'll have it all clear in my head!

I have had a structural engineer round to work out some beam sizes and spoke to him about the footings. He told me dig an inspection hole so he could take a look next time he's round. He also said that he's sure that its gravel/stone/rock not too far down and for a single storey it should only need about a 200mm thick footing .
Obviously I'm no expert but I've laid 7"-8" concrete footings for only a 2ft garden stone wall.

But anyhow, digging the inspection hole I hit clay about 700mm down. I have not had the engineer back round yet because of the Xmas break.

After reading many posts already, I had come to the conclusion that a 2ft wide x 18" deep footing would suffice for a single storey build, of course I will be consulting the struct eng and the BCO before starting....
...Oh yes and you guys!

 

[denso13]

He also said that he's sure that its gravel/stone/rock not too far down and for a single storey it should only need about a 200mm thick footing .
Obviously I'm no expert but I've laid 7"-8" concrete footings for only a 2ft garden stone wall.

It depends on the loading and bearing capacity of the ground but, generally, in decent soil 200mm is fine.

 

[Nail_it]

It depends on the loading and bearing capacity of the ground but, generally, in decent soil 200mm is fine.

Well, bearing this in mind, I am leaning towards option 2 -> 2 courses of trench blocks of a 475mm deep footing arriving me to 75mm below ground level.


jmc200 - Just out of curiosity why don't you like trench blocks ?

 

[noseall]

In about 90% of all extensions we build, the concrete level is 525mm below dpc, i.e. two courses of block + one brick on the inner skin and one course of block + four brick on the outer skin.

The thickness of concrete is usually about 550mm or greater depending upon the trench bottom. The trench bottom is never shallower than 1m down.

All of the above is dependant upon ground conditions and ground levels.

 

[Nail_it]

Thanks Noseall. In my post #9 (above), you will see that I have already been reading and taking your advise, as my option 1 is directly taken from your earlier posts to other members.

Can I ask you though, what do you think of my option 2 ?

Thanks.