Blocks for first course

[Will1986]

Im starting building a two storey side extension on my property. The concrete has been poured and im ready to start building up to DPC. The trench is 1m deep and ive poured concrete up to 450mm. What blocks do I need to use to get up to DPC. What compressive strength should they have etc? Any advice welcomed.

 

[noseall]

7n dense concrete or trench-blocks if you prefer.

 

[nickjb]

Two rows of trench blocks then 1 row of 7kn blocks or 3 of class b bricks depending on the look. You could do it all in 7kn blocks but I found the trench blocks much easier.

 

[tomfe]

That's just not true. (Sorry read this as you need a 7n trench block)
Building regs state you can use a 2.9n+ block if the height of your footing to floor level is less than 1m if it's 2 storey.
You'll also find most available trench blocks are 3.6n

 

[^woody^]

Building regs state you can use a 2.9n+ block

And are these blocks frost resistant?

Do the "building regs" (or is the approved documents?) mention that?

 

[tomfe]

What so all these thousands of houses built with 3.6n trench blocks are going to fall down now are they?
And BBA must have had a lot of back handers to pass 3.6n blocks for use below dpc....

 

[tomfe]

Do the "building regs" (or is the approved documents?) mention that?

Woody you already know the answer to that as the building regulations basically say 'The building should not fall down' and that's it.

I think wires have got a bit crossed, I am answering the title of the op and it's for the first row of blocks, I think woody thinks I am saying build the whole thing in 3.6n blocks.

 

[^woody^]

I am answering the title of the op and it's for the first row of blocks

You've only half answered the OP's question, as compressive strength is not the only criteria for blocks in the ground.

 

[tomfe]

OK but for the first course which is actually protected from frost as its 450mm below you could use any block. All areated blocks are suitable due to their material nature so you can use 3.6n block you can also use an aggregate block in the inner leaf of a cavity wall below dpc.
So to make a blanket statement that all blocks below dpc must be 7n plus is not true.

 

[Footsoldier888]

To me it seems a little worrying the OP is being advised because we haven't seen his plans.

My Haynes manual says 'The drawings will specify blocks of the correct strength (eg 7N), density (eg 2000kg msq), and thermal effeciency (eg 0.15W/mk) designed to support loadings and meet energy targets.'

 

[noseall]

Meanwhile back in the real world.....

7n dense concrete blocks will be fine. Show class b's or Staffy blues above soil level on the outer leaf.