Correct blockwork layout for extension

[SpecialK]

Hi,
Attempting to self build an extension.
Is this the correct blockwork layout for the internal wall or is there a better way to lay it out?
The 75mm will be celcon coursing block, the 225mm are celcon internally and 7n concretes external. Can I use concrete commons externally?
Will I require padstones for an IG type 3m span cavity lintel?
How many courses less will the external wall generally be or are they the same height? (pitched roof)
When I form a cavity tray over the lintel is it OK to use DPM or is there a specific material to use?
Thanks....

 

[kingandy2nd]

You can use concrete commons externally, but assuming you are planning to render the walls, you might want to consider that two different materials might lead to the render behaving differently on the different surfaces. Maybe a minor consideration.

Personally I just cut my exterior blocks to the right levels for the lintels, and at the top of the wall. You should consider that as a non-pro bricklayer you may not get your courses to be exactly 225mm.

For my extension, the outer skin was one brick course lower than the internal.

Cant comment on the IG - but the specs for the actual lintel may specify.

I used 450mm damp proof course for my cavity trays.

 

[23vc]

You don’t need padstones for the lintel, I used conc. commons and rendered, no issues
Agree with Andy re. Not getting too hung up on courses if you’re new to it, by the time you get to 2.1m you’ll have learned a lot.

 

[SpecialK]

Thanks
Yes learning a lot and getting quicker, shame about the rain and the swimming pool!
I think I will cut the blocks for the lintel and again at the top.

Is it right to stop the internal wall at 2.4m and then the wall plate goes on top? The architect plans I have are very poor for measurements!

Currently 9 courses up on the end wall with a small return...
Fingers crossed for a dry day tomorrow

 

[23vc]

You need to plan your roof with whatever pitch you want, top plate height below windows (assumption based on your pics, could be wrong) to set the wall plate height, don’t go off architect drawing. Mock up an actual rafter with birdsmouth cuts etc and set the heights accordingly