Concrete Foundation height

frank999 · 23 Jul 2019

Am about to start a rear ground floor rear extension to a 1930's semi.

DPC will be running just under the vent here.

The existing DPC should be 150mm above ground level, but here it is 70mm above ground (at the party wall), and zero at the flank wall.

I need to know at what level I should set the concrete pour, I am told to make an allowance of 400mm for foundation course bricks/blocks.

Looking at this I should really run the new extensions DPC 150mm above the existing DPC (As it is zero at its lowest point, and then calculate down from that for the 400mm, and 600mm for the concrete.

Would that seem wise.

Any advise on how I step and meet the existing DPC with the new would be appreciated.

Some pretty pics attached.

^woody^ · 23 Jul 2019

frank999 said:
I am told to make an allowance of 400mm

Was that by Lego?

Bricks are 75mm gauge, blocks 225mm. Non of which divide into 400.

frank999 · 23 Jul 2019

Thanks, I did think that it was an odd number ... so he should have said either 425 for Brick, or 470mm for 2 courses of block (inc. 10mm mortar).

I will run the new DPC 150mm above the old, so that it ends up 150mm above ground, I know I need to connect the two DPC's somehow, raking out about 50mm of mortar above the old DPC - and jointing the slate to the polyester DPC with 50mm double sided lapping tape ?

I understand the lapping tape sold by Visqueen is like Blue Tack, so as long as its pressed into the slate firmly, can't see why it shouldn't create a good seal.

Unless there is a better method ?

stuart45 · 23 Jul 2019

frank999 said:
View attachment 168247

Thanks, I did think that it was an odd number ... so he should have said either 425 for Brick, or 470mm for 2 courses of block (inc. 10mm mortar).

Still not quite right. 75mm for bricks or 225mm for blocks include the joints.

frank999 · 23 Jul 2019

Just measured a brick ... 65mm, and a block 215mm ... thanks for the correction.

I cannot find any details to join the two DPCs, the LABC Tech Manual: http://info.labcwarranty.co.uk/technical-manual
s12.2 suggests a vertical DPC on the inner leaf of a cavity wall, placed behind wall starter ties, but surely a DPC like that will reduce the strength of any bonding between new and old walls

Notch7 · 23 Jul 2019

frank999 said:
s12.2 suggests a vertical DPC on the inner leaf of a cavity wall, placed behind wall starter ties

Sometimes a 50mm slot is cut out -which effectively joins the cavities and stops thermal bridging.

Otherwise a cut and a vertical dpc. There are wall starters both sides.

frank999 · 23 Jul 2019

Notch7 said:
Sometimes a 50mm slot is cut out -which effectively joins the cavities and stops thermal bridging.

Thanks, yes I have seen details of that - but the existing is solid 225 mm brickwork here - so without cavity

Do we have a vertical DPC, - to isolate the old wall from the new, right up the wall, as any horizontal lap between old and new DPC can only be limited, I read 150mm over lap is required, can't see how that could be achieved with a slate mortered into an existing wall.

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Concrete Foundation height

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