New Porch- Vertical DPC and Foundation Questions

Driftwood

Hi there, I am in the process of planning and building a small front porch (2mx1.5m). After much research, I have decided to make the foundations 1 metre deep by 500mm wide (the walls will be block and brick with a 50mm cavity and no insulation), as the ground is what I consider to be a heavy clay. The porch will be in a sheltered position. The new wall will be tied into the existing with wall starters, and I intend to use mastic instead of mortar to fill the vertical gap between the new and existing walls. I have the following questions:

(1) Where the new cavity wall joins the existing external wall, do I need to make a cut in the existing wall (the height of the new walls down to the existing house dpc) and install a vertical dpc/dpm? This is supposed to stop damp bridging between the porches external and internal walls. Also, is it possible to make the vertical cut accurately without going through the existing DPC?
(2) I have dug down to the top of the existing house foundation and it sticks out from the main wall of the house, as expected. When I dig my new foundation it will go deeper than the existing. Do I need to cut through the existing foundation that sticks out, in order that I have my trench running right up to the existing wall, or can I leave it in place, dig round, to the required depth, then pour in the concrete? This will save me having to get through it to make the new trenches. If I need to take part of the protruding foundation away, could I use a ‘breaker’ or would this undermine the existing footings?

I would really appreciate the advice as it has taken a long time to get to this stage.Thanks

iamcornholio

You only need to go as deep as the house foundations, not 1m, but normally about 600-700mm will do. You don't need to cut your existing foundations, pour up to them

You don't need to install a vDPC. Moisture bridging is a theoretical possibility, but if the porch walls are just 100mm thick then moisture is just as likely to get through anywhere in the wall, and if the porch wall is 215mm or cavity, then moisture wont get around

Driftwood

Thanks, iamcornholio. That will save me a bit of digging. Just to clarify re: the vDPC (I am learning quite a lot about this), it was going to be installed in a cut through the existing external caivty house wall between the wall starters/profiles/alligator/crocodile clips to prevent dampness bridging. Is this correct?

iamcornholio

Yes, that's where it goes