# Extension wall with partial earth

I have a structural engineer however he’s really only done the structural calcs for the steel works of the build.

I’m designing the extension myself and getting a builder to build it.

I don't think Celcon do a 'dense' block, their 7N block is not that all that dense and mass is what you want for a mass retaining wall. A rough rule of thumb for a mass retaining wall is that the depth at the base should be about 1/4 - 1/3 of the height. There's a fair chance BC will expect the retaining wall to be designed by the SE.

This is what i has envisioned in the attached picture.

I believe I read somewhere that retaining walls over 1 meter would require SE calcs. However the retaining part of my wall is just 0.6 - 0.7m from ground level.

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• 39 eastleigh Road - Extension(Recovery).jpg
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Whats that waste pipe doing? Waste of time?

And if that's the height you are overthinking this. A normal cavity wall will do. But if you like, two course of 100mm blocks on the outside, separate from the house wall with some clayboard/clayshield between it and the house wall. You don't want to bear an external ground slab on a house wall. Keep it separate.

Where you have "Retaining Wall 2OOmm" on the left. See that diagonal line at the end of it? The wall is holding up the ground to the right of that line. Thats all.

That waste pipe is an air brick and vent. It’s to provide air back into the existing void. Probably have 1 or 2 z shaped vents and 2 going straight out the side of the extension.

Well the length of it, and the bends will mean that it has an effective diameter of less than an overflow pipe and is a condenstaion risk internally due to that design. That should need re-designing using 100mm pipe, 2x 68mm pipe or equivalent square duct. With flexible joists to allow for movement.

I think his waste pipe as you put it is just his crappy drawing and it's actually gonna be a telescopic air brick. As his note says in fact.

It's all in the detail.

Are you designing this yourself or is there a designer involved?

I think you and FMT are designing it

What else is there to add?

Guys, as per the designed wall in my drawing, the contractor mentioned he’s going to lay a high load DPC. I would expect this to be in line with the existing DPC which would mean it would need to be partially under ground on the retaining wall by 300-400mm.

I believe to appropriately waterproof it, is to apply tanking slurry between the top of the foundations, the retaining wall up to ground level, French drain near the top of foundations, and tanking slurry just around the corner. Have a layer of tanking membrane leading to the French drain.

Generally the DPC layer is 150mm above GL. Understandably, we can’t split the DPC to different levels as that will just let moisture through?

Therefore we need to apply dpc to prevent rising damp from the ground, and tanking to prevent penetrating damp/moisture from the retained soil. Am I correct in thinking we need to make sure that the tanking slurry should at least go up to 150mm off GL, then have the wall rendered with sand & cement & plasticiser?

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• Side View.jpg
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The contractor is starting next week, have the plans been approved?

The contractor may well be saying he's going to do this and that, but he should be following the approved specification.

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