We need to lower external ground levels to below DPC and have been advised that we need a good 800mm width excavation. Does it really need to be this wide? I can't see how 800mm will be any more effective than 400mm with a grate on top.
Its simple really, the idea behind lowering the ground outside, is that heavy rain bounces, depending on your location and how exposed you are, the rain will bounce and make the wall wet above the dpc.
The recommended level is six inches below the dpc, the width whatever you like, if your guess is wrong you can always deepen or widen the hole. (or both) I find that having pea shingle in the hole stops the rain from bouncing.
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