FFL at ground level

[Olf]

As the original house is barely above the ground and the garden is sloping towards it, so the finish floor level of the extension will be at ground level. That maybe makes stepless patio door and Part M happy, but causes issues elsewhere. There aren't too many details available for this case (all the stock drawings cover typical case where FFL is well above the ground and the life is easy), but I found something and translated into this:

 

So it uses a 'secondary DPC', although as I look at it, what is marked as 'secondary' is actually primary >150mm from the ground DPC. The other layer right at the ground level doesn't seem to make much sense, but I believe it is there to restrict travel of moisture at least internally, as if it somehow crossed the cavity, there is nothing else to protect the inner skin.

Is my thinking right here? Is the original idea sensible? Does the inner DPC need to end at FFL, could it not go one course up ?

 

 

 

[Jason L]

What about DPC like this with a cavity try over and do away with the external lower dpc 

[George]

My advice is don't do it, at least, not all the way round. Make the door threshold level, use a vertical and secondary DPC/DPM to bring things up to FFL and then slope the ground level down after the threshold.

 

If you must have ground level equal to FFL, installed a gravel trench / french drain 200mm wide at the brickwork, 

 

You'll have a digger to sort the foundations so a few metres of rough landscaping at that stage is easy.

 

I'm not saying it can't be done... but I've seen a lot of attempts and they are always messy. They might work, but I've also seen too many old buildings with spalled brickwork and penetrating damp! Level threshold and slope away is invariably the most straightforward solution.

[markc]

@Olf A DPC is there to act as a barrier to prevent damp/water travelling upwards. In your initial sketch/detail there is nothing to stop the water going around and over the broken DPC. If your Ground level is equal to or above your floor level then you really need to look at it as tanking/waterproofing as opposed to a DPC

[Temp]

I would also put a french drains under the extension and on the side with ground sloping towards it. 

[Olf]

A drain seems like the most sensible solution, that will simplify the outer leaf DPC (leaving only the 'standard' layer at 225mm in my case) and will protect the wall from any surface water - the slope of the garden, however shaped, will still be towards the house in the end

I've found some ideas here https://www.pavingexpert.com/threshold01#doorway-drainage (there is a patio door to consider as well, will just extend it all around the wall).

 

One question still unanswered though: can inner leaf DPC be also positioned at 225mm (1 block course up), rather than at the floor level? That would make my life so much easier to have that floor level joint available for wall ties.