Sandstone bedrock drainage help!

[GeorgianProblem]

I have a Georgian property in the northwest of England, built around 1820 i believe. There are no drains on 2 sides of the property and the walls are 32% damp inside now 4-5 courses up now i have taken off the concrete render inside! i have uncovered the footings and it is sat on bedrock, in parts only one course of brick as in pictures. The external level will be 200mm lower than internal floor. My plan with the 100mm french drains/footings is as follows:

1. To come off at a 45 degree angle (angle of repose i believe) as best i can considering the bedrock about 300-450mm below the external finished floor level. Some bedorck will need to come out about 500mm from the footing to fit the drains. The land drain will be wrapped in non textile and 10mm pipe bedding used. There will be a storm water drain as well for the gutters with gullys in each corner (currently no drains on 2 sides of the house!)

2. To finish i have read permaboard (eggshell board) that has been recommended to keep the moisture away from the footings. On the footings first is a bitchemen primer, then a self adhesive roll, then the permaboard.

3. clean 20mm stone to finish and then compacted and flagged.

As the diagram in images but im not putting insulation in.


Any advice/anyone done anything like this before? Does this sound like the right thing to do? I am conscious of protecting the footings but also aware i will have to cut some away to install the gullys in each corner. Considering a structural engineer but not sure what different info they would give me from what i have read on here and other sites. many thank and happy sunday!

 

[Kearn]

Bin the moisture meter - it’s for timber and timber alone.

If you’re able to get external solid ground level 4-6” lower than inside, and ensure all the correct materials in and out (lime render / pointing) then give it time before investing time/money/effort - allow a month per inch thickness of the wall if saturated.

This is a strange area - which wall is the house and what is the nearby wall? Both look quite possibly pointed in sand and cement. At least the lowest couple of feet needs to be raked out and redone in lime. Even just raking out and leaving will help massively until you get round to pointing.

Is there a fall on the area between the walls? Can one be introduced so any water drains?

If it were me, I’d be carefully grading the bedrock from both walls to a central channel, then cut this this with a fall through the alley to drain elsewhere away from the buildings. You’ve already enough delta inside to out. This could be an aco if needed for heavier flow rates.

French drains are not a very good idea at all. They encourage moisture to pool against the footings and entice damp to the walls. They also clog up and don’t really shift a lot of water anyway. The best course of action is always to grade the surface away from the building and install an aco drain a few metres away if natural run off won’t be adequate/space doesn’t allow. Cover in gravel against the building to minimise splashback. Soil/grass also good if levels allow.

Most importantly, why do you think you need drainage here? Where is the water coming from? I assume both walls have guttering, and it’s a small area of rainfall to impact. Is it this that needs to be discharged?

I’d also be considering ground water levels and how much water the porous sandstone may naturally have in it. We are built onto limestone bedrock, and if I cut into the limestone, through the horizontal layers of natural structure, water will find its way through the layers and pool the hole I’ve dug, pretty quickly when the water table is high.

Few more questions to be resolved I think and some time to wait and watch the impact of what you’ve done internally and likely need to do externally on the walls. Most importantly understanding the source of moisture and if you actually have damp walls. And then allowing the walls first to dry, then to manage liquid movement to the air ongoing. And finally addding drainage if needed.
Diving into carving through bedrock is detrimental to the building, may create more issues than it solves and will be a decent chunk of time and money that’s probably not needed.