French Drain?

[jungle_jim01]

I'm reluctantly looking into the option of a French drain...a lot of digging!
I have an early 19thCentury Yorkshire sandstone house, the back of which has a patio which is up to 6inches (in places) above the internal floor level. The walls are obviously damp and have previously been tanked with cement. This was in a bad way so i've completely removed the plaster and cement render. Now i need to find a solution before re-plastering which will probably be next year.
I'm seriously looking into the Electro Osmosis option but there's such a range of opinions i'm not sure although it would be relatively cheap and easy for me to install myself (anyway i digress!)

French Drain: i understand this and suspect it's the best solution but i have few questions:
1. I'm guessing that the foundations for the house are shallow so i'm not sure how far down i will be able to dig or whether a drain can be below the level of foundations without risk of movement?
2. I want to make a neat finish with the patio and i'm wondering if once filled with gravel i can re lay the patio stones on top, essentially creating a roof for the french drain. I haven't seen this done so i'm presuming there's an obvious reason?
3. Do i need to take the drain into existing drainage? ...(i'm not sure how i'll achieve this and keep a steady fall the entire length as it's approx 15m from furthest end of the wall).
4. Any additional advice greatly appreciated.

Thanks

 

[jungle_jim01]

Just another question i forgot to ask...

I'm going to be repointing the inside of the wall in lime. At the base there are a lot of large gaps and looks like the internal may be rubble filled. Am i best just pointing up the stonework or should i attempt to pour grout into the higher holes and let it fill the inside?
Thanks

 

[Zebra]

I can't be of much assistance but can answer your first question, since I asked it myself on here not so long ago. If you imagine a line at 45degrees downwards and away from the bottom corner of the lowest foundation stone, you can dig down to that line but not below it. Does that make sense?
I can have a go at answering your second question. I think that this would cut down the effectiveness of the thing so much as to make it pointless, because it would prevent moisture from evaporating away from the wall and out through the stones. It would drain but it wouldn't dry. For the same reason, you should try to use the largest stones you can for your French drain, so that there are plenty of air gaps. I made one, and it didn't work, partly because there wasn't any air circulating between the tightly packed small gravel stones that I used, and so the wall wasn't able to dry. It doesn't look bad though just with the gravel exposed.
Your third question I understand is a yes. However, much may depend on the local soil conditions, where the water table is, and whether the ground slopes towards or away from the house. If there is likely to be a greater volume of water than can drain away through the soil by itself, then you would need to carry it away somewhere, and into the existing drain is the obvious place.

 

[Lime]

IMO french drains are always a compromise, some work well for a short period some never quite do the job.
In many soils their life expectancy is particularly short and need the infill renewed to open up the air spaces again.
It is a battle against nature to keep the air spaces free.
It will always be better to bite the bullet and lower the entire outside surface if possible (for several metres distance from property) so that the walls can "breath" again.
On some occasions when I have done this I have been surprised at how much the soil has risen/been raised.
I have found buried stone steps (2) leading upto a doorstep and several underfloor airvents at differing levels in the same wall.

 

[AMc]

1. I'm guessing that the foundations for the house are shallow so i'm not sure how far down i will be able to dig or whether a drain can be below the level of foundations without risk of movement?

Don't assume your old house is on shallow foundations, though of course many are.
As part of my building control inspections and for my structural engineer we had to determine the depth of the foundations. The expectation was quite a shallow brick foundation on a rubble base. The obvious way to do this is to dig a couple of "test pits" along the length of the wall, a spade's width or so and as deep as you need. Once we'd dug down about a metre and were still finding brick the inspector and engineer were happy that the foundations were more than adequate. They also required a deeper pit further away in the garden to check the soil/subsoil composition, that might help to answer your question about drainage.

I agree with Lime that if you can then you'll be better served by lowering the ground level to where it should be to keep the house dry.

When I built my patio (almost) to the house I left the last 6-8inches unpaved and filled over the hardcore base with gravel to ensure water could flow down rather than get dumped against the wall. I also made sure the fall was healthy and away from the house. We've not had any damp there before and I wanted to ensure I didn't create any.

 

[MatthewC]

1. I'm guessing that the foundations for the house are shallow so i'm not sure how far down i will be able to dig or whether a drain can be below the level of foundations without risk of movement?

Don't assume your old house is on shallow foundations, though of course many are.
As part of my building control inspections and for my structural engineer we had to determine the depth of the foundations. The expectation was quite a shallow brick foundation on a rubble base. The obvious way to do this is to dig a couple of "test pits" along the length of the wall, a spade's width or so and as deep as you need. Once we'd dug down about a metre and were still finding brick the inspector and engineer were happy that the foundations were more than adequate.

My 1840s stone house foundations were far deeper than I expected. A pear tree 10" diameter was growing right beside the house (you couldn't get a chainsaw between the tree and the house!) and so I was seriously worried about the roots under the house. When we dug down to lower the ground levels (and do the modern extension), the stump just pulled out as the original foundations were almost as good as the new foundations we were laying! As AMc says, do a test pit.

 

[88v8]

Regarding the wall, make it sound by repointing before you plaster... obviously don't bother with how the appearance of the pointing, but work it well into the joints. Don't rely on the plaster to do the pointing for you.
Do not fill up holes in the rubble core, this is important in separating the inner and outer skins so that damp does not pass through. The air gaps in the rubble prevent water tracking.

Ivor

 

[ChrisD]

The above advice is applicable to a cavity wall construction but I suspect yours was constructed as a solid wall. The 'solid' wall would have been constructed with an inner and outer skin with the centre having a very basic rubble and mortar construction. If this is the case in your situation then it would be sensible to ensure the long term structural integrity of the wall by filling with grout, as you suggest.

 

[MdB]

I believe Ivor is talking about a nominally solid rubble wall too. Not cavity. If you grout then you will improve the structural strength but also improve the ability for moisture to move from the outside face to the inside.

For our wall, I decided against grouting as it will produce a force trying to push the wall apart until it sets. It is important not too fill too much of the wall in one go due to this issue if you do decide to grout. It will also introduce a lot of water into the wall which isn't great when your mortar is made of mud. Instead I fitted a lot of stainless steel ties to hold both sides together.