House perimeter drainage and connections

[battenburg]

I live in a victorian terrace. There is a historic utility room extension which is suffering from damp problems. Beyond the utility room there is a former outdoor toilet which is now just used for garden storage. The whole garden is (badly) concreted over, right up to the walls of the house. I have had two builders suggest that one of the ways to help the damp issue would be to install french or aco drains around the perimeter. I think there could be multiple things contributing to the overall damp problem, but this seems to make sense as a good first port of call. So I have a few questions whilst I am waiting for quotes:

1) Is there any particular benefit to either french or aco drains?

The combined sewer pipe for the whole terrace runs through the back of the garden and I assume under the old outdoor toilet. Unfortunately the drain cover, if there is one, has also been concreted over but I can see the drain covers in the neighbours gardens in about the right place. This means that if I were to install drainage around the perimeter then it would be likely to cross above the combined sewer pipe.

2) Is this a problem or would it need any special permission? I don't know the depth of the sewer pipe. I'm guessing that french drains need to go a bit deeper than aco drains and therefore might end up meeting the sewer pipe.

I have 3 downpipes from the roof. 2 currently just flow onto the concrete and I assume these would get connected into the new drainage. 1 downpipe is right next to the soil pipe and releases water straight into a grate which I am guessing then connects to the sewer pipe along with waste from the soil pipe. This is shown in the photo below.

3) Similar to 2), would any permission be required for work that involves changing pipe connections in the vicinity of the sewer? I'm assuming that the grate and the soil pipe both join the sewer, but I can imagine that things could need to be replaced/adjusted once the concrete is up and I don't want to do something wrong to the sewer pipe that serves an entire row of houses.

Any pointers about these questions or in general would be much appreciated! I'm at the stage where I have procrastinated for so long about how to approach the damp problems that I would really like to do something. Thinking about all of these different issues (which I still think need to be considered!) mean I never manage to get stuff sorted and all the while the damp problems carry on.

 

[Mike13]

First thing is to find where the damp proof course actually is. If it is less than 150mm from ground then you need to plan around sorting out the level outside. render can blow while still giving the appearance of being intact and can allow water to penetrate through to the bricks. Until you know where the damp proof course is laid you cannot really decide best course of action. A drain would help I suspect.

 

[battenburg]

The concrete is below the air brick (to the left of the image but not actually shown in the photo) but not by much. The bricks below the render are painted with some kind of bitumen or barrier paint so its difficult to see if there difference in the mortar between any of the bricks. Presumably to identify the DPC, the concrete would need to come out anyway so you could see lower down, or the bitumen would need to be removed.

Would I be right in thinking that the DPC would likely be below the airbrick?

I've been doing a bit more reading since I posted and one other thing that I'm worrying about is whether digging out a trench for a french drain is the right approach at all. My understanding is that the houses in this area often have very shallow foundations, or none at all. It seems a little bit risky to be digging out around the base of the house if that's the case, though I'll be the first to admit that I don't know much (anything!) about structural engineering. I've read that soakaways should be 5m away from building foundations - isn't a french drain just like a mini soakaway, and this proposal would be like putting it right next to the foundations?