French drains...

[Sylar]

We have damp issues on one of our walls. Outside, is a French drain with gravel (fairly old though so could do with cleaning out and refreshing). It doesn't lead to a drain at either end from what i can see from the surface.

Surely this is just a holding place for water to collect next to the wall? I've never been convinced by French drains, especially when it doesn't appear to be draining anywhere?!

What would the best solution to this be? Relocating a drain would be difficult as there's nothing nearby. Maybe just removing it and back filling with concrete? I need to check the fall of the alley way if doing this.

Any tips would be appreciated.

 

[RedHerring2]

You haven't supplied enough information to make any kind of diagnosis/suggestion.

How deep/wide/long is the French drain?
Where is it in relation to DPC (if there is one?)/internal floor level?
What is the wall made of?
What is the ground like around the French drain?
Why is it there?
What were the conditions like before?

And, you'd be better off in the Building Forum. Ask the mods to move it for you.

 

[Sylar]

How deep/wide/long is the French drain? Approx 11m long
Where is it in relation to DPC (if there is one?)/internal floor level? Floor level is low, top of airbrick is at outside floor level (not ideal!)
What is the wall made of? Brick and rendered
What is the ground like around the French drain? Rendered wall one side, concrete alleyway other side
Why is it there? Was there when we bought property, previous owners have had damp issues too, obviously not resolved
What were the conditions like before? As above, like it when bought, always had damp on wall.

 

[RedHerring2]

My apologies for the sarcasm Sylar, but getting information out of you is a bit like extracting teeth.

How wide/deep is the drain?

What are the ground conditions like, not the surface?

How does the airbrick function if it's below ground level?


It seems obvious to me that the French drain was an attempt at resolving the damp issue and to simply remove it and reinstate conrete will excacerbate the situation.

Is there any dpc?

Does the render go all the way to the ground?

 

[Sylar]

Hopefully a picture (or two) speaks a thousand words!...

I cant see any visible sign of DPC.

I replaced last week the fascia/soffit and guttering so that is now all good all the way along.

 

[RedHerring2]

That's the ticket!

I'd probably extend the airbrick arrangement all the way along the wall.
At least 150mm wide and the depth of the airbrick. Obviously you'll need some arrangement to avoid it filling with soil from under the concrete.

Monitor it to see if it fills with water during rain. This is kind of checking on ground conditions.

Don't dispose of the gravel just yet, if the channel does not fill with water you could re-use it.

I'd also hack off any render below the top of the airbricks. Even a little higher.

You say floor level is low, I asume you mean internal ffl (finished floor level) where precisely is it in relation to the external ground level?

Do your neighbours have any problems? They might have hacked off the render to allow the wall to breathe.

 

[Sylar]

Thanks for the advice RedHerring!

Internal floor level is approximately level with the outside. Neighbours had issues when they moved in and had DPC injected, although they are not sure if that has resolved the issue.

So you reckon dig out the trench and monitor during rain? Will planks of treated wood be suitable as a temporary barrier?


So theres two scenarios...

If the water soaks away in the channel, then is this acceptable? Does the channel need a splash guard at the edge? Hack off the render and backfill with gravel? Is it worth getting DPC injected? (there's a possibility it already has been done and is under the render however).

If it does fill with water? Then what?