French drain against external wall DPC?

[Mat_J]

Hi,

At the back of my property, there is about a 20cm wide gap from the external wall and patio paving's, this was filled with small gravel. I presume this was to form a type of French drain. however this is also higher than the Walls DPC, and I think has been causing bridging of the DPC. I have taken out the gravel to have a look (See photos attached), you can see the level the gravel was where the sir brick is shown and lover half had gravel covering it.

I was thinking of putting a drainage channel U-drain in, but I am unsure if I should have this right up against the wall to help protect the DPC, or do some thing else to protect the DPC and prevent any bridging.

Thanks

 

[Doggit]

Most people put gravel in a french drain, and then dirt and dust fill the gaps, and water tracks across; they should have stones in with at least an 1" diamater, and the depth should go down below the dpc. You can install a drain channel, and any water from the patio will fall into the drain, and then get taken round to the drain in picture 4. As the drains are made of plastic, they will provide a barier up against the wall, but you need to be mindful that the bottom of the drain when set in place, will be below the DPC.

 

[Mat_J]

Ok, would I benefit to treat the outside of the wall before putting in the drain? The area where the air brick and door is, looks to have a render over the DPC, of the right of the drain (pictures 3 and 5 ), the bricks are exposed and the DPC visible. Is there a specific product that is used to render over the brickwork and DPC without bridging?

 

[Doggit]

The render shouldn't be bridging the DPC, so you'd need to hack it off to above the DPC, and to the level that the drain channel is going to go. The render should always start ebove the DPC, and even if it has a waterproof admix in it, it shoudn't really bridge it.

 

[Mat_J]

Thanks for the info Doggit.

I have started to remove the render, and dig out the rest of the channel. I think I will create a french drain, but as you say with at least 1" diameter stones, and just bring it around a inch below the DPC ( there is not much depth, and the footing of the wall starts to appear just one brick course down from the DPC. so I am taking it down to just past the first footing e.g. 2 courses below DPC).
I was going to put in a perforated 2inch pipe, just holes at the bottom.

Rather than having the brick left bear to the elements, would it be best to apply some type of breathable water proofing to the brick surface?

 

[Doggit]

If you're trying to use a perforated pipe system, then the procedure is completely different. Look at pavingexpert.com, and search for land drain, as well as french drain, and you'll see you're looking at more of a land drain design. You'll either go with the french drain, or with a channel drain, and that'd protect the wall, and divert any ater that runs into it.

 

[Mat_J]

I had a look at the paving expert, and they say the a "proper" French drain uses a pipe. looking around the web it seems that using a solid PVC pipe with holes just at the bottom (not the flexible perforated pipe with holes all around the circumference ) is part of a French drain installation. something similar as depicted here https://www.wikihow.com/Build-a-French-Drain , but I was going to just put decorative stones on top instead of soil.

Should I not use the pipe at all, and just 1" stones?

 

[Doggit]

Most times nowadays, it's just taken to be a trench with rubble or large stone in it. Most people put gravel in it, but that clogs up with silt and dust, and the water then tracks across. If the trench goes down into the earth, then there's no need for the pipe, and if you do the pipe, then you need a slight slope, and have to be able to get the pipe into the surface drain system.