Damp in Victorian terrace - French drain advice

[bridgetc]

Hi everyone,

I've just come across a few useful threads on here about french drains and I'm hoping to get some advice on how to go about creating one.

We recently moved into a late 19th century ground floor garden flat in a terrace in north London. Our pre purchase survey indicated that we had damp throughout the rear of the property and I suspect this is caused by the concrete slab that forms our garden and a lack of drainage. As you can see on the photographs, the garden level has been raised to the air brick and the interior level is now lower than the exterior. There is also cement render which is bridging the damp proof course - assuming there is one. At the back of the garden there is also a very high decking over the concrete which meets the rear wall just below the window, where rain water is presumably running straight into the brickwork. There is an old flower bed running along the garden wall which I've dug around in to see how deep the concrete is - I think its about 10cm.

I have tried to look through the cracks in the decking and it looks as though the concrete is highest in the back corner meaning that all the rain water flows towards the house.

Ideally, we would remove the concrete in its entirety but we do not have the funds for that.

As an interim solution, I have been thinking about removing the decking and concrete render, repointing in lime and creating a french drain along the perimeter of the building. My questions are:

1. Where does the collected water go? Is the idea that it soaks into the ground below the house and if so, how doesn't it just make the wall wet?
2. How deep/wide should it be? What should it be lined/filled with?
3. Should it be positioned right up against the brick work?
4. Should the brickwork be lined with anything protective or does the gravel go right up against the brick?
5. How do I build around the existing drain? (which is open and connects to the kitchen/bathroom of upstairs flat)
6. Would it be better to create a drain with a half pipe that connects somehow into the existing drain?

Any advice much appreciated!

If anyone knows a reliable and knowledgeable builder who is experience in period properties and operates in north London, please do let me know.

Many thanks in advance

 

[Cubist]

Sorry but I don't think a French Drain is at all appropriate for your property and may, if you were to install one, create more problems than you would fix. In your questions you've identified the key constraint insofar that water gathered into a French Drain has to go somewhere and, in your case you would have to break into one of the extant waste water drains to make a suitable connection to it. Whist there do seem to be several of these to hand the problem would be whether a sufficient slope (fall) could be established from the French Drain to allow rainwater to escape into the main channel to then flow away from the property.

Looking at the photos you've included above there seem to be adequate surface rainwater drains sited around the walls and these should be sufficient to capture and remove any direct rainfall. However, the photos also reveal that the concrete floor and slabs laid in this space have been positioned just below the bottom edge of the air-bricks that are visible - the top surface of any concrete or slabs in this space should be at least two courses of brickwork below the bottom edge of the air-bricks to ensure that in severe storms rainwater cannot find its way through the air bricks and into the interior underfloor voids. If you have suspended wooden floors in the rooms abutting this courtyard there is a possibility that damp could have already spread into the timbers of the floor to induce rot. I think you would be wise to consider what options you may have to reduce the current height of the concrete and the slabs whist also ensuring that you do not compromise surface water drainage into the open drains shown and consider investigating under any wooden floors in those romps to verify the condition of the timbers.

These will help to reduce future damp penetration into the building from the courtyard and re-prointing the brickwork of those walls will also be beneficial.

 

[alexander]

the ground level need reduced by at least 150mm, possibly stones rather than concrete maybe better, the old cement render needs to be removed & the painted brickwork needs to repointed with lime mortar, if the brickwork behind the cement render is compromised then its needs re-rendered in lime render,

 

[LeonUk]

1. Where does the collected water go?

The collected water in a French drain should be directed away from your property. It is not meant to soak into the ground below your house. Ideally, the drain will channel the water to a safe location such as a storm drain, an existing drainage system, or a place where it can naturally disperse without affecting your foundation.

2. How does the water not just make the wall wet?

The French drain works by collecting excess water and funneling it into a gravel trench, where the water flows away from your home. The gravel allows the water to flow freely, and this system prevents moisture from continuously making direct contact with your brickwork. If the water was left in place without a drainage solution, it would indeed seep into the wall, leading to dampness.

3. How deep/wide should it be?

For a French drain, a common size is around 30–45 cm wide and at least 30 cm deep, but it could be larger depending on the extent of water pooling in your garden. Since the concrete seems to be around 10 cm deep, you might want to dig at least 20 cm deeper than that to ensure it has enough capacity to handle water flow.

4. What should it be lined/filled with?

The trench should be filled with:

A layer of landscaping fabric at the bottom (optional but helps prevent dirt from clogging the gravel).

Gravel (ideally, 10-20 mm stone, or larger pea gravel) for the main drainage area.

Perforated pipe: You can use a 100-110 mm perforated pipe at the base to help with water movement. It’s better to have the pipe near the base to direct the water.

The pipe should be wrapped with another layer of fabric to keep the gravel clean and avoid clogging.

5. Should it be positioned right up against the brickwork?

Yes, you can position the French drain directly along the wall, but the trench itself should allow water to flow freely and not build up against the brickwork. The key is to direct the water away from the wall, so if the drain is slightly offset, that can also work.

6. Should the brickwork be lined with anything protective, or does the gravel go right up against the brick?

The brickwork can go directly up against the gravel. However, if you are concerned about water directly touching the wall over time, you can apply a breathable waterproofer (such as a breathable sealant for brick or stone) to the base of the brick wall. This can help prevent long-term moisture buildup. That said, the French drain should divert the water enough to make this unnecessary.

7. How do I build around the existing drain?

For the existing drain that connects to the kitchen/bathroom, you need to plan how the water from the French drain will merge with it. If the current drain is open, ensure that the French drain system doesn't lead to overflow, which could cause other issues.

If the existing drain leads into a larger drainage system (sewer or storm drain), you could connect the French drain pipe into it.

Using a half-pipe that connects to the existing drain might be a good idea if it's not functioning well enough. If you go this route, ensure that the half-pipe is placed at the correct slope so the water flows smoothly.

8. Would it be better to create a drain with a half pipe that connects somehow into the existing drain?

Yes, a half pipe might be a great solution, especially if you have existing drain lines. If the French drain is connected to this, it would ensure the water doesn’t pool and instead is funneled out. The half pipe can easily be linked to your current drainage system if it’s positioned properly.