External drainage

[erikwoolf]

Hi all, we have been back and forth speaking to various trades to finalise how to go about reducing the water ingress that comes through the coal hole and kitchen. As a background the front part of the property is on a slope facing the front garden. There is a coal hole just under the front steps and a kitchen right next to the coal hole one of the kitchen walls is covered up to 2 meters high by the soil of the front garden externally. When we stripped back the walls of the kitchen the wall that is next to the coal hole and covered externally by the front garden soil was wet (picture below is from this weekend after 2 months without any plaster internally). No just damp but wet. We started with big plans to retain the coal hole (marked as storage in the diagram below) but money got in the way and decided to sacrifice the space by building a masonry wall between the coal hole and the kitchen and close it off from the rest of the house.
We still need a solution though for the kitchen wall that is essentially under ground. The options that we are considering at the moment are:
1) to install a very deep French drain outside the wall that is underground and hope that by improving external drainage we will get humidity down to levels that we can manage with good ventilation
2) to install a French drain as explained above and add a waterproofing membrane externally on that wall such as a Delta membrane to improve our chances of getting as less water ingress as possible especially if in the long term there is a chance that the French drain might fail
3) to do number 1 or 2 above but not just on that wall but the whole perimeter of the coal hole
We got a plumbing company that specialises in external draining to do the works. Does anyone know if we also need to involve an engineer to design the drain (if depth of the drain is a consideration)? I have been reading different opinions online and don't want to overcomplicate things if this is a straightforward task that an experienced tradesman will be able to do. Does anyone have an experience with either of these options that you can share?

 

[Cubist]

Hi Erik,

You say that the garden slopes away from the coal-hole wall but make no comment concerning by how much and whether that ground is free draining toward the boundary. I appreciate that you may not wish to keep that wall fully exposed but I'm thinking that significant improvement could be achieved by ensuring that rainwater escapes away from the property as efficiently as possible via a network of French drains for which the key design considerations include any extant drains to the boundary of the property, slope and soil composition and adequate collection of rainwater falling directly onto the fabric of the building. These may not need to be more than a foot or so down from the existing or desired level of the external ground.

Cheers,
Cubist

 

[erikwoolf]

Hi Erik,

You say that the garden slopes away from the coal-hole wall but make no comment concerning by how much and whether that ground is free draining toward the boundary. I appreciate that you may not wish to keep that wall fully exposed but I'm thinking that significant improvement could be achieved by ensuring that rainwater escapes away from the property as efficiently as possible via a network of French drains for which the key design considerations include any extant drains to the boundary of the property, slope and soil composition and adequate collection of rainwater falling directly onto the fabric of the building. These may not need to be more than a foot or so down from the existing or desired level of the external ground.

Cheers,
Cubist

Hi Cubist at the moment there is a very small (barely seen in the photo) landscaped slope with soil and shingle. This offers 0 protection as the wall covered by soil on that side is continuously wet (worst on rainy days). The current plan is to excavate (along the side of the front steps in the photo) down to the foundation level (2 meter) to allow the installation of a drain that will be connected to an existing gully. Because of the required depth this is quite the undertaking practically and financially.

 

[88v8]

The drain may offer an improvement but the kitchen wall will still be underground and against the damp soil.

What is the fall from the proposed drain to the gully?

Having excavated, I would build a retaining wall to create a clear space between the soil and the kitchen wall. Set it back just far enough to allow for maintenance access.

The open space thus created will have to have a drain at the bottom, and the fall to the gully will determine how far down you can go. Ideally to the top of the footings.

It would be good if you could put some sort of lift-up roof over the space to minimise rain ingress, like a car port but perhaps only six inches above ground, hence the lift-up. Hinge it on the house wall so you can get a ladder down.

This would be more likely to produce a good outcome, although you may still find that the foot of the wall remains damp if the drainage fall doesn't allow you to excavate all the way down to the bottom.

 

[erikwoolf]

The drain may offer an improvement but the kitchen wall will still be underground and against the damp soil.

What is the fall from the proposed drain to the gully?

Having excavated, I would build a retaining wall to create a clear space between the soil and the kitchen wall. Set it back just far enough to allow for maintenance access.

The open space thus created will have to have a drain at the bottom, and the fall to the gully will determine how far down you can go. Ideally to the top of the footings.

It would be good if you could put some sort of lift-up roof over the space to minimise rain ingress, like a car port but perhaps only six inches above ground, hence the lift-up. Hinge it on the house wall so you can get a ladder down.

This would be more likely to produce a good outcome, although you may still find that the foot of the wall remains damp if the drainage fall doesn't allow you to excavate all the way down to the bottom

The drain may offer an improvement but the kitchen wall will still be underground and against the damp soil.

What is the fall from the proposed drain to the gully?

Having excavated, I would build a retaining wall to create a clear space between the soil and the kitchen wall. Set it back just far enough to allow for maintenance access.

The open space thus created will have to have a drain at the bottom, and the fall to the gully will determine how far down you can go. Ideally to the top of the footings.

It would be good if you could put some sort of lift-up roof over the space to minimise rain ingress, like a car port but perhaps only six inches above ground, hence the lift-up. Hinge it on the house wall so you can get a ladder down.

This would be more likely to produce a good outcome, although you may still find that the foot of the wall remains damp if the drainage fall doesn't allow you to excavate all the way down to the bottom.

Good point, we did a drain survey and has showed that the drainage system crosses the foundations at the end of the slope where the drain will be connected to the existing pipes. I think our worse case scenario is that we have a minimum of 8% slope to work with from the top of the footings.
I think that no matter what we do we will get some damp on that wall so the question is how to manage it without internal tanking..

 

[johncraig8229]

We chose to have a cctv drain survey before planning any new drainage systems as it can be so costly on older properties. You could always ask an expert what they would suggest to be the best option for you? We used a company called DASA and they were great help. https://www.dasa.co.uk/

 

[88v8]

Good point, we did a drain survey and has showed that the drainage system crosses the foundations at the end of the slope where the drain will be connected to the existing pipes. I think our worse case scenario is that we have a minimum of 8% slope to work with from the top of the footings.
I think that no matter what we do we will get some damp on that wall so the question is how to manage it without internal tanking..

Well as I said, I would create an external air gap with a set-back retaining wall.

If your drain works to a degree you may find that an internal mesh membrane will suffice. This is spaced off the wall to create an air gap and then plastered over. A slot a half inch or so high is left at the bottom. Any damp runs down behind the membrane and evaporates from the slot.
Our cellar has this arrangement, installed 20 years ago and the wall is dry, although we do have a dehumidifier down there running 24/7.
The merit of this system is that it does not trap damp in the wall, but it would need to start rather drier than the situation you currently describe.