French drain required...?

[eddie_ed]

Hi all, I've got loads of useful info from this forum but this is my first post.

I'm looking for some advice regarding drainage around my house. It's a 1911 build, single-skinned semi detached and we've had damp issues since we bought it back in November 2014.

Initially we were told that a DPC would cure all our woes but after buying I found out about the single skinned wall and that a DPC would be utterly useless for us. After months of research and heartache we finally took the plunge and had the house re-rendered in lime. This has cured 90% of the damp issues (water ingress, saturated walls, mould, condensation) but we still have issues with damp getting to the foundations and causing issues with the space underneath the wooden floor in our front room. The lime render also makes it easy to see that all the external walls seem to be sucking up water from the ground. I should mention that the ground level outside is pretty much level with the internal floors, the surface is tarmac.

I believe a french drain around the perimeter of the house would address these issues by stopping the ground water getting to the base of the wall. I am after some advice as to how to ensure it is done correctly. I know we need to make sure the foundations are not too shallow or we won't be able to position the drain close to the wall but i don't know how to find this out.

Does anyone have any handy tips/advice please?

Many thanks.

 

[Ian H]

The you think the damp is coming from surface water on the Tarmac or ground water under it?

 

[vinn]

It might look to be single skinned but it will most probably be a 9" solid wall. Measure your walls at window openings.

My first handy tip is for you to post ground level pics of the external walls in question.
And pics showing any internal damp signs.
And a pic showing the state of the sub-area soil.

Is the external ground level tarmac all around the semi?
Is the property on a hill or a slope?
Are any internal floors solid?

 

[eddie_ed]

It might look to be single skinned but it will most probably be a 9" solid wall. Measure your walls at window openings.

My first handy tip is for you to post ground level pics of the external walls in question.
And pics showing any internal damp signs.
And a pic showing the state of the sub-area soil.

Is the external ground level tarmac all around the semi?
Is the property on a hill or a slope?
Are any internal floors solid?

I'm on mobile at the moment and can't see how to get pics up, will get on my laptop later.

I don't think it's surface water only because the walls remain wet at the base even when there's been no rain. We've not had a long enough dry spell to see if they dry out completely yet.

We've got no internal damp issues above floor level since the new render but we have one room downstairs with floorboards over a cavity which is visibly damp below floor level. The internal floor level is the same as the ground outside but there is a surface drain running along it which stops surface water getting to the wall hence why I am leaning toward it being a ground water issue.

I've been a bit crap with my description as it's only tarmac against two (out of 5) elevations. One other has concrete whilst the other two have a raised brick floor.

The property is on a very slight slope from front to back of the house.

The lime render company were confident that a French drain would be needed but I'm happy to be corrected!

Thanks for taking the time to help

 

[^woody^]

If the water is being absorbed from the ground, then no French drain or anything will stop this other than a DPC in the wall, or 'tanking' the inside of the wall for any room below ground level.

 

[eddie_ed]

If the water is being absorbed from the ground, then no French drain or anything will stop this other than a DPC in the wall, or 'tanking' the inside of the wall for any room below ground level.

I don't agree, a DPC is no good for a single skinned stone wall and, as i understand it, the primary purpose of the french drain is to create a way for water seeping into the wall below ground level to drain away before it reaches the wall so it would appear to be the ideal solution for the issue I'm having. I just need to understand how to assess the depth of the foundations to ensure the drain isn't so close that it destabilises the wall.

How does your understanding differ?

 

[Ian H]

I just need to understand how to assess the depth of the foundations to ensure the drain isn't so close that it destabilises the wall.

You will have to expose the foundation. Might as well do it level with the dampest bit of wall.

 

[eddie_ed]

It might look to be single skinned but it will most probably be a 9" solid wall. Measure your walls at window openings.

My first handy tip is for you to post ground level pics of the external walls in question.
And pics showing any internal damp signs.
And a pic showing the state of the sub-area soil.

Is the external ground level tarmac all around the semi?
Is the property on a hill or a slope?
Are any internal floors solid?

I think i may be mixing terminology here, it's a solid stone wall (I thought that single-skinned meant the same thing) and it is indeed very thick. It had a failing concrete render on it when we bought the house which we've had replaced with lime. The wall was utterly saturated when the old render came off!

 

[eddie_ed]

You will have to expose the foundation. Might as well do it level with the dampest bit of wall.

Ok, so basically dig down and have a look, i thought it might be as simple as that but thought best to ask! What sort of depth would be considered shallow/moderate/deep for a foundation?

 

[^woody^]

How does your understanding differ?

Ground soils are permanently damp, and walls absorb this moisture and it travels into, through and up the wall depending on the amount of moisture and pressure available.

A French drain is just a drain for collecting surface run off. It does not magically dry ground.

 

[Ian H]

Old ones will be shallower than new ones. Our house has them about a foot deep and 9" thick. Our kitchen extension has them a foot deep and 4 foot thick.

 

[eddie_ed]

Ground soils are permanently damp, and walls absorb this moisture and it travels into, through and up the wall depending on the amount of moisture and pressure available.

A French drain is just a drain for collecting surface run off. It does not magically dry ground.

I think you may have the wrong idea about a french drain mate, https://en.wikipedia.org/wiki/French_drain

It does deal with surface water but also ground water too and one of it's primary uses is to prevent water reaching building foundations

 

[^woody^]

I think you may have the wrong idea about a french drain mate

I think you might have the wrong idea about dampness.

I don't get my info from Wikipedia btw.

 

[^woody^]

one of it's primary uses is to prevent water reaching building foundations

So you are saying that foundations will be dry with one of these then?

What does Wikipedia say about the water table, gravity, hydrostatic pressure, ground pore pressure and moisture transit through materials?

 

[eddie_ed]

So you are saying that foundations will be dry with one of these then?

What does Wikipedia say about the water table, gravity, hydrostatic pressure, ground pore pressure and moisture transit through materials?

I am going on what I've been reading, wikipedia was simply the first link that came up earlier.

It's this kind of article (http://www.1stassociated.co.uk/french-drains.asp) which has led me to believe that it is the solution I need. There's many other examples out there.

Are you saying that a french drain will not stop moisture from entering the foundations? If installed to the depth of the footing it would seem to, at least in theory, address the issue. Can you please explain why this is incorrect? I don't want to do unnecessary work!