Lowering Ground Levels

[Anonymous]

Hi All,

ok so next in my mind of the 50,000 jobs I have to do on this rennovation is to lower the ground levels which have been inexplicably raised all the way around the cottage. If I was being charitable I would say it was done to provice wheelchair access for this previously estate owned rented out property.

What I'm left with is concrete about 1 foot above floor level (and maybe more) as if the drain cover is anything to go by at the rear right hand side of the it's going to be about 2 - 3 feet.

So my hope / plan would be to break the concrete where it's been sloped up against the house at the front with just enough gap so that the water can run in. This would leave a trench approximately 8 - 12 inches wide and I suppose a foot deep... Obviously that will need to be filled with something but a conservation builder who came around advised not to put in a french drain as this just keeps the water in place....

So what's the best solution do you think ?

There's clearly been a problem as the dining room had the walls boarded, all the skirting boards were rotten, the dining room (again) has had a concrete over dpm installed (that's also coming out) and there's a lot of cement pointing which has to be removed too.

Lastly can I do any damage by removing the concrete ? or put another way is there ever a legitimate reason for doing it ?

I'll try and take / dig some pictures out.

Paul.

 

[Flyfisher]

In no particular order . . .

I find it hard to believe that a French drain will trap water as its very reason for existing is to allow it to drain away. Having said that, if you can reduce ground levels to well below inside floor level (preferably at least 6 inches) then a bit of standing water shouldn't be a problem anyway.

I'm not surprised about the skirting problems and wouldn't worry about it. We've had a similar thing in a room where the ground was about a foot higher than the inside floor, which makes such things almost inevitable. I've yet to replace the rotten skirting because I felt there was no point until the ground level had been reduced and the wall had dried out. I finally got around to finishing the ground works last year and am unlikely to be decorating the affected room this year, so I'm expecting the wall to have thoroughly dried out by the time I do replace the skirting.

We also had a 'patio' of concrete against the back of the house that was higher than the inside ground level - to the extent that we had two steps UP to exit the back door! Again, I removed all of this so that our kitchen extension floor was the same level as the inside floor level. This has worked very well but, depending on the ground around the house, it can result in the outside ground either sloping towards the house or stepping up at some point, which we didn't want.

Our solution was to 'roll back' the new ground level quite away from the house, which left us with a step up of about a one foot onto the grass, which again we didn't want. So we literally rolled back the grass (using a hired turf cutting machine), dug out the ground down to the required level and then rolled back the grass! It was quite a lot of effort but we now have proper ground levels and I no longer worry about damp around our sole plates.

We're fortunate to have plenty of space around the house to be able to do this amount of landscaping. Other situations may require a slightly different approach but I'm sure your instinct to lower ground levels by some means is the right one.

Some photos would be helpful.

 

[JBean]

On your last question, we removed gravel either side of our large Edwardian bay, hoping to find soil and be able to plant - and found what looked like about a foot of concrete. We called in a structural engineer and he feared it was structural support for our bay foundations, which apparently are separate from the main house foundations and can fail. He advised removing a few inches from the top if we could, as only the lower part of the concrete would be doing anything helpful - the rest was just overkill. In the end our landscaper levered up one of the slabs and it was only about 4 inches thick with some odd deeper concrete shuttering round the edges. We concluded it was the world's most over-the-top weed suppressant!

So maybe, based on our structural engineer's experience, sometimes concrete is intended to do something structural...

 

[malcolm]

I dug up a French drain and replaced it with a trench because of damp walls and rotten wood inside. The gravel gets rid of surface water but mine was quite damp only an inch below the surface (which was still a foot above internal floor levels) and prevented the walls from drying to the outside.

Here is a thread where I ask similar questions to you and then eventually get around to doing something: http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=14273

 

[MikeG]

......I find it hard to believe that a French drain will trap water as its very reason for existing is to allow it to drain away...........

Water will only drain away if the end of the pipe in the French drain leads to somewhere lower and dry. If, for example, the French drain leads to a ditch, and is above water level in the ditch, then it will work. If however the end of the pipe is below a static water level, then the drain is a waste of time and effort. French drains, and other land drains, aren't a magic solution: they actually have to take water away by gravity to a place where there isn't water. For this reason, lowering ground levels around buildings is hardly ever as straightforward as people claim.

 

[malcolm]

True enough our French drain no longer did the draining part especially well - the plastic tube was full of mud as was the drain it was supposed to empty into. I dug a soakaway to make sure the new trench had somewhere to dispense water. Still don't think they are effective for drying walls when they are higher than internal ground levels.

 

[Flyfisher]

Water will only drain away if the end of the pipe in the French drain leads to somewhere lower and dry. If, for example, the French drain leads to a ditch, and is above water level in the ditch, then it will work. If however the end of the pipe is below a static water level, then the drain is a waste of time and effort. French drains, and other land drains, aren't a magic solution: they actually have to take water away by gravity to a place where there isn't water. For this reason, lowering ground levels around buildings is hardly ever as straightforward as people claim.

Yes, that's all true but I'd argue that a 'French drain' that doesn't actually drain because of local ground issues is not actually a French drain in the the first place! :wink:

I agree that lowering ground levels may not always be simple, which is why I described the ground works I had to do to get our ground below our floor level. As it turned out, I ended up having to extend the works about 20m from the affected rear wall and I appreciate this might not always be easy or even possible.

 

[noj]

A french drain (if it does drain) could help stop the trench from turning into a moat depending on how impermeable your soil is. But the surface level of the drain backfill still needs to be below your internal floor level otherwise all you're doing is replacing some wet soil against your wall with some less wet aggregate.

 

[Zebra]

Paul,
Is the raised concrete on your property? How much freedom do you have to alter it? If it's on neighbour's property / pavement / public land, then the issue is more complicated.
Reducing the ground level, and having air adjacent to the masonry is always going to be better than any type of gravel-filled trench, with or without pipework in. Although the gravel allows water to drain away, it still holds moisture, and doesn't allow evaporation from your wall in the same way that air does. I tried the gravel-filled trench, and took it out and built a retaining wall with grating on top.
You need to consider the slope of the land, and whether there is any significant quantity of water draining towards your house from the surrounding area. If the ground doesn't significantly slope towards you, and the only water you are dealing with is the water that actually falls in the trench, that is not very much, and you won't need pipework to take it away.
When considering digging a trench around the house, you have to consider the depth of your foundations. How old is your house again? Does it have foundations as such? The rule, in any case, is that if you imagine a line drawn downwards at a 45 degree angle from the lowest foundation course, you don't dig below that, if you know what I mean.
Also, you have to consider the height of the water table. If you were to dig a hole in the garden, would it fill up with water?

 

[Anonymous]

Hi All, Hi Zebra,....weeeeeelll... All good questions and good advice.
The house has a concrete (I think) retaining wall to the left hand side of the property, which has next doors house plonked on top of it... so if you imagine a slope running left to right, next door on one level and ours on another normally they would have been stepped but ours has a slope (I think) of concrete, so higher on the left han on the right. ...something like this
(in cross section)

On the right hand corner is a 3 / 4 brick plinth that's clearly visible at the foot of the house, there's nothing visible on the left hand front corner...
My plan as suggested is to cut a trench down to what I think should be the correct level approximately 6 - 8" below the original floor (concrete inside with plastic DPM under it will have to come up to do that calculation...

The back of the house is further complicated by an extension and a bathroom with a step up (on the left hand side of the property) and this is where all the drains are (the rain water drains are all on the right hand side (makes sense draining down hill)!!

I'll post photos once I start breaking concrete!!
thanks again for the tips...

Paul.