Exterior Ground Level Decision

Currently the ground levels outside the north wall of our house are about 280mm higher than floor levels inside. This is because someone built a car park outside. There is a French drain but we do have solid walls and damp problems inside. The house was messed around with in the 1920s. There is a damp proof course but this appears to be more than 200mm above inside floor levels.

I want to remove the car park and build a garage and driveway instead. Lowering the outside ground level by 180mm would make the whole site flat and match the levels next door. But it would still be 100mm higher than inside. Ideally it would be a further 175mm lower.

It would be good to get this right. I could maybe slope a little in the vehicle access between the garage and house- maybe 100mm drop over 3m and keep the French drain with a pipe to a soakaway. And if I use properly permeable materials for the driveway (gravel) I might get much of the rest. Does that sound sensible to anyone? Or anyone have any better ideas?

There are some plans in http://www.the-salutation.co.uk/blog/plans/ to show roughly what I'm talking about. The garage is the folly at the top of the site plan and the north wall of the house just underneath.

Do you mean slope the ground towards the house, in order to have the ground level lower near the house? Doesn't sound like a good plan if so, as it will only bring more water in your direction, French drain or no.

I faced very similar problems at the back of my house, where someone had built a darned churchyard, and buried people in it over many centuries! The ground was 400mm higher. The pic above was my solution. I have to say though, although it undoubtedly helped, I still had wet walls a year after its construction, and the thing that really solved the problem was cutting a new dpc into the brickwork all the way round the house.

Well, it's a lovely looking building.

Presumably the walls are 9" solid? or are they stone?
Surely the dpc would not originally have been above floor level... if of course there was a dpc originally.

I would not rely on anything short of lowering the ground to below floor level to sort out the problem. Ideally, 6" below dpc which in turn should be below floor level.. all of which is easy to say...

Zebra's solution of digging a gully alongside might work for you. It could be a year though before you know, it can take that long for the walls to dry.
And that wouldn't deal with rising damp (await customary picture of house built in large puddle ). Are the walls lime pointed, or cement...

If you intend to leave the French drain as part of your solution I'd be inclined to take a jolly good look at it. Some people have the idea that digging a trench and filling it with rubble constitutes a French drain.

We just spent £8,000 here digging out high ground and putting in a land drain, all a bit of a leap in the dark really.

Sorry to be rather vague.

Ivor

Thanks for the thoughts. A slope probably wouldn't be sensible then.

The structure on that side was originally timber frame built on a brick plinth dating from somewhere from 1646 (according to pub records) to 1715 (according to maps). Some of the timber remains on the corners and interior partition walls, but much has been replaced with brick (cemented) built on the original plinth. Looks like the damp proof course is on top of the plinth hence the distance above ground.

We removed a lot of interior wood panelling and flooring and left the wall to vent, but it is still damp. I still haven't dared to stick a screwdriver into the timber frame to see how rotten it is.



There is an extension to the side built in about 1850 (mostly lime mortar) which seems to be 9 inch solid brick.

I have the opportunity to build drainage and a soakaway - the existing gutters drain onto the pavement, so could tie in some sort of land drain at the side.

Dropping the outside level down 200mm to 90mm above inside floor will make the outside the same height as the neighbours (8m from the house) and the garden. Already that is difficult as I'm restricted in garage foundation excavation depth. I'm only supposed to dig 500mm deep because of a pretty willow tree that I'm not allowed to remove. I could re-plan the garage but it would be a pain. Top of the foul drain pipe that runs close to the building is 300mm below interior ground levels.

I'm going in circles. Could probably make the outside slightly lower still, but I think I'm looking at a trench beside the house draining into a soakaway to get below inside floor levels.

Just a note for anyone considering French drains. I had assumed mine was just a trench with gravel infill as the bricks inside are still wet to the touch. I found out today it is (or was) a French drain, and the plastic pipe dates it to not very old.

I said to the wife "what do the French know about drains". She is French as it happens and it turns out I'm not going to be fed tonight. To be fair I suspect French drains would work better if they aren't gravel filled above internal floor level.



I figured out my ground levels in the end. A 25m plastic pipe filled with water to do a survey was really helpful (have one end at a reference level 1m above ground and measure the height at the other end). I think the site must have always been on a slope, and I'd have a 1 in 6 slope if I lowered ground levels around the house without lowering ground levels on the adjacent barn or undermining my annoying but once beautiful willow. Looking at a trench similar to Zebra's solution but with a sloping drain at the bottom.

Zebra said:

Do you mean slope the ground towards the house, in order to have the ground level lower near the house? Doesn't sound like a good plan if so, as it will only bring more water in your direction, French drain or no.

I faced very similar problems at the back of my house, where someone had built a darned churchyard, and buried people in it over many centuries! The ground was 400mm higher. The pic above was my solution. I have to say though, although it undoubtedly helped, I still had wet walls a year after its construction, and the thing that really solved the problem was cutting a new dpc into the brickwork all the way round the house.

Click to expand...

How exactly does one install a physical dpc in an exsisting building?

malcolm said:

I said to the wife "what do the French know about drains". She is French as it happens and it turns out I'm not going to be fed tonight. To be fair I suspect French drains would work better if they aren't gravel filled above internal floor level.

Click to expand...

Ze drains are not ffrench in the sense of "from France", but in the sense of "invented by Henry French (1813–1885)".

Roger440 said:

Zebra said:

Do you mean slope the ground towards the house, in order to have the ground level lower near the house? Doesn't sound like a good plan if so, as it will only bring more water in your direction, French drain or no.

I faced very similar problems at the back of my house, where someone had built a darned churchyard, and buried people in it over many centuries! The ground was 400mm higher. The pic above was my solution. I have to say though, although it undoubtedly helped, I still had wet walls a year after its construction, and the thing that really solved the problem was cutting a new dpc into the brickwork all the way round the house.

Click to expand...

How exactly does one install a physical dpc in an exsisting building?

Click to expand...

I think most of its talked about here http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=13077

LadyA beat me to it. It's extremely laborious, but one follows middi's detailed instructions. Luckily for me, my house is very small!

Builders turned up today to make a start on the drain. It looks like ground levels were originally more sensible as we found a step down from a door. Unfortunately there don't appear to be any foundations to speak of under the Victorian extension. We'll put a sloping drain in there to a soakaway and a metal cover on top.

I had originally planned to get rid of the shed before doing this work. Building regs insulation requirements have altered the schedule so we've gone underneath the shed instead. Damp levels are currently 20% in the exposed brick inside and off the scale in the failing plasterwork in the kitchen behind the wall in the photo. Hopefully they will start going down.

I think we're looking at concrete at the bottom of the drain about 6 inches below interior floor level, cement for the retaining wall, and lime pointing the existing foundations. Does that sound reasonable?

malcolm said:

I think we're looking at concrete at the bottom of the drain about 6 inches below interior floor level, cement for the retaining wall, and lime pointing the existing foundations. Does that sound reasonable?

Click to expand...

Yes for the lime pointing, yes for the cement mortar for the retaining wall, but what would the purpose be of concreting the bottom of the channel? What is this drainage channel for again, remind us? Is it taking the water from somewhere else? Or will it take just the water which falls in it? In which case, there would be no more need to concrete the bottom of it than there would be to concrete a strip that wide adjacent to the house if the ground were not higher.
My drainage channel (I'll post another picture of it in a minute, I didn't use a metal cover in the end, it was too expensive, but found a good alternative - more later) doesn't have a concreted bottom. It has drains running through it which take all the water from the guttering from the back of the house, but the water which falls in it is allowed to soak into the ground naturally.

Have you considered adding some external insulation to the wall exposed by the trench? E.g. EPS?

The concrete is mostly something to build the retaining wall on. If there is only 6 inches or so between the retaining wall and existing wall the builder's thought was to make a drainage channel out of the concrete footing (which would make it easy to clean out when it fills with leaves from the willow) and run it to an existing soakaway. Top of the concrete will be 6 inches below internal floor levels. I'll not send any more water through than drips in there.

I think we have foundations down to at least 12 inches below internal floor level, but doubt they go much deeper. I felt a bit of concrete would help negate the effect of digging a 0.5m trench beside the house in case the walls felt the change was unusual. Exterior levels were 300mm higher than interior floor levels.

I did consider external insulation when I thought I had the budget and wanted to save on heating. Building regs want me to spend the insulation budget on less sustainable insulation so I don't have the funds to save on heating any more.

Where did you get your cover Zebra? I need to decide on one before next week.

Oh dammit I forgot to take a picture and now it's dark.
I used fibreglass gridding. It comes in black, and with its gritty top, it looks really quite OK, better than it sounds. The company will send out free samples.
http://www.fibregrid.com/moulded-fibreglass-grp-grating.php?bc=Moulded%20Grating#tabview=tab2
I made a galvanised metal frame to fit inside the trench, bolting it to the retaining wall, and making legs for the back so I didn't have to bolt it to the weak house wall.
Then screwed down the fibreglass grid inside it.
I paid I think about £280 for the fibreglass, 220mm wide, 10m long. Metal would have been around four times that, and actually, unless you got some really nice looking church grating, would have looked quite industrial.
Here's the thread where we discussed my trench....
http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=14035

Here's a pic of offcut of the stuff, with mug for scale.

Thanks Zebra. It might make sense to go a bit wider on the cover like yours. It would allow the concrete at the bottom to slope more away from the wall I'm trying to dry out which should cover all bases I've thought about so far. Also good dimensions in case I don't get around to choosing something before the wall gets built. Not sure I'll make a good project manager.

Concrete went in to the bottom of the trench today. Ground levels are now as much as 7 bricks lower at the front of the house and 5 to 8 inches below internal floor levels. We had a downpour this afternoon and the water didn't puddle in the trench so fingers crossed the fall works well. I spotted wear in the flaunching on the existing drain collector which means the drainpipe drains into the kitchen. I have installed a temporary fix using a bit of wood.

I've not figured out how wide to make the trench cover. I could have over 12 inches if I wanted, but 8 to 10 inches feels more sensible for looks and it would still be wide enough for access to clear the willow leaves out.

I've been looking at trench covers today. Your grille looks good Zebra. Do you remember which one it is? FG18 maybe? The holes look small enough to prevent willow leaves from dropping through where alternative type with long thin holes would be no good as willow leaves are long and thin and would fit through easily. Did they supply in the width you needed or did you have to cut to size? I'll give them a call tomorrow.

It was ... (rummages in folder full of receipts.....) Ah, here it is
"Grit Top TwinGrid Grating Panels (FG18), 10,000 x 300mm x 14mm, Black, (Supplied in 3 pieces) Cost £178.00, Carriage £ 23.50"
They supplied in the width I needed, although clearly not in one piece at 10m long!
Here's a picture of the gully.


I made the frame for it myself, out of galvanised angle sprayed black. It's hung from brackets screwed into the top of the retaining wall, with the screw heads mortared over. To avoid screwing it to the house wall, the back rail is on legs.

Thanks once more. I've asked for a quote for the thicker version (FG12). The retaining wall hasn't been built yet so hopefully I can incorporate a step at the top for the grating to sit on, then build a frame to support the other side. My conservation officer is allergic to plastic things, but the cast iron versions I've found so far are expensive (£5k for the length). Hopefully I'll get away with it.

Very neat job there Zebra.