I’m after some confirmation that what I’m proposing to do is, if not correct, then at least won’t cause more problems than it’s supposed to solve.

I have a stone-built outbuilding (or Mancave), of which one corner is set back into the hill behind it. As a result of this, when it rains, the Mancave has water coming in through the wall in that corner. There’s no DPM or anything.

I’m proposing to possibly batten and DPM the two walls that form that corner, but we may also have problems with water coming onto the floor slab.

To alleviate this, I was proposing to excavate about 6″ to 1′ around the back corner down to just below the footings (about 1 – 2 feet or so), put a DPM against the wall, and then backfill with graded aggregate to make a sort of French drain against the wall in the hope that will direct runoff away from the walls.

Will this work? Or should I just be looking at tanking the inside instead?

Sounds reasonable. You need to make sure the DPM isn’t arranged so that it holds moisture against the wall. Also for drainage you want singled sized aggregate, not well graded.

Ah, cheers. I was assuming you’d use bigger stuff at the bottom, then progressively smaller as you moved up. That’ll probably save some cash then! 🙂

Why not bury a drainage pipe in there and lead the unwanted water elsewhere?

A French drain doesn’t have to be against the wall and you risk weakening the foundations if you go too deep at that point. A proper French drain with a pipe with holes in the top part only further out from the wall is an alternative.

I’ve got a 25m coil of perforated plastic pipe for that, which you can have if you can collect it. Lancashire. It’s going to the dump on Saturday.

Edukator – Troll
A French drain doesn’t have to be against the wall and you risk weakening the foundations if you go too deep at that point. A proper French drain with a pipe with holes in the top part only further out from the wall is an alternative.

TBH I’d be very surprised if there were any decent foundations at all. I’m not proposing to excavate too far below the footings – possibly just down to the edge of the slab to allow room for the DPM.

The main problem is that there is a grass/earth footpath that runs nearby up the hill which bends just at the corner of the building, meainng water can run unopposed down the path straight against the corner of the building.

A French drain doesn’t have to be against the wall and you risk weakening the foundations if you go too deep at that point. A proper French drain with a pipe with holes in the top part only further out from the wall is an alternative.

+1

I put a French Drain mid-way between the road and the buildings. It’s not obvious from the photo, but the internal floor in the buildings are about 12-18″ below the road surface.

Went down about a metre for the drain, plus there is a natural flow towards where the photo was taken from – into a deeper ‘sink’.

The main problem is that there is a grass/earth footpath that runs nearby up the hill which bends just at the corner of the building, meainng water can run unopposed down the path straight against the corner of the building.

Do something further up to hill too?

b r – Member
Do something further up to hill too?

I’d like to, but it’s not my land.

I’d like to, but it’s not my land.

And, who’s is it?

If nobody’s or nobody will care if you do a bit of groundswork – do it.

We’ve all the right to protect our property against run-off IMO.

What @br says. I would focus on reducing the water flow and/or diverting it away.

So this is it:

http://goo.gl/maps/imwUx

The footpath follows round to the left – technically we don’t own the grassy bit next to the building, but we have a right of way across it for the cars. The path goes up all the way to the top of the hill, and it’s used by residents up there to access their garages and back gardens, so if I start messing around it with I suspect I may find myself meeting Mr D.S.Gruntled in short order.

Your original suggestion looks like a good option there. DPM on the outside.

We’ve been here before, it might be worth a search. A slotted or perforated pipe can go in with the holes down*, and using more than one might save money on aggregate. Have some sort of liner between the gravel and the soil to stop dirt washing in and clogging up the works. You may be taking away support that the ground gives to the wall, so you need to consider doing a short length at a time and filling before moving on.

*Think about it, it can lower the groundwater level to the pipe base, not just to the pipe top, if you put it in with the holes that way.

Yup we’ve been here before and proper French drain pipes don’t have holes in the bottom because they are intended to remove water from the trench rather than irrigate the whole trench. If one part of the wall is wet and you want all the wall to be wet use a land drain with holes top and bottom. If you want to dry the wall use a French drain.

I like how it’s “Marshfield” 🙂

Lol – sadly I don’t think it’s meant descriptively, but Marshfield village is a couple of miles away!

OP I don’t think the other residents will complain if you install your pipes/soak-away and then return the surface to the original condition, ie overgrown grass ?

jambalaya – Member
OP I don’t think the other residents will complain if you install your pipes/soak-away and then return the surface to the original condition, ie overgrown grass ?

Agreed on that part visible on Google, but it’s the bit going round the corner and up the hill that would probably be a problem – it’s not very wide, and everyone else’s gardens etc back on to it.

There’s also the legal issue of if I DO divert water which ends up in someone else’s garden will I be responsible for any consequences? I think I’m probably okay with the limited scheme of works I propose, but if I start getting more involved up the path that could cause problems down the line.

What you proposing to tank it with?

Life’s too short, but there’s a nice demo here from a drainage contractor…

He’s not French and that’s not what the French call a French drain. I checked out suitable pipes when I thought of installing one, went on a French DIY forum to check when you said put the holes facing down on the last thread and the advice is consistent – use a pipe that will carry water away rather than irrigate the whole trench. A field drain is different to a French drain.

Depending on the age of your building it may well have no footings to speak of, in which case, digging down vertically immediately adjacent to the wall is cutting through the load bearing area. I’ proceed with caution and not go far unless it’s built directly on bedrock.

The way you put it make it seem like your’re going to put a square of DPM down and then put the drain in after. Firstly, this would be the wrong way round, but isn’t going to do much unless the drain has somewhere to go. Also , Gravity is the driver here. This may mean that your DPM just gets bypassed.

The devil is in the detail for this stuff, and you haven’t really given enough.

I’ve only skim read this, does the water pool at the base of the wall for extended periods of time?

If so then you need to install a catch drain or even something like the ACO linear channel (or one of the many cheaper plastic alternatives).

If the soil from the hill side directly abuts the wall then it either needs to be reprofiled to prevent this or you need an impermeable layer between the wall and the soil. Maccaferri make a product that will provides an impermeable layer but more importantly will drain the water. You would need to get rid of the water to a drain or discharge as overland flow. http://www.maccaferri.co.nz/Building/Drainage.html

Post a photo and it might be easier to visualize.

Scienceofficer – Member

The way you put it make it seem like your’re going to put a square of DPM down and then put the drain in after. Firstly, this would be the wrong way round, but isn’t going to do much unless the drain has somewhere to go. Also , Gravity is the driver here. This may mean that your DPM just gets bypassed.

The devil is in the detail for this stuff, and you haven’t really given enough.

Well, I was intending to put the drain near to the wall with the DPM up against the wall above the effective ground level. The drain will discharge onto the rest of our drive. Can’t see if I put the DPM vertically against the wall how gravity can affect it, unless it can push it uphill?

amesca – Member
I’ve only skim read this, does the water pool at the base of the wall for extended periods of time?

No. It’s coming in through the wall during heavy rain, I assume due to groundwater pressure, but it doesn’t pool, it’s just consistently damp.

If the soil from the hill side directly abuts the wall then it either needs to be reprofiled to prevent this

Not an option as it’s a footpath.

or you need an impermeable layer between the wall and the soil. Maccaferri make a product that will provides an impermeable layer but more importantly will drain the water. You would need to get rid of the water to a drain or discharge as overland flow. http://www.maccaferri.co.nz/Building/Drainage.html

Or, given it’s only an outbuilding, some impermeable plastic membrane would do…

Post a photo and it might be easier to visualise.

I dunno if it’s just me, but I thought it was fairly clear from the Google Maps link what was going on?

In a real world scenario which is where I reside the best plan of action would be to excavate down and use a bitumen based membrane such as bituthene along the legnth of the wall. However youll struggle to seal it at internal floor level as you would normally tie this in during construction, however I appreciate that this is not an option. To do this you will also need to excavate a trench SAFE to work in as you will be bending down etc to adhere whichever method you use to the wall. I would then lay a wavincoil flexible perforated pipe wrapped in a permeable geotextile membrane to allow water in but no fines. This would sit on a thin bed of single size 20 mm limestone/pea gravel and then be back filled to the top of the trench with the same material.
There are also paintable systems such as Heidi k11 from sovereign.
Its always easier and more successful if you can damp proof on the positive pressure side (I think its that way round) as in external wall with water pressure pushing against the protection rather than the internal wall where the pressure is pushing it off the protected wall.
But in all seriousness do be careful in excavated trenches against buildings especially with the current rain we’re having.