2 Nov 2010
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Isle of Wight
United Kingdom
Hi there,
I am after some advice on what to do about my old retaining wall. My house was built in 1887 and is a solid stone built property. I don't have any damp anywhere else in the house, but the old outside toilet was built with two walls retaining, holding up the garden! I have damp coming through, and don't know what to do about it, I have dug down about 4 ft and there is no damp course whatsoever, just big stone blocks and then earth.
does anyone have any advice on what I can do to help this problem that won't break the bank?!

Many thanks

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A bit more clarity is called for, I think Gemmy.
How high is the soil against the wall?
Is it directly against the toilet wall, or is there an airgap? i.e. is there another retaining wall?
What finish is there to the inside and the outside of the walls?
How high is the damp rising?

Obviously the soil should not be against the tiolet walls.
Many old stone buildings do not have a damproof course (dpc). They are intended to 'breathe'. That is they will take up some moisture, from the air and when it rains, etc, but will expire that moisture, also, inside and outside.
Hi thanks for quick reply.
The wall is about 6 1/2 feet high with soil retained to the whole height.
There is no airgap at all, stone blocks, some old render on the inside, and obviously soil on the outside. I have applied two coats of permaseal tanking slurry on the inside wall which seems to have helped a bit (it is a breathable cement based product), but really want to sort out the problem properly.

The damp is sort of in patches along the inside wall, and I suppose not that bad really.
an adjoining retaining wall (built later) does have a damp proof course, and hence i don't have a problem on that section of the wall.

I just need to know what I can do? Should I continue digging down the whole 6ft and then put a damp proof membrane (that plastic stuff you can get?) and then back fill again with soil?

I would be very grateful for any advice I'm trying to do all this myself, and digging is hard work!
Wow, that's a whole heap of soil against that wall and to be honest, it's beyond my DIY skills for a proper solution.

I hope the more experienced people on here can help you out.
I would add: Don't dig down too far without shoring up the trench. I think more than one metre depth it's advisable to shore up the trench.
Your 4 ft is about the limit! :eek:

Incidentally, don't misunderstand the damp proof course (dpc) and the damp proof membrane (dpm).
A dpc is a horizontal layer laid in the wall to prevent damp from rising via the bricks/blocks/stone/mortar. It separates the upper wall from the lower wall at or about floor level. (not to go into high level dpc).
A dpm can be laid horizontally under the floor or vertically against a wall, perhaps as in your case, to prevent damp penetration from the adjoining material.
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Most likely the damp problem is caused by damp penetrating in through the wall rather than simply rising damp. As the wall has no cavity it is just one large mass that is sucking moisture from the wet soil.

If this was being built today a wall retaining that much earth and especially one which forms part of or is adjacent to the house would be designed by a structural engineer and inspected by building control. I only mention this to highlight that it is important to make a thorough job.

Digging down and simply putting a polythene dpm type sheet up the wall is a bad idea as whilst the soil does press against the wall and exert pressure on it the water pressure is the real problem. So adding an impervious barrier will simply let the pressure build until it damages the wall.

As i see it you have two options.

1. Dig on down and line the wall with polythene or synthapruf but install proper drainage at the base and right up the face of the wall.

2. Tank the inside linking in the floor DPM (if there is one)

It is up to you which you think is easier or cheaper.
Hi thanks for the reply. when you say install proper drainage at the base do you mean on the 'earth' side (outside of the wall) or on the inside?

and sorry to be awkward but could you explain a bit more what option 2 actually means practically?Tank using what? and I don't know what 'inside linking' is!

I really appreciate the advice, thank you.
Hi again Gemma.

Fist a couple more questions, perhaps for more clarity again.

You say the problem is not that bad, is this before you applied a tanking slurry or since? And, if it's not that bad I suspect that you already have good natural drainage and/or the stone is reasonably impervious, like granite or similar.

What type of floor do you have inside the house, and how old is that floor?

R896neo did mean that the drainage should be outside the house against the base of the wall, something along the lines of a french drain, but constructed properly and continued around the house to an outlet downhill.
I don't think it's advisable to rely on good natural drainage.

You have already applied a tanking slurry on the inside and this is what r896neo was referring to, but I'm not familiar with the appropriate materials or methods.
'Linking it' to your floor dpm, means just that, making sure it is continued through, without a break, to your floor dpm. Obviously, if your floor does not have a dpm, e.g. rammed earth, quarry tiles on earth, concrete slab without dpm, then this is going to cause further problems down the line bbecause you may just re-direct the damp into the floor.

A couple more points;
If you can/choose to go down the tanking inside route, it can be a DIY fix.
However, I would suggest that the dpm/drain outside is not really a DIY fix because of the amount of digging required, you'll need a mini-digger at least, and not all mini-digers can reach down 2 metres. And because of the dangers of collapse at that depth. You may even need two diggers, one to dig and one to move the spoil away to reduce the danger of collapse. Plus the technicalities of constructing the drainage properly and applying the correct dpm to the outside wall.

I suggest that you have a long hard think before choosing a suitable resolution. The dpm outside is probably the better option, IMHO, but obviously more expensive. The tanking applied inside, I suspect will need renewing periodically.

The choice will also depend upon the answer to my first question; How bad and when?
To be honest it wasn't awful before, but the tanking slurry has worked, I have a few patches of damp coming through but probably only covers about 15-20% of the whole wall if that.

The floor I think is concrete and because it is the old outside toilet there is still drainage etc below. I don't know how I can find out if there is a DPC there, but I suspect not if the wall hasn't got any!

Because of where my garden is I can't physically get a mini digger up there!

The joys of owning an old house! Thanks for your comments, I'll have to give it much more thought . .
Gemma :)
There may be other options, gemma, but I'm not fully clued up on them, such as, building some kind of stud wall on the inside, with an airgap between the new stud wall and the stone wall, which perhaps should be ventilated.
Or there is a type of membrane that is often used in cellar damp proofing. it consists of a membrane with drainage channels bult into it fixed against the inside of the wall, but then some drainage is required along the bottom of the wall.

Again depending on the floor height, you may be able to apply a sand/cement screed with a dpm under. This would raise the floor level by about 50mm but would give you the opportunity of linking the wall tanking to the dpm in the floor.

The choice really depends on quite a few things: the other factors (floor dpm/height, wall space), how severe is the problem, how much you can/want to spend on the problem, how permanent or maintenance free you want the solution to be, and of course which is the most effective solution.
You have to treat this wall as if it were a cellar because that is effectively what it is.

As RH said the option on the outside is completely exposing the wall and then installing a land drain along the base and graded clean gravel up the face of the wall as you fill it back in so that you end up with a mass of clean gravel against the wall and the water takes the path of least resistance and flows away through the gravel rather than trying to fight it's way through the wall. This then drains through the perforated pipe into a gulley or soakaway.

The big problem with this in your situation is the wall construction. A land drain would certainly cure the damp problems but a stone built house with thick mass walls may have unusual or small foundations and i wouldn't advise running a land drain near the base of the wall for small but possible risk of
undermining the foundations.

In your case with poor access to the rear of the site tanking the inside is probably your best option. As RH mentioned though you need to create a full unbroken envelope including the floor because if you tank the wall and the floor has no dpm the water pressure created behind the wall will almost certainly begin to force water up into the floor.

Whilst your slurry will work a bit you will probably need to go the whole hog and use a tanking membrane. You could read up on it using the search function as it's been discussed a lot or Get a couple of guys out to quote for tanking it and at the very least they will give you an idea of what's involved. It can definately be done diy though once you've looked at it a bit.

Wowza that was a bit long
Thank you so much for your advice. I'm not against getting someone in to have a look but I know that damp related problems can be very expensive! So I wanted to know a bit more about it so I would know if I was being ripped off or not!

your right for me digging up behind the external wall would be a nightmare for me!

Thanks very much again

Best wishes
Gemma :)
If it were me I'd simply add battens to the inside wall and then nail plasterboard with vapour barrier (the stuff with the shiny aluminium on one side), skim it and paint it. Not only will it stay dry but will be a lot warmer due to the air gap.

Forget about messing around with the wall outside - complete waste of money.

Do it my way and it'll cost you around £150 - £250.
But you'll just ride off on your horse after she's paid you the 250 quid...
You aren't living in the real world are you? It's a genuine fix. The wall has been that way for over a hundred years - and will stay that way over the next hundred. All they have to do is board over it with the right materials and it's fixed. Digging out gardens etc is just plain daft. :p

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