Green Building Forum - French drains at the bottom of solid walls

French drains at the bottom of solid walls

Citrus — Fri, 20 Apr 2012 16:33:40 +0100

Hello everyone -

I'm checking something for a friend, but as it will also be relevant for the house I'm buying, I'm really interested in all your answers.

So, the wall in question is an approximately 1' thick stone wall with a cement render. There is a poured concrete slab which adjoins the wall all around the house. It is below floor level in most places, and in some places it's hard to tell as I haven't checked it with a level or anything. There is clear evidence of rising damp in most of the walls (as with most houses locally) and it has caused some rotting of the joist ends (which she will be addressing).

My friend is planning to renovate the house, and was going to dig up the concrete slab and then replace it (it is quite damaged). I suggested a french drain to encourage water away from the walls, as the cement render on the wall will be staying. Was this the right thing? And should the drain be right next to the (probably foundationless) walls or further away, as someone suggested on Babs' thread? How deep should it be in relation to the wall?

Also, if the render doesn't currently extend to the bottom of the walls (below ground level), should it be extended? I would love to suggest EWI to her but it's completely outside her budget and I'm trying to break her in gently with some more basic things like insulated the suspended timber floors ;) )

So far she has had people suggest injectable DPCs to combat this problem, as well as painting the bottom half of the internal walls with bitumen paint or lining with a DPM (under the plaster). These suggestions seem mad to me if water is still getting into the wall from outside and being held there by the cement render/concrete slab. Surely the damp will just appear in different places in it's quest to escape??

Many thanks for your answers, Hannah

p.s. she's also been told by numerous people that it's not the render that keeps water out, it's the paint on the render. She has some unpainted render and has been told by some tradespeople who've looked at the house that if she paints the render it will fix the damp?

French drains at the bottom of solid walls

tony — Fri, 20 Apr 2012 16:56:31 +0100

How old is the house? Is the wall uninsulated?

French drains at the bottom of solid walls

Citrus — Fri, 20 Apr 2012 17:07:42 +0100

Completely uninsulated. Building is circa 1890-1900. Very exposed maritime position (in case that's relevant).

French drains at the bottom of solid walls

tony — Fri, 20 Apr 2012 17:26:31 +0100

I would like to some insulation go in

Ideally EWI, breathable, a lot of the damp could well be from condensation within the very cold wall and it's inability to evaporate externally, likely the wall was originally built with lime which means that for most of its life it has been dry.

The French drain would do no harm, but the house has many more pressing problems.

French drains at the bottom of solid walls

Joiner — Fri, 20 Apr 2012 17:57:34 +0100

But don't hold your breath on Tony's "originally built with lime which means that for most of its life it has been dry."

Doesn't take long for cement render to make itself felt. But I suspect you already appreciate that.

French drains at the bottom of solid walls

Citrus — Fri, 20 Apr 2012 18:02:33 +0100

I entirely agree, but there is no way that EWI will work with her budget and the other work which definitely needs doing. From reading the forum I got the impression that the next best thing is to focus on getting the wall dry and increasing ventilation on the inside of the house to handle any moisture that does come in. Let me know if I'm barking up the wrong tree though. In a month or two I'm going to have my own damp dwelling to deal with :)

Although it could be condensation, it doesn't look like what I thought condensation-based damp problems looked like. There are also typical signs of condensation damp around windows, in the corners of rooms, etc, but this damp is all at floor-level or just above, and has those little white crystals growing on it which she's been told are salts which have been dissolved from the stone (would love to know if this is true or not).

I could ask her if I could take some pictures if that would help?

French drains at the bottom of solid walls

windy lamb — Fri, 20 Apr 2012 18:43:13 +0100

Any type of stone rendered with cement is asking for; 1. Damp internal walls, 2. rotten woodwork in contact with said walls and 3. deteriorating stone.

First thing I'd do is knock some of the render off and look at the stone. If it's in reasonable condition (i.e and not held up by the concrete render!), I'd knock the lot off and either re-point with lime mortar or, if funds allow, lime render it. All that the cement/concrete render is doing is holding water within the stone wall. If you paint this (the render) you just make it worse.

Also, it's a GOOD thing if the render doesn't go down to ground level - if it does, then it forms a bridge for rising damp to go up and also prevent any moisture in the stone draining out.

The concrete around the base of the wall (forming a path?) could be a good thing but only if it slopes away from the wall AND does not actually touch the wall. Otherwise dig it up and a shallow french drain would work fine.

Remember - if you put anything on the stone it has to be breathable. Definitely do not inject a Damp Proof Membrane as this will almost certainly not work and will most probably ruin the stone.

These old houses were never supposed to be draught proof and relied on significant air movement (rattly windows, ill fitting doors and big fireplace) to reduce condensation. If they put in plastic doors and windows then cement rendered the outside all you've done is make a big fish tank!

French drains at the bottom of solid walls

Cerisy — Fri, 20 Apr 2012 19:05:01 +0100

Hannah - from my experience looking after old pubs we had a similar major problem with a stone basement wall - unfortunately with a brand new Brewers Fayre kitchen on the inside! The french drain idea did the trick. The perforated drain pipe needs to be just below the bottom of the wall to take the pressure off the wall. Put semi-rigid glassfibre insulation batts against the wall to remove any likelyhood of water pressure against the stonework and backfill the trench with pea-gravel to allow the water in the ground to trickle down onto the drain. While I totally agree with removal of the cement render as old stone walls were designed to breathe, you will have to remove all the cement between the stones which could be difficult and costly.

I'm one of those building professionals that don't believe in rising damp - totally made up by a fraudulent industry ... apart from in old stone walls that were constructed with mud (as in our French house - rats!). The damp will rise up earth / mud, but it is impossible to recreate rising damp in brickwork in any properly undertaken test. Injected DPC's are a nonsense, especially when suggested for an old stone wall that has sizeable gaps within it. Most damp in houses is either condensation or caused by too high external ground levels.

Hope that helps, Jonathan

French drains at the bottom of solid walls

Citrus — Fri, 20 Apr 2012 21:11:45 +0100

Posted By: windy lambAny type of stone rendered with cement is asking for; 1. Damp internal walls, 2. rotten woodwork in contact with said walls and 3. deteriorating stone.

First thing I'd do is knock some of the render off and look at the stone. If it's in reasonable condition (i.e and not held up by the concrete render!), I'd knock the lot off and either re-point with lime mortar or, if funds allow, lime render it. All that the cement/concrete render is doing is holding water within the stone wall. If you paint this (the render) you just make it worse.

Also, it's a GOOD thing if the render doesn't go down to ground level - if it does, then it forms a bridge for rising damp to go up and also prevent any moisture in the stone draining out.

The concrete around the base of the wall (forming a path?) could be a good thing but only if it slopes away from the wall AND does not actually touch the wall. Otherwise dig it up and a shallow french drain would work fine.

Remember - if you put anything on the stone it has to be breathable. Definitely do not inject a Damp Proof Membrane as this will almost certainly not work and will most probably ruin the stone.

These old houses were never supposed to be draught proof and relied on significant air movement (rattly windows, ill fitting doors and big fireplace) to reduce condensation. If they put in plastic doors and windows then cement rendered the outside all you've done is make a big fish tank!

Actually her plan is to remove the render from the front of the house and get it re-done, so she would perhaps be into the idea of repointing with lime. The issues there will probably be cost and finding a tradesperson. And would it properly resolve things if the whole house wasn't done? The rest of the render is in good nick and she didn't mention plans to do anything on it (except paint it to make it waterproof )

I did go on about having heard that injectable DPCs are a daft idea so I think that idea's out of the window

Posted By: CerisyHannah - from my experience looking after old pubs we had a similar major problem with a stone basement wall - unfortunately with a brand new Brewers Fayre kitchen on the inside! The french drain idea did the trick. The perforated drain pipe needs to be just below the bottom of the wall to take the pressure off the wall. Put semi-rigid glassfibre insulation batts against the wall to remove any likelyhood of water pressure against the stonework and backfill the trench with pea-gravel to allow the water in the ground to trickle down onto the drain. While I totally agree with removal of the cement render as old stone walls were designed to breathe, you will have to remove all the cement between the stones which could be difficult and costly.

I'm one of those building professionals that don't believe in rising damp - totally made up by a fraudulent industry ... apart from in old stone walls that were constructed with mud (as in our French house - rats!). The damp will rise up earth / mud, but it is impossible to recreate rising damp in brickwork in any properly undertaken test. Injected DPC's are a nonsense, especially when suggested for an old stone wall that has sizeable gaps within it. Most damp in houses is either condensation or caused by too high external ground levels.

Hope that helps, Jonathan

Interesting about the rising damp, and I agree about the DPC. As it appears that the ground level is not higher than the floors, could it really all be condensation? If so, why is it all so noticeably around the bottom of the walls? Are they somehow colder than further up the walls? Houses are fascinating :D

I sort of understand what you describe for the french drain, but can I just check a few things? Was the glass fibre batting just lain along the wall or covered in some way? When you say "below the bottom of the wall", do you mean the above-ground wall, or wherever the bottom of the wall turns out to be below ground level?

The concrete slab runs right to the wall, which is something I have seen on nearly every house in this area. There's no discernable slope, and it's pretty obvious that when the concrete needs repairing people just pour a new layer on top

French drains at the bottom of solid walls

Citrus — Fri, 20 Apr 2012 21:13:27 +0100

Oh! Have thought of a slight issue with the reverting-to-stone idea though. There's been a later addition bay/porch put on the front, which I'd guess is brick (from the wall thickness), so the house would look a bit of a mish-mash. Guess that might be why it's rendered in the first place

French drains at the bottom of solid walls

Gotanewlife — Fri, 20 Apr 2012 23:51:12 +0100

Almost identical issue here. I have been asked to advise a client of mine but I don't have the experience. Same concrete path abutting the external wall. No render but the stone has been recently 'heavily' pointed with mortar containing a little lime. Inside the walls have been sort of cross between rendered/plastered (breathes a bit) but then painted with impermeable paint - yup! the mould returned! Not really lived in much (1 w/e a month in the winter) but from the outside one can clearly see the wall damp up to 2m in some places. I really cannot accept that this is not raising damp. It seems clear though that a liquid DPC will not work. French drain then the only way? Obviously, remove the vapour barrier paint inside. The only other thing that occurred to me that might help Hannah and my client was a 24/7 extractor set in the affected wall (like the Vent Axia Lo-Carbon one (non-HR))? Surely this will assist the ventilation, especially in a largely locked up house.

French drains at the bottom of solid walls

CWatters — Fri, 20 Apr 2012 23:57:21 +0100

If she goes for the french drain make sure she doesn't just dig a long trench right next to the wall. House might just "fall" into it. There is a correct way to trench near existing buildings.

French drains at the bottom of solid walls

Citrus — Sat, 21 Apr 2012 00:05:04 +0100

This should be relatively easy to find out, though, if people here are roughly agreed it's the right course of action. Although what goes between the trench and the house, then?

French drains at the bottom of solid walls

Cerisy — Sat, 21 Apr 2012 10:39:50 +0100

Okay - the glassfibre insulation takes the pressure of any water away from the wall as water will run down through the insulation. Cover the top of it with a thin layer of pea gravel for protection. The trench must go to the bottom of the wall - the foundations if it has any or as close to the bottom as you can, being careful to do it in sections to prevent the structure being affected. It is much better to have 20cm of gravel against outside walls rather than concrete paths as it allows water to drain away and stops excessive splashing.

Damp at the bottom of internal walls reflects the fact that the bottom is always colder than the rest of the wall, hence moisture condenses at that level. Any "moisture meter" will detect this condensation and you get the classic "it's rising damp" - NO!! I had to write a report for a friend who was buying a house and the Building Society survey had identified rising damp - the walls were only one brick thick so very cold and the ground level was higher than the internal floor level - so we had condensation and penetrating damp. My report said that the kitchen would be mechanically ventilated and the ground level reduced. Building Society happy - damp proofing company pissed off - client smiling with extra money in the bank!!

As has been emphasised many times on this excellent forum - the keys are insulation, air-tightness and controlled ventilation. Old stone building weren't designed with large gaps around the doors and windows - that was just the way they built them. If we want to close up the gaps, then we have to address the whole building issues - please!!

French drains at the bottom of solid walls

SteveZ — Sat, 21 Apr 2012 12:14:50 +0100

Hi Citrus, I hope the following thoughts help.

The concrete path around the house will be bouncing falling rain up onto the wall, making the lower part wet and cold. A temporary and easy measure to reduce the bounce-back of the rain might be to lay an energy absorbent material next to the wall. I think strips of glass fibre batt insulation, or a layer of pea gravel spread at the base, or boards at an angle against the wall should stop the rain bouncing and cause it to drain quietly to wherever it goes. If this reduces the damp then you have found at least part of the problem and the concrete must be removed from the base of the wall and the slot filled with pea gravel.

If it is intended to remove the concrete anyway, which seems a good idea, just excavate the base of a small section of the wall to see if there are any foundations. If the foundations are there - great! If not, then digging the ground from the base of a wall without foundations sounds really risky, even in sections. This is the method used to underpin walls and is difficult. I agree a French drain is a good way to remove the water, but if the wall has no foundations, then I would put it a reasonable distance from the wall, with the soil sloped from the wall towards the drain. Use permeable geotextile under and over the pipe and pea gravel, finishing with a top layer of gravel. If the path is still needed, it could go partially over the drain once installed. leaving the gravel at the base of the wall exposed. Use paving rather than cast concrete.

The cement render, although sound, will be an ongoing cause of damp, as previously stated, as there will be no path for internal moisture to escape from the structure. I think it would be better removed, but you could wait to see if improving the drainage makes a worthwhile difference to the damp problem, as it might be tolerable once the penetrating damp is fixed. If the wall is only one foot thick, removing the render might/will shake the wall - probably OK if solid stone blocks, not so if random stone.

Your friend has an interesting problem to solve, and I wish her success.

French drains at the bottom of solid walls

Citrus — Sat, 21 Apr 2012 13:57:50 +0100

Thanks for all of your advice. I think I'm getting a picture of what to recommend that she does now. She definitely wants to remove the concrete (though I'm not 100% on what she was planning to replace it with) so this all sounds like stuff which would be easy to incorporate into that work, and if it resolved the damp, that would be brilliant. I fail to see how it couldn't at least improve it.

I have to go now but will reply more fully later. Thank you again!

French drains at the bottom of solid walls

djh — Mon, 23 Apr 2012 13:52:29 +0100

Posted By: CitrusOh! Have thought of a slight issue with the reverting-to-stone idea though. There's been a later addition bay/porch put on the front, which I'd guess is brick (from the wall thickness), so the house would look a bit of a mish-mash. Guess that might be why it's rendered in the first place

Stone walls on the house with a rendered porch wouldn't look bad. Modern houses often make artificial changes - brick, render, timber panels - just to break up the facade.

French drains at the bottom of solid walls

Citrus — Mon, 23 Apr 2012 21:04:51 +0100

Very true. I'm going to invite myself over to hers for a cuppa this week, and will make sure I revise this thread first. Thanks, guys :)

French drains at the bottom of solid walls

fostertom — Wed, 25 Apr 2012 13:52:27 +0100

Posted By: CWattersmake sure she doesn't just dig a long trench right next to the wall. House might just "fall" into it. There is a correct way to trench near existing buildings.

That's alarming - something I've been worrying about - pse say more?

Of course, not to trench any deeper than the base of the wall,
but are we saying don't remove heavy material from the outside face while heavy stuff still remains against the inside face?
OK once backfilled?
What about backfilling with light material e.g. EPS or Leca,
albeit wedged tight against the remaining solid ground of the trench wall?

French drains at the bottom of solid walls

fostertom — Wed, 25 Apr 2012 13:53:19 +0100

Posted By: djhStone walls on the house with a rendered porch wouldn't look bad

or even a brick porch.

French drains at the bottom of solid walls

fostertom — Wed, 25 Apr 2012 13:57:02 +0100

Posted By: Citrusremove the render from the front of the house and get it re-done, so she would perhaps be into the idea of repointing with lime

That sounds expensive, labour intensive - wd go a long way towards the cost of EWI incl patent render, over the extg render left in place. That, as well as insulation, wd solve dampness, whether penetrating horizontally or due to internal condensation, and wd also solve rising dampness in conjunction with french drain (EWI carried down to base of found) - but see my Q above based on CWatters' warning.

French drains at the bottom of solid walls

tony — Wed, 25 Apr 2012 15:15:23 +0100

I generally just pot a layer of 20 or 30mm stones in the bottom never more than half full.

French drains at the bottom of solid walls

Citrus — Fri, 27 Apr 2012 23:44:11 +0100

Tom, you're right. I was forgetting how pricey paying someone to remove the existing render and re-point with lime would be

She's definitely not getting EWI as there is not enough money in the pot to even re-render the whole house right now, I believe. So would removing the concrete, adding a french drain and improving internal ventilation go some way to improving the dampness, or do people think I'm barking up the wrong tree entirely? They just seem like affordable things which she can do to go some way to improving things.

In our new house (similar site, issues and previous mistreatment) we will be removing the concrete slab around the front and back (it's higher than the internal floor level), and are planning to install EWI in the medium term (5-7 years). Unfortunately we have some structural stuff which needs to be done first, or we'd be having it sooner :(

Everyone's going to be fed up of me asking questions once we finally get the keys ;)

French drains at the bottom of solid walls

CWatters — Fri, 27 Apr 2012 23:57:10 +0100

Posted By: fostertom
Posted By: CWattersmake sure she doesn't just dig a long trench right next to the wall. House might just "fall" into it. There is a correct way to trench near existing buildings.
That's alarming - something I've been worrying about - pse say more?

Of course, not to trench any deeper than the base of the wall,
but are we saying don't remove heavy material from the outside face while heavy stuff still remains against the inside face?

I was thinking of the former as some stone houses have pretty shallow foundations.

French drains at the bottom of solid walls

fostertom — Sat, 28 Apr 2012 10:16:36 +0100

Thanks CWatters - but even when going no deeper than extg found, what about risk that removing heavy material outboard whilst leaving the heavy material inboard, the wall might with time slide outward, due to unbalanced pressure - perhaps 1.5 tonnes per m2 at base of 1m depth of soil?

If replacing the heavy material with broken stone, that wd approx re-balance it.
But if replacing with lightweight Leca, or running EPS down the face before backfilling, both those leave the outer trench very light.
Maybe the Leca or EPS, if well consolidated, would provide a 'strut' brace across the trench to the undisturbed trench wall, but I have a feeling that this brace would 'give' by heaving upward, with time and pressure (if any).

That's been my worry. Any comments?

French drains at the bottom of solid walls

Peter_in_Hungary — Sat, 28 Apr 2012 11:26:47 +0100

Posted By: fostertomThanks CWatters - but even when going no deeper than extg found, what about risk that removing heavy material outboard whilst leaving the heavy material inboard, the wall might with time slide outward, due to unbalanced pressure - perhaps 1.5 tonnes per m2 at base of 1m depth of soil?

So how would you put a french drain on such a building? Perhaps limit the depth to say 50cm (if the founds. are that deep!) or maybe some %age of the foundation depth - if so what %age?

French drains at the bottom of solid walls

CWatters — Sat, 28 Apr 2012 14:27:48 +0100

Posted By: fostertomThanks CWatters - but even when going no deeper than extg found, what about risk that removing heavy material outboard whilst leaving the heavy material inboard, the wall might with time slide outward, due to unbalanced pressure - perhaps 1.5 tonnes per m2 at base of 1m depth of soil?

If replacing the heavy material with broken stone, that wd approx re-balance it.
But if replacing with lightweight Leca, or running EPS down the face before backfilling, both those leave the outer trench very light.
Maybe the Leca or EPS, if well consolidated, would provide a 'strut' brace across the trench to the undisturbed trench wall, but I have a feeling that this brace would 'give' by heaving upward, with time and pressure (if any).

That's been my worry. Any comments?

I can see it might be an issue. Not sure how to answer though! Perhaps best use gravel rather than a lighter back fill material?

I know compressible board (like Celotex) is used as a slip layer on clay soils. My own house has 50-75mm boards lining the out side wall of the foundation trences.

French drains at the bottom of solid walls

malakoffee — Sat, 05 May 2012 10:57:59 +0100

CWatters said :
<< I know compressible board (like Celotex) is used as a slip layer on clay soils. My own house has 50-75mm boards lining the out side wall of the foundation trences. >>

Colin, could you explain about the "slip layer" and why it is suitable/necessary for your circumstances ??

I ask this because I have dug trenches around three sides of my (previously damp) house.
I'll be laying porous pipe alongside, just below, the top of the foundation, but I'm no longer sure that I want to backfill with much/any gravel.
I was thinking of using paving slabs to maintain the void and provide a suspended sloping run-off from the brick wall.
i.e. vertical slab sitting on top of the foundation, against the brickwork :
off-vertical slab, sitting alongside the pipe ( at a slightly lower level ) on the outside of the trench :
off-horizontal slab, sitting on top of the verticals and sloping away from the wall.

It might be ever easier to do this with Celotex slabs . . . nice and light to handle ( but necessary to find a way of stopping the top horizontals from blowing away in the wind.