Penetrating Damp in your Traditional House (Damp 3)

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Originally posted 2013-06-17 20:37:29.

Penetrating damp is the result of  water coming through the walls.

Once you’re sure no water is coming through the roof by following the previous articles in this category—and the saving grace of that kind of leak is that it is very obvious and marks its presence clearly—the next issue is this one. Here’s an excellent overview of the problem.

I’ll take time for another of my provocative asides here. I’m pretty convinced—actually I am totally convinced—that there is no significant problem of rising damp in most traditionally built houses, at least as long as they have been left that way. Note that last bit. I’ll come back to this later.

 Meantime, if we discount the possibility of rising damp in most cases, we must look elsewhere for the source of water and there are two issues to address here.

Typical French house with damp walls Pic: Rod Fleming

Ground and floor levels

The first issue is  levels. I think you might be amazed how often I have spoken to someone who complains bitterly about their rising damp problem, and shown me a suitably soaked, blackened wall as evidence. And when I went outside I found that the exterior ground level was higher than the interior floor level. Rest assured that this condition will guarantee a wet wall due to water penetrating it, especially if there are also gutter or drainage problems.


 So the very first task you have, when you get the keys to your dream-home, is to go all the way around the exterior walls and measure the levels of both the internal floors and the external ground. Start at a window. where you can get a datum for both inside and out and mark levels in chalk on the walls. A laser-type level is perfect for this, and a basic one will do nicely. Try here.

For the wall to be dry all floors should be at least six inches or 150mm higher than the exterior ground-level. They won’t be. Promise.

If the ground-level outside is higher than the floor level inside, then penetrating damp is inevitable.  So, if the ground outside is earth, you need to dig it away until the level is right. If doing so means that you have to create a moat around your walls, then you must ensure that the water is carried away by a ground drainage system.

The problem is much worse when the ground outside is paved, and it really gets complicated if the ground outside is not only paved but not yours. This is frequently the case in town houses, where the ground outside the wall forms the public trottoir. Unless your mayor is a saint, there is no danger at all of the Commune recognising the fact that for years they have been building up the level of the pavement without reference to your predecessors, far less doing something about it, and you will have to lump it.

Careful with that digger

 Be very careful when digging close to old walls and on no account use a mechanical digger near them. It is very common for the wall footings to be only a few inches below the interior floor level and they must not be disturbed. Dig it out by hand. Here the attentive reader will note a potential problem—what if making the outside ground level six inches lower than the internal floor level exposes the wall footings? Well, I can tell you from personal experience that this is a common occurrence and all the owner can really do is dig down to this level, back fill with a couple of inches of gravel and ensure that the slope of the ground is gently away from the wall. These old walls were built straight onto beaten clay subsoil, and you must not disturb this or undercut it. That is one reason why you must not use a mechanical digger.


Guttering and Rainwater Drains

Once you have dug away all the earth that you can, and arranged the levels such that any water that falls onto it is led away from the house, the next thing to look at is the guttering and drainage.

These two items, when working properly, are a source of easy sleeping and contentment. When they are not, nightmares ensue. This issue is compounded by the fact that in many areas of France and elsewhere, no roof guttering was traditionally used, leaving the modern owner with the choice of blighting his beautiful dream-home with nasty modern gutters or tholing the damp.  If faced with this, you have to remember that unless you plan to use traditional open-fire heating methods and traditional non-hermetic windows and doors, then that damp will get worse and worse. I see no alternative to fitting proper guttering and just trying to make it as neat as you can.

Zinc Gutter

 Where guttering is traditionally used, the matter is much simpler. It is still possible to buy traditional zinc gutter in France, and at reasonable price, but it is an expert’s job to fit it. I’m not saying you can’t, but joints have to be soldered in place, which means using a blow-torch up beside those nice roof-timbers. You could use an epoxy cement instead, but whereas solder is hazardous and finicky, epoxy is messy and finicky.

Plastic Gutter

Personally, where the gutter is not on a prominent façade, I would use plastic. Give it a week and you won’t even notice it. It is easy to cut, fit, joint and repair, and it is genuinely as cheap as chips. Make sure that there is adequate slope to ensure good flow—1:100 is enough for deep-section gutter, twice as much for shallow, and make sure downpipes are installed properly and at the right distances apart. The shallower the gutter the more you need, so the added cost of deep-flow is offset by the reduction in the number of downpipes, fittings and connections that are required.


Clearly the water collected by the gutters has to be got rid of, and you will have to lead this into proper rainwater drains; you can’t run it into either your own or the communal septic tank or other treatment facility. This will cause problems if your house is in a village with no street drains, as is often the case. All you can do is drain off to the lowest point and then run the water into the street. This is what all your neighbours will be doing anyway. It can make for spectacular results as the streets flood under the assault of a torrential summer thunderstorm, of which central France sees many.

If you live in the country, things are easier; run the rainwater drains off to an existing ditch or waterway, or if all else fails, dig a soakaway, dry well or French Drain at least six metres from the house, preferably more, and run into that. Bear in mind it will have to be a big one.

18 Replies to “Penetrating Damp in your Traditional House (Damp 3)”

  1. Hi Rod, thank you for your engaging blog – in particular I have really enjoyed your article on penetrating Damp. You say “….it really gets complicated if the ground outside is not only paved but not yours” which describes my problem to a tee – but then you say that this will “guarantee a wet wall” and “you will have to lump it…”. Does this mean that there is no natural solution, and that the repellent route of invasive chemical injections and interior tanking treatments to stem the symptoms (in my case, serious damp and dry rot), is the inevitable course of remedial action?

    1. Hi Grainne

      Thank you for your kind comments.

      The problem is that you can’t go digging out a public footpath that is not yours. Depending where you are you could appeal to the local authority (in France, the Mairie) and put the case to them. They might let you install a French Drain — with large stones recovered with smaller — to let the water drain away and infiltrate less, but since the issue is usually that the road itself has been raised over the years, there is always going to be a problem there. It would be better if you could at least dig out enough to tank the outside of the wall rather than the inside, if you are going to have to tank. Be careful using diggers though I have to say I hired a mini-digger last month to do some work and was surprised how delicate you can be with it.

      Chemical injections, if your house is a traditional stone-built one, are a waste of time. In any case a dpc will only work as long as it is above the ground level — and if that is significantly above your interior floor level, it’s not going to help.

      Tanking the inside of the wall should also be last resort. The trouble is that the damp will just rise up the wall.

      Feel free to come back, I’ll do my best. A few pics of your situation would help.

      Best Rod

      1. Hi Rod,

        Thank you very much for your response.

        The situation with the adjoining laneway is complicated, and I am afraid to say too much on a public forum, however I can say that I am very uncertain as to whether
        I will be in a position to install a French Drain (or even an external tanking solution, which I had not identified as a possible option) – however I will do everything I can to try to pursue one of these two options.

        There is what I believe to be a cement render on the outer (end-of-terrace) wall (my uncertainty is because I have never seen lime render on a house, so don’t know what it looks like up-close). Regardless of whether or not I have the opportunity to follow up on either of your laneway-modification options, it might help to have what I assume to be cement render stripped from the outer wall and replaced with a breathable alternative.

        On chemical injection effectiveness, my understanding from the damp proofing company is that they would strip off the plaster internally up to 1.5 meters, tank the walls up to this height with a cement/sand mixture and a tanking agent, and then – following the exposed mortar line at the level where the tanking ends – inject chemicals that would diffuse through the capillaries/pores of the mortar to form an impervious layer. I have just realised from your response that I would need to be absolutely certain that the external laneway is no greater than 1.5 meters higher then the ground floor of the house….

        I was struck by the damp proofing company’s attempt at reassurance with reference to their 22 year guarantee (i.e. as evidence of chemical treatment effectiveness), however I couldn’t help seeing it as just the opposite – as evidence of its ultimate ineffectiveness.

        I would like to run a possible hybrid option past you, in particular with reference to your comment “…Tanking the inside of the wall should also be last resort. The trouble is that the damp will just rise up the wall….”? Let’s say that, despite best endeavours, I find that I am not in a position to make any alterations to the adjoining lane way. Under these limiting circumstances, if I were to go with an internal tanking solution, and then, instead of chemical injections into mortar from the point where the tanking ends, what if I were to resolve the external cement render problem. Wouldn’t this mean that, even though the wall would now be partially blocked from ‘breathing’ on the lower internal section, nonetheless by restoring breathability on the outer wall (i.e. to the surface area above where the adjoining laneway intersects with the outer wall, it could compensate?…Or do you think the newly exposed ‘breathing’ wall sections would be over-burdened and would be unable to sufficiently compensate (without impact) for the non-breathing, tanked wall sections?

        PS: Apologies, but I cannot figure out how to upload attachments with my response, so I hope it makes sense in the absence of photos.

        1. Hi

          Well, easy first. Lime render is soft so take a 4-inch nail and give a decent scratch, firmly, somewhere unobtrusive. If it’s lime the nail will dig in and leave a clear indent, if it’s cement it will chatter over the surface. Might leave a scratch but it won’t dig in.

          You house sounds like it is very much interred. IMO whatever they say (how do they come up with that 22 year guarantee I ask myself) and even if their attempts succeed that is going to be a really cold wall. The problem then becomes condensation from the vapour produced by living in the house.

          I think you might be better to double the wall, installing a stud wall with 100mm glass-wool insulation and finished with plasterboard.

          Leave a 25mm air gap between the studding and the exterior wall and install breather vents, through to the outside, which will help suppress any dampness. You should investigate this approach before spending a fortune on a chemical treatment.

          If doing this you must ensure that nowhere does the timber structure come into contact with the wall or the floors; use 4-inch pvc dpc, available in rolls for very cheap. The timber itself must be properly treated against fungus etc. Basically you lose 6 inches of living space but you would have a dry, well-insulated interior wall.

          Everything you can do to get a flow of air over the exterior wall is good. Install breather vents to the outside above ground level and then again just below ceiling level. You can use air-bricks with a section of tube in between. Then install hit and miss vents in the plasterboard wall just above floor level; again, this will encourage airflow, and you can close them when it’s too cold. This air will flow up the air-gap between the stud wall and the masonry, promoting a healthy environment, before escaping to the outside through your upper air-bricks.

          Basically what you need to do is develop a multiple belt and braces strategy using a number of complementary techniques. And remember: free-flowing air is your friend.

          BTW, I don’t know where you are — are you in France? If you’re actually in UK, and if you are lucky, then your local Building Control Officer can be a tremendous ally and a real source of knowledge. Don’t be shy to ask his or her advice. (There are some jobsworths but the most I’ve dealt with have been really great.)

          1. Hi Rod,

            I live in Ireland. Re: ‘Building Control Officer’, up to last year, the building industry here operated a system of ‘self-certification’ – whereby builders could certify their own compliance with the building regulations. You might imagine how this worked out.

            Thank you for the alternative perspective to managing damp in an old house. I may know little about houses and construction (though I am learning as quickly as I can), nonetheless your advice makes intuitive sense – and there are ‘first principles’ steps I can take immediately, such as tackling ventilation (in line with your comment “.. free-flowing air is your friend…”).

            I hope others in my situation find this blog and similarly benefit from your insight.

            Thank you,

  2. Hi Rod,
    We bought a small townhouse “holiday home” in Aude in 2013. Being retired I can spend about 13 weeks a year there with a clear conscience! In Spring 2014 I noticed a damp patch about 1 metre up our front wall by the door. Bit concerned because it was near a light switch. I had wondered why there was some strange upvc cladding on the lower part of that wall and began to have my suspicions.
    The house has been shuttered up and empty since early November, but we pay a management firm to do fortnightly security checks (nice English couple). On the most recent visit Dave took his “damp meter” and has reported that the patch on the front wall has reappeared and there is evidence of damp in the cellar. The floor level is about 50cm above street level and so the damp is at about 1m50cm!
    Suspect it’s an old rubble wall which has been plastered over and then the cladding was put on to disguise a problem!!
    Really need to get an accurate diagnosis before shelling out and wondered if you could advise who to contact and how to approach getting advice. Fear being pressurised into spending on an expensive damp proof course when that would not solve the issue???
    Would be grateful for any guidance.
    Steve R

    1. Hi Stephen

      Well if it’s a rubble wall then no damp proof course will really work short of hacking out a line of masonry the full thickness of the wall and then inserting a pvc one and rebuilding. That will likely cost as much as the house and it might cause terminal damage if not done carefully. Do not waste money on one of these solutions. The only one that really does help is to punch through th ewall at 60cm nentres and insert terracotta pipes. The se must be porous and unglazed. It does definitely help but it is not a cheap procedure and needs to be done by someone who knows what they are doing, which in my experience excludes most local French brickies.

      If there is evidence of cement rendering then this should be removed but you have to be careful, these walls are very fragile. Remove any gypsum plaster, it’s a magnet for damp.

      The best solution is to provide for a really good flow of air through the affected parts. This may involve punching through and putting in airbricks. You have to remember that the walls in these houses are placed on the earth and there are no real foundations, so any efforts you can make to lower the water table around the house, eg putting in French drains, may help but do please remember the 45 degree slump rule.

      These houses were built when everyone had a huge fireplace with hundreds of cm3 going up the chimney an hour so they were airy but stayed dry; with the move to closed stoves and CH which far less air changes they get damp.

      You could try running a dehumidifier constantly in conjunction with better aeration — leave your windows open behind the shutters when you’re not there for example. Don’t be tempted into a proprietary DPC; I have seen em all and apart from the clay tubes they don’t work.

      I am in the Phils right now so its a bit tricky to respond sometimes but please contact me again if you need more advice. In general however, I think you will have to manage the damp problem rather than actually cure it.

      Good Luck


    2. An afterthought: the standard French procedure is to line the damp wall with polystyrene backed plasterboard. It’s crude but very effective. Ihave had to do that on a couple of walls where I just could not prevent the damp.

  3. Hi Rod

    Very grateful for your response and apologies for not thanking you sooner.
    I was actually at the house between 9th and 16th January, but did not have access to wi-fi.

    There is some dampness, but I didn’t think it was actually as bad as last year in terms of moistness, although it may have spread to a couple of other adjacent areas! I had our standard electric heaters on during the daytime and managed to borrow a friends dehumidifier which did seem to make a difference. Got pints rather than gallons in the reservoir. We don’t have a wood burner or open fire, only standard radiators and the two in the downstairs kitchen/living room can struggle on a really cold day. I expect the main problem is ventilation and had independently thought of the shutters closed windows open approach. Issue then would be security as we really are directly onto the road of the side street in which the house is situated near the centre of town. I know some Brits don’t think there is any crime in France, but I don’t want to tempt providence!

    I am due back in March to do some decorating and will see whether I can do a successful cosmetic job then with a combination of your helpful hints. Not sure if there are any particular paints which might work??

    Thank you again for your reply, and I may seek your wise counsel again as we spend more time in the magnificent Socialist Republic!

    Steve R

    1. Hi Steve, for some reason this comment got caught in my spam trap, I don’t know why. My apologies. I had thought that once approved, you would remain that way… hmm. I will tweak my spam filters.

  4. Hi Rod,

    Thanks for informative article. It took a lot of hunting around to find someone who was clearly describing this problem.

    We, like many others above (although I haven’t read them all: sorry) have bought a house in France, which has exactly the problem you describe.

    “If the ground-level outside is higher than the floor level inside, then penetrating damp is inevitable. So, if the ground outside is earth, you need to dig it away until the level is right. If doing so means that you have to create a moat around your walls, then you must ensure that the water is carried away by a ground drainage system.”

    Specifically, the house is on a slope with a gable end wall at the higher end of the slope, and the ground there probably a foot or so higher than internal floor level. It’s an old barn so almost certainly no DPC.

    I wondered if you could advise me:

    1. How far to dig down (below internal floor, or down to foundation)?

    2. How wide the ‘moat’ should be?

    3. What, if anything should the moat be filled with?

    4. Should I try to build a retaining wall to hold back the soil on the other side of the moat?

    5. Is a French drain appropriate to put in the moat?

    I’d really appreciate your advice and any further observations. I’ve got pictures, if they’d help.

    We complete in October and it’s the first thing on my wife’s list for me to do!

    Many thanks in advance.


    1. Hi Ash, well, your problems are not unusual.

      1 The foundation is probably only a few inches below the internal floor. Do not go further than that. Remember the 45 degree rule at all times.
      2. It’s not so important how wide, but to form a walkway, around 80cm is needed.
      3 If you’re backfilling, gravel, not hardcore, allows quick drainage
      4 yes, if the moat is anything over 30cm deep, certainly
      5 French drains are useful but you risk undermining foundations if you do it there; if you were to take that approach, do it in short sections, say 1.5 metres and consolidate as you go. Better might be to install surface drains (caniveau) or perforated drainpipe at say 60cm depth, running into the rainwater drains. Get as much rainwater as you can away from this area.

      Where is your dream home, by the way?


  5. I have to say that your article on penetrating damp in traditional houses was a joy to read and the best article yet (of my searches on the internet for similar subjects). A half day spent in the tax office (also a French resident) followed by collecting a 3rd trailer of gravel to fill a trench that was dug at back of house with hope of getting rid of damp (inside floor level a few inches lower than ground). Only to find that all the ‘gravel’ I put in already (not too much, a tonne so far) was possibly the wrong type and shall have to dig out again and think of somewhere to put/use elsewhere. I was advised to buy a largish roughcut gravel mix that was grey in colour; it didn’t seem right, but he assured me that it was gravel with sand and great for drainage. But today, when I went back his colleague said it was just rough cut gravel I needed as the other would consolidate to form a cement and the water would not penetrate that, but it could be ok too if I slope it away from the house. I don’t know what to do; dig up all I have put down and return the 3rd trailer and put down a free draining pure gravel instead (don’t think underground drainage pipes necessary; there are gutters, it’s more a question of the outside level being slightly higher than inside) or continue with the ‘gravier secondaire’. I would really appreciate your advice on gravel type for the trench (profile of trench is 30cm x 30cm). Many thanks!

    1. David the gravel/sand mix is wrong. You need the gravel only. I’m on mobile phone so can’t reply at length but any gravel roughly 10mm is fine. It sounds like you got ‘crusher run’ aka road metal. That will indeed compact and form a solid mass. Great for roads not for drainage

  6. Many thanks Rod for your quick reply and info. Good to at least know it is the wrong stuff and can now get on with scraping out what’s been laid and returning the third trailer.

  7. We live in south west France modern bungalow with no problems at all that we can tell until late September into October when we get inundated with woodlice!! We have many in the garden as we are very rural living by a vineyard and plum trees. We can’t see any damp in house but you would not believe the amount if woodlice that we get and can’t stop other than and powder a round house which does act as barrier. Please please help

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