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.
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.
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.
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.