We have a massive moss problem on the patio paving around the house, particularly in the north/north-west facing area which is in the shade for most of the day, and so far I have found no other viable solution than physically hoeing it every so often to lift the moss. I tried burning it off with a miniature flame thrower last year, but even that was only mildly successful. As we have fish, I don’t want to use algicides (not even sure you can still buy them now, anyway). I read a suggestion to use bicarbonate (baking soda) and a brush to scour it, but that would require significant amounts of powder.
So far all the “French Roundup” that I have come across (post the glyphosate ban) has been pelargonic acid based.
It kills the green tops of plants very quickly - within a few hours - but does not get taken up into the root so deep rooted weeds grow back very quickly.
Also formulations like “Pathclear” in the UK contain a germination retarder or residual weedkiller such as diflufenican - also banned in France
Pelargonic acid is not quite “useless” but pretty close.
A formulation containing just acetic acid would be close to a complete con IMO given that you can by large bottles of clear spirit vinegar in most French supermarkets for just a few cents.
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my point exactly… I reckon they have to recoup large sums to makeup for the fines they have had to pay… 
You need to improve the drainage. Sounds like it could be compacted underneath the gravel? It’s a big job, but should save you effort in years to come.
We selectively hand weed where ok to have height (mainly verbena, sedum, aquilegia and valerian), use a gas gun on the main area we need to keep clear and leave it at that. Salt and vinegar run-off can kill things further afield.
We do use lots of purins, and some we add a tiny amount of savon de marseille where using them to control aphids. Nettle purin works wonders.
Jane… what do you suggest I use to deter the weeds which send their roots into the cracks between wooden doorsurround and the walls… they must have really tiny, spreading roots which go in deeply… up and all round the door
… and I scrape the tips of my fingers trying to grab enough of the top/flowers to pull them out by hand … all I manage is some bits… never enough.
can’t use a gas gun … (not that I have one…)
If it is just around the door, and no other beds nearby I would just salt them. Make a strong solution of boiling water and salt and pour it over them (maybe use a funnel so you target them).
tried similar…but as they are all around the doorway… it just runs off/down… not indepth… although the ones below get a double dose… 
got to make it stay put a bit…hence the soap…
Therein lies the problem, I wouldn’t be too bothered about lifting the dalles autobloquantes or even replacing the sand, but we’re on a granite outcrop so improving drainage by digging channels is a “not worth the effort and expense endeavour”, I’ll just stick to hoeing, keeps me fit 
Ditto, I made TNT and picnic acid, went on to be a navigator on Vulcans!
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Picric
People in France often had a bottle of picric acid solution mixed with god knows what at home as a remedy for burns. Picric acid solids, lots of fun but possibly 
I recall picric acid was very good at destroying lab coats.
Was picric acid mentioned in the picric papers by Charles Dickens?
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@Graham_Lees asks “Was picric acid mentioned in the picric papers by Charles Dickens?”
No, but it was described, mixed with flowers of sulphur, as a remedy for Mrs Gargery’s chesty cough in Dickens’s ‘Great Expectorations’.
We’ve not tried it yet but have come up with the idea to use our large (think mini shop vacuum size) steam cleaner on the terrace. We have 10 x 10 tiles and lots of weeds everywhere (particularly now we have free range chickens who are obviously fertilising the cracks!). Steam gets to such high temps and we get a long steam time so we are going to test it as a totally natural option! I’ll update after I do.
Before I got a gas gun I used a wallpaper stripper - one of the ones with a rectangular plate that exuded steam. It worked, but no faster than hand weeding!
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Oh well that is good news, I think the cleaner we have probably has a stronger steam and there is a small nozzle attachment that I thought may work well to really get on each weed.
Who dreams up the meaningless brand names ?
Why “Roundup?”
Why “Frontline?”
Does someone get paid for that? They give no indication of what the products are for.
What if you get confused and use the wrong one?
Suggestions on a postcard . . . . .
I use the boiling water method; sometimes, after having boiled spuds in the water. It works for localised weeding, ie small paved areas. For the rest, it’s a mixture of hand weeding, enthusiasm, & eventual boredom that determines the appearance of things here