I had a look at the uk building regs part H, they are on line so you can have a look to see if they help you out, I think french poo performs more or less the same as the English stuff.
they state the minimum drop should be 18mm per m and a maximum 6m run for a single toilet.
you will need to put a vent after the furthest toilet from the main drain and fit a rodding point.
Can we have a link to your site please?
Thanks
j
I’m inclined to agree with Andy on the length of the run, and with Scott on the loo, composting toilet may be the way to go. Any links/info on those Scott?
j
I have a small amount of info on my site, we have one and it works well for us!
Hi Andy even with a 1 in 40 drop you feel that the pipe runs are too long?
So the 1m soil pipe laid in the concrete is then exiting to a down pipe/vent stack?
I’m no expert Johnny i’m an ex heating/gas engineer but those ‘horizontal’ runs sound far too long to me, i’ve just had a quick through my ‘comme un pro’ plumbing book and the only thing i can find is where soil pipe is laid in concrete the maximum horizontal is 1m!
Hi Scott, Good point about the gas so thats answered the question as where to place the stack really.
I haven’t come across self contained composting loos, sounds interesting, do you have anymore info or websites etc?
Hi Johnny
I would imagine that the stack should be at the highest point as gas rises and runs uphill. Have you looked into fitting a self contained composting toilet which just require a small vent pipe only?
Spanc now see them as an accepted alternative and save on water bills too!
Just a thought
Scott
Thanks James, just want to make sure that horizonatlish run of that distance with a bend is realistic and as you said won’t cause blockages.
Hi Johnny
I’m not sure of the French regs without looking it up but 1:40 is generally recommended, anything less and the solids could hang around in the pipe and eventually cause a blockage 
j