With that sort of spacing between the flags, you cannot prevent weeds - they will be inevitable with anything that is windblown or dropped by birds. But you have gone about as far as you can go to minimize them with the landscape fabric (this is a situation where I would recommend the fabric over any cardboard!!) and a proper compacted sand base. Weeds are well able to root and grow in gravel so I doubt the size of the gravel will have much bearing - IME, they can pop up just as easily in 1/4" minus as they can in 5/8" minus. They just are easier to remove from the larger rock :-)

I agree. The only thing that helps a little is to put your flagstones more closely together if you can. I find a torch or a kettle of boiling water takes care of the weeds that pop up in my gravel and flagstone paths.

thanks for both of your comments. i will probably go with the weed block fabric. it just isn't a long pathway (about 25-30') and I would prefer not to buy a full roll just for that. Plus cardboard is readily available and has a 'recycling' component to it. But I also know it will degrade over time where the fabric won't. and yes, i know i won't prevent - just trying to minimize. But i am thinking of putting in clean rocks instead of any with 'minus' in it, figuring that any weeds that do root in the clean might be easier to pull out.

I also found one site that said the boiling water kills only the leaves, not the root. I will probably try it tho, along with weed b gone and maybe preen in the spring.

Spray. There are products that prevent weeds up to 12 months.

Preen!!