Help on this trouble grass

Jimmy, the grass didn't "poke through the fabric;" dirt has settled in the cracks (on top of the fabric) and seeds have taken root. Two of the reasons why the grass has not grown from under the fabric: no sun to encourage germination, not able to grow through the fabric (providing you didn't damage the fabric when laying the bricks).
The "young ones" are different seeds that were ready to start when you sprayed; Roundup does not affect anything that has not sprouted; it acts on the foliage.
Cement would have helped quite a bit but not eliminated the issue since cracks soon start happening and soon the weeds are there to greet you.
This is a time to use your tea kettle. Pour boiling water wherever you see the grass appear.
Another solution is vinegar.
There are other things also but are either toxic and/or harmful to the soil; some would have worked if applied prior to the fabric and only for a year.
The big thing people miss when using landscape fabric, bark dust, etc., is that the current vegetation might be addressed but there is dust/dirt that accumulates on top and settles down for a place for seeds to take root. Weeds are successful because they can grow in minimally "desirable" conditions. Surely, you have seen something growing in a crack on the side of a building and definitely in the cracks of sidewalks.
If all trace of dirt/dust/soil could be kept out, those common weed seeds would have no place to take root. That is qualified with "common" because there are plants which need no dirt.
Of course, you could let them grow to tall enough to pull them, water the bricks to loosen the soil and pull the weeds by the roots - a solution for someone with more time on their hands than most of us have and certainly as exciting as watching snails race.
Keep pouring the boiling water which will be more effective than the sprays; it will cook the about-to-sprout plants as well as those sprouted and perhaps cook the seeds as well. Unfortunately, the problem will be ongoing as dirt will collect and seeds will be scattered. We welcome it with our lawns and hate it with our sidewalks and drives; contrary lot are we humans.
Good luck. I feel badly for you, all that work and now the weeds; it stinks!
Glenna
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