I need to kill all grass and weeds before laying a paving in my garden - how????

smoke'em!

Use a sheet of Viscuine, (plastic), place sand on top, then set the pavers. You can use weed killer chemicals first if you want to.

Try TUMBLEWEED it is neutralised when it hits the soil so wont do damage elsewhere.Will also kill all weeds especially if you use a strong solution. Then put down viscuine a plastic membrane that lets water through but not weeds.

use neat vinegar on them, then cover them with wet newspaper- which will suffocate and kill them

Edward's answer worked for me. My plastic sheeting was black so the light couldn't get through. Then I used a nice white sand. And roundup is good to use before you lay the plastic.

Get some weedkiller called Round Up. It's the dogs.

Round-Up for sure... just be sure if you have kids or pets to keep them away from the area that you treat.

I would NOT use Round-up. It turns to fertilizer after it is diluted. Get a broad range brush killer.

What we did was level the4 area. Then layed down layers of newpaper in the entire area then we covered with the black weed plastic and then the sand and pavers. It's been 3 years and not one weed. We prefered to do it without chemicals because rain water can make the chemicals flow to surrounding areas and possibly kill other grass and plants not to mention pollute the groundwater.