dead ground

Agent orange?

Why would you want to 'kill' the ground Badgerbrock? You will kill all the beneficial insects, worms etc that live in it if you do.

i just want to stop anything to grow (grass/weeds) as i am decking a section of gar den

Badger - you can get heavy duty, weed impermeable landscape fabric designed for exactly this purpose - i.e. laying under decking.

weed membrane will do the job and is permeable letting rain etc through. or heavy duty black plastic that will do the job but the rain will run off into other areas.