The Medway Handyman coughed up some electrons that declared:

Looking for a weedkiller to kill off grass under decks. Weed fabric
usually
does the job, but I want to be sure. Sodium Chlorate?

How far do they/does it spread sideways?




Weed fabric alone will be enough - cut the light off and nothing grows.
People on allotments use bits of old carpet - though I suspect your
customers may think that a bit cheap ;-

Sodium chlorate will poison the ground quite effectively - it won't spread
far though, probably a foot at the most in the wet, depending on the
strenght of application.

A Glyphosphate based killer will get in the foliage and travel down to the
roots in the same plant - but this isn't helpful in the case of grass,
though it does kill grass quite effectively.

Cheers

Tim