Hi everyone hope your all well

I have a company doing some decking for me.  The decking is getting put on top of the existing lawn.  However I have just went out to check on the progress of the deck (as the builders arnt here today) and noticed that they have put a think weed supressing fiber mat over the top of the grass and then put the frame down.  Will this be strong enough to stop the weeds and grass comming though or should i tell them to take it back up and take up all the grass first?any help greatfully appreciated

I would say taking up the grass is pointless, garden air is laden with grass seed that is why bare earth will soon sprout more  :)  the thick fibre mat sounds excellent and will kill the grass well enough underneath it.

Forgetting all of that the wood should never be put directly down on grass or anything that water will get through. The wood should be sitting on a proper membrane even if it is treated wood to stop it absorbing the ground water and rotting. It is OK to go onto of the grass IF the protective membrane is there. Quite often people put slabs at the joist supports.