I use a piece of kite Tyvek cut to 24" x 70" and use it under my tent floor where I sleep. The Tyvek weighs 1.5 oz and I use it mainly to protect the tent floor from sticks, stones and abrasion from my weight. I see no advantage to having a ground cloth cut to the tent footprint since damage to tent floors is largely confined to where body weight is located. I have never found a ground cloth useful in keeping the tent floor dry. YMMV.