but how old is the deck, they do rot quite rapidally compared to enclosed frameing
i seen 10 year old pressure treated, redwood and or maghoney decks rotted like you would not belive
so perhaps you would relate the age and general condition of your deck, height off the ground and other such information you may feel relivant
places i notice decks rot(or bugs infectation) are at ground contact points, the rim joist that run agnist house, top side of joists under flooring < and many other plases
mostly where moisture after rains< sprinklers sits
take a good look < relate the real world facts and the figureing guys will let you know about the weight
but hell if your going the expense of walls< windows< roof it would not be out of the picture of building from the ground up new
also its going to be considered a structure requireing permits< inspections< tax increase