How do I waterproof a deck which is on top of a room. The deck has regular OSB flooring.?

We have a deck which is on the second floor and right under the deck there is a room. The deck has a roof over it and the floor is just regular OSB, supported by floor joist. The floor is FLAT and is not waterproof. Our roof has to go, it is becoming very unsafe and unstable. What I would like to do is replace the windows and put French Doors to gain access to the deck and would like to get rid of the roof to make the deck open. How do I go about having the proper drainage for water and water proof the deck so it does not leak below and also how do I make sure the water does not get into the room?

Answers:
Paint will not waterproof your deck!
The proper way would be to first remove all of structure that is unstable.(roof,walls)
Hopefully you have a difference in height between top of deck and floor of house(7"or more would be ideal)
Start by covering OSB deck with a rubber membrane which is typically glued down with rubber cement(be very careful not to put any holes in it because this will be what keeps water out of your house)Be sure to run rubber up existing wall as a flashing.
Next you will have to make some "SLEEPERS" A sleeper is a piece of lumber cut with a taper to it.This is how you achieve pitch so water can run away from house.Get a piece of lumber(2x4 or 2x6)the width of your deck.Cut it aprrox.1/4" to 1/2diff from end to end(example:2" on one end and 1 1/2" on the other end.
Next lay sleepers on rubber membrane DO NOT NAIL!!!
Glue them down with construction adhesive.(16" on center)
Once glue has set apply a piece of metal flashing(bent at 90 deg. angle)on top of sllepers and up house wall(4-6")
Then apply new decking on top of sleepers.

Hope this helps.
GOOD LUCK!

Other answers:
Paint it, it seals it and look better than stain
Paint it, it seals it and look better than stain
sight unseen,,it sounds like (as long as it could support the weight) you may be able to apply a viyl undercoat and then skreet the floor at an ever-so-slight pitch to deter the water,,then possibly a skid resistant ceramic tile.This would look great and not be outrageously expensive
Well if your going to remove the roof remove and replace the deck with typical decking these are usually spaced 1/8 to 3/16 of a inch apart to allow water to pass through. This might be ok if the room below has a normal type of roof. Now if you'd like to keep cost down and not remove the deck I would suggest paint but there is a paint that is used around community pools that is thick and will seal and resist water. It also comes with non skid material in the paint like sand . This is also good for use on interior or exterior stairs.
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