So nobody gave any ideas on a waterproof membrane.Tisk.Tisk.
well first is to slope the floor.
you have quite a few options.
then obvious like everyone stated is use a wedge under the plywood.since your cutting it out and replacing it. that would be a good option.
All slopes that interact with water.
(except roof pitch and pools) Building code minimum 2% slope(1/4 inch per foot) That goes for a balcony,concrete patio or soild floor deck.To Slope away from the house or to a drain(outdoor,indoor or shower).Also the pipes for sceptic,sewer and runoff drains should slope a minimum 1/4"per.ft.
Now the floor boards depends on what type of waterproofing you decided to go with.
I will start at at the top.
durable least likely to fail
longevity neefing replace,normal breakdown of materials
#1 Gavinsized corregated(flooring) sheet metal with flashing on sides. Then 1/2" concrete board on top. Waterproof ready to tile.It wont fail,warp,leak,mold,rot,get termites ,withstand 180mph hurricanes,moving flooding water(maybe a small tsunami) that can last, possibly centuries. Although I cant say the same for rest of house. Unless its also a galvinized steel framed home.(like Australian homes.Which honestly was sure how they would be made on the future(our future)Im baffled we dont have them here. Because I was learning how use the framecad program for the machine to make steel framed homes. Way Back in 1996 taking advanced drafting in high school.
, #2 Replace OSB and roll on the OSB coats of Aquaguard,Redguard or another water proof membrane. to trowel on and dry enough to roll 3-5 coats on top. Nexf day start tile.
#3 Remove OSB put down 3/4 plywood. then roll on a cheaper waterproof membrane(Henerys roofing elastomeric, or go oldschool and slop a ton of asphalt emulsion everywhere.Then cover it with roofing tar paper. stalpe down rows startng af bottom and overlap one above it 3-4"inches.Then screw down 1/2 concrete on top of that.but after screting thinset on bottom side of each concrete board first.
(technically you could just use the elastomeric, or alsphat emulsion in sections without tar paper or thinset on the bottom if the boards before screwing down. but it would be extremely messy and wasteful.)
Without a coatinf like redguard thinset wont (doesnt really) bond to a wood surface or tar paper. it is mainy used to fill any holes or voids under the concrete boards so that they are leess likely to flex and crack tiles or pop tiles off because houses move alot Especially a 2nd story.
#4 Keep exsting OSB and fix dips and create new slope on top.
get.thinset and screet OSB filling voids, use 1/4" concrete board on top cut around any humps and screw board down then fill and flatten between boards around humps with thinset.
since balcony is 5ft before edge and aleady have one concrete board over entire surface .
heres an easy way to make a 1/4" per ft slope.
because each concrete board is 1/4" thick. just cut a row of concrete boards 4ft wide and put on top agajnst the house . then put a 3 ft wide row on top aganst wall. Then a 2ft and 1foot so it looks like 1ft 1/4"inch high steps. cut one more strip less than 1 inch wide at the top put againstw wall and screww them all down.
Mix up some thinset a bit thicker to fill in steps. screet it with level or flat stick and you got your perfect 1/4" per ft slope
next day after it dries roll on waterproof membrane on top and its good to go for tile next day after it dries.
you can also use the step technique on the boards under the OSB To make a new slope without having to try and make a bunch of giant long skinny wedgdes all the same hopefully correct slope. With out cutting your fingers off as well.
Either buy some 1/4 wood strips or cut 1/4 strips from a 2x4 on a tablesaw.
then just cut each one on top 1 ft shorter with a 1 inch piece on top.staple nail,screw, the end of each step so if wont ever move.Then place your board on top. it works totally fine and the wood just flattens over time anyway.