I have a flat roof over my garage roof, it used to have the membrane and rock deal. It would pool water whenever it rained, usually leaving puddles until they eventually evaporated away. When it started to leak (it was ~20 years old) I looked into the alternatives:
-put another membrane down on top of the non-sloped roof
-build a little bit of slope by ripping dimensional lumber and putting another membrane down
-building a lot of slope (4/12 to match my house roof) and putting shingles down
The building a lot of slope looked like a headache and likely would not be approved by my HOA. Ripping 2x6s to make a 1/2 inch per foot slope seemed doable. Putting anther membrane down on the flat roof just to pool water for another 20 years would work too, but would eventually fail again and leave puddles.
I found a fourth alternative, which I ultimately went with, using hard foam insulation boards that were made with a taper. The are relatively hard to find in my area (VA Beach) but I found a building supply company that had them. They are like giant legos - 4'x4' that you stack in place and screw into the roof subfloor. I got ones that are 1/4" slope per foot. The make them that are multiple thicknesses, so if your roof is 12' you would get one piece that starts at 1/2" -> 1 1/2" then butt it up against a piece that is 1 1/2" -> 2 1/2" then a third piece that is 2 1/2" -> 3 1/2" thick. Alternatively, following our example 12' roof, you can put one 1/2" -> 1 1/2" then butt it with a non-sloped 1" piece and another 1/2" -> 1 1/2" piece on top of the flat one, and then put down 2 1" boards with another 1/2" -> 1 1/2" on top.
Overtop these sloped foam insulation boards, I put a 1 inch thick high density foam board (forget the actual name, but can get it if you need it) overtop. The sloped insulation is pretty dense, you can walk on it and it will not dent, but I wanted something harder and more protective on top. if you live in an area with hail, or the roof is under a pine tree or something, you can get the foam boards that are topped in plywood (or I guess you could just put plywood over the sloped insulation).
Everything gets screwed into the subfloor with screws that have giant washers on them so the screw head doesn't eat into the foam.
Overtop the foam boards I put the membrane. It is EPDM 60 mil thick. It seems to be the standard, it is fully waterproof (it is used to make pond liners), 60 mil is the middle thickness, but I saw it recommended as sufficient. It comes in black or white, I used black even though it really heats up in the summer because I read that the white will chalk up and wash off. The black matches the existing better too, so there was less risk of running afoul with the HOA.
It is easy to put down, like a carpet almost. I glued it down using EPDM bonding adhesive. I can get into the specifics if needed, but basically, you use a paint roller to roll a section about 5'x5' at a time. put the adhesive on the back of the EPDM and the foam board, wait a minute or two for it to get tacky, and then fold the EPDM back over onto the foam board. You have to be careful to roll it back smoothly, the adhesive will almost instantly make a permanent hold and if you aren't careful you will end up with ridges or bubbles in the membrane.
If you need to splice two pieces together (I got a roll 10 feet wide by 50 feet long, so I had to glue a few pieces down to cover the 30 x 15 ish roof), you just put the bonding adhesive on the bottom of the piece that will be overlapping, and the top of the piece being overlapped - overlap the two between 4" and a foot", and then press them together. You can use EPDM caulk to caulk the seem, or get a roll of splicing tape that is self adhesive and just goes over the seam where they overlap.
At the edges of the roof, I overlapped the EPDM a few inches, put aluminum flashing around the edge, and then put the seam splicing tape around the end of the flashing that was on the roof.
The materials are easy to work with. The EPDM is a bit heavy, the 500 sq foot roll I got was probably 200 #s. I cut it into smaller pieces on the ground before moving it up to the roof. It is easy to move around and fold over, it really is just like a carpet. The adhesives and cleaner (I forgot to mention before you adhere it to the roof you have to rub cleaner on it - it is specialized EPDM cleaner you get it with the membrane and adhesive) are hazardous materials with fumes and avoid skin contact-type warnings.
Timeline wise, once the old roof was off, I had all the foam pieces in place and screwed down in an hour or two and most of the membrane down a few hours after that. I had to wait for the seam splicing tape to arrive in the mail, otherwise, I could have been done (did it by myself) in one long day.
I hope this helps, I remember when I was looking for info on doing this, it was hard to find any good guidance. let me know if you want more information or you get stuck with a detail.
Cost wise, I forget how much exactly every was (all my literature and receipts are at a different location than I am now), I think the foam pieces were in the neighborhood of $10 each, and the membrane I remember was $350 for 500 sq feet. The adhesives and cleaners were probably a couple of hundred more, and you will need a lot of disposables (rags, rubber gloves, paint rollers, wide broom to smooth out the epdm, etc.). Again, I can look it up if you need, but for my ~450 square feet of roof it was probably ~$1000.
Good luck,
Ed