One of our balconies is over the master bedroom. We needed a way to make the floor of the balcony completely waterproof. Talking to local roofers and roofing supply companies was unproductive. It was hard to get advice and the supply companies seemed reluctant to sell to owner/builders. There is a company in ST. Charles, MO. which had the supplies and the guidance we needed. They put together a package, we paid for it and then they shipped it to us.

Installing the sheet in one piece eliminates seams which mitigates, or eliminates, leaks.
The sheet waterproofing turns up the wall forming a curb at the door opening.
Hows EPDM waterproof membrane
The finish balcony deck will “float” on the waterproofing and will be even with the bottom of the siding.