Proper placement of the scupper prevents standing water.

Low slope roofs with parapets require two drainage paths. A primary that is the lowest point and an overflow which sits above the lowest point. The purpose of the primary is draining all the water.

The secondary has two purposes:

1. Drain water if the primary drain is clogged without exceeding the live load of the roof structure.

2. Alert the building owner that the primary drain is clogged. For example by dumping water out a visible scupper on the facade or in the case of internal secondaries by triggering an alarm in the building's management system.

Standing water on a new roof is a symptom of improper design. Roofs are a place where it makes more sense to pay for architectural services because nobody ever gladly exchanged roof leaks for cost savings. And that roof is going to leak.

TPO, properly installed can make a fine roof. But standing water will void any manufacturer's warranty because the roof is not properly installed. Coal-tar pitch is the only roof that can reliably resist standing water...and you won't save money installing coal-tar pitch, it's carcinogenic and a specialty trade.

Finally, if you really want a reliable roof pay for a twenty year no-dollar-limit bond. The bonding company will come out and closely inspect everything before issuing the bond...and the contractor doesn't get retainage...so yeah you want a good contract, too.

I'm sure the Redditor felt clever with all the money they saved. But if you are sitting at the table with contractors, to near statistical certainty you are the dumb money.