What counts as flat, and why the distinction matters
There is no such thing as a flat roof. Every one of them has some pitch, and the code treats anything under a 2:12 slope as low-slope. Below that line a covering made of overlapping pieces stops working, because it relies on gravity moving water off faster than the laps can let it in.
So low-slope roofs use a membrane instead: a continuous sheet or a monolithic coating, sealed at every seam and every penetration. The roof is only as good as its weakest joint, because there is no second chance further down the slope.
That is the whole reason flat roof work is a separate skill. A crew that is excellent on shingle can do real damage on a membrane, because the instincts are wrong. Fasteners go in different places. Laps run a different way. The details that carry the roof are not the details a steep-slope roofer is trained to watch.
Ponding is the thing to understand
Ponding is water that is still sitting on the roof after the rain has stopped and the roof has had time to drain. On a Florida low-slope roof it is the single most common route to failure.
It does three things at once. The standing water works on every seam it touches, continuously rather than in bursts. It holds dirt and organic debris that hold more water. And the weight of it deflects the deck slightly, which deepens the low spot, which holds more water next time. The problem builds on itself.
Ponding is usually not a membrane fault. It is a slope fault, a drain fault, or a deck that has sagged between supports. Which means recoating over it solves nothing — the new surface ponds in exactly the same place. The fix is drainage: tapered insulation to build slope, a drain or scupper that actually sits at the low point, or structural work where the deck has moved.
If someone quotes you a coating for a roof that ponds and never mentions where the water is going to go, that is the question to ask before anything else.
Standing water is not a membrane problem. It is a drainage problem wearing a membrane problem's clothes.
The membrane options, plainly
TPO is a single-ply sheet, usually mechanically fastened or adhered, with seams heat-welded so the laps become one piece of material. It is light in color, which matters under continuous Florida sun. Its failure points are the seams and the flashing details at every curb and pipe.
Modified bitumen is asphalt-based, in layers, torched or self-adhered. It is tough underfoot and forgiving on a roof that gets walked on, which matters if there is equipment up there. It handles a complicated roof with lots of penetrations well.
Silicone and acrylic coatings are restoration systems, not new roofs. Applied over a sound, dry substrate they seal a weathered surface and reflect heat. Applied over wet insulation they trap the water and the roof keeps rotting underneath while looking fine from the ladder. Silicone tolerates standing water better than acrylic does, which is why it shows up on Florida jobs.
There is no best one. The right answer depends on what is already up there, whether the insulation is dry, how many penetrations the roof has, how much foot traffic it takes, and how the water is meant to leave. Any of those can rule an option out.
Where flat roofs actually leak
Almost never in the middle of the field. The field of a membrane roof is the part that was installed under the best conditions with the most attention, and it is usually fine.
They leak at the edges and the interruptions. Perimeter terminations where the membrane turns up a parapet and has to be fastened and sealed. Pipe boots and equipment curbs, where a flat sheet has to wrap something round. Drains, where the membrane meets metal and a clamping ring. Any spot where two different materials meet and both move at different rates in the heat.
This is why photographs from the ground tell you nothing about a flat roof. All of the decisive information is on top of it, in places you cannot see without walking it.
It is also why we will not give you an answer over the phone. We do not know what is up there, and neither does anybody else who has not been on it.
What to ask before you hire anyone
Ask whether they are going to fix the ponding or coat over it, and how. If the answer is vague, you have your answer.
Ask whether the insulation underneath is wet, and how they established that. Cutting a test area is the honest way. Guessing from the surface is not.
Ask which system they are proposing and why that one for this roof. A contractor who fits the same membrane to every job is selling what they buy, not what you need.
Ask what the warranty covers and who carries it — the manufacturer, the contractor, or neither. On membrane roofs the distinction matters, because a material warranty does not cover a seam that was welded badly.
Ask for it in writing. Scope, system, drainage, and what happens if the deck turns out to be worse than it looked. We put all of that on the estimate, including the part where opening the roof might change the answer.