The short answer: a puddle that clears is the roof working; a pool that is still there in dry weather is the roof telling you something. Standing water rarely causes a leak by itself, but it concentrates wear, holds dirt and growth against the surface, and marks the exact spot where drainage has stopped happening.
Why is some standing water normal?
Because a flat roof is not flat. It is laid to a shallow slope towards wherever the water is meant to leave, and a slope that shallow does not shed everything instantly. Surface tension holds a film, slight irregularities in the deck hold a little more, and after real rain there will be wet patches for a while. That is the design behaving as designed.
The distinction that matters is between water that is on its way out and water that has arrived nowhere. One shrinks between showers. The other reappears in the same outline every time, and is still there when the rest of the roof has been dry for days.
What does it do to a covering over time?
Nothing dramatic and quite a lot cumulatively. Under a pool the surface stays cold and damp while the roof around it warms and dries, so the two ends of the same sheet live different lives. Mineral finishes soften and shed faster where they are permanently wet. Silt collects, then moss and algae behind it, and the mat that forms holds yet more water against the surface.
None of that punctures a sound covering. What it does is make an existing weakness fail sooner: a lap that would have lasted, a hairline split that would have stayed shallow, a patch repair whose edge would have held. So the pools on an old roof are not a diagnosis, they are a map of where to look hardest, which is one reason a survey photographs them before anything else is discussed.
Where does the water actually come from?
Not from the sky, in the sense that matters. Persistent pools are almost always a drainage story with two chapters.
- The outlets. The commonest cause and the cheapest to fix. An outlet packed with silt, leaves or a nest drains slowly or not at all, and the roof backs up towards it. A gutter running the wrong way after settlement does the same thing more slowly. Half the pools we are asked about disappear once the drainage is cleared and left clear on a maintenance schedule.
- The falls. The slope itself, either never built properly or lost since. Decks deflect between joists over decades, insulation compresses under foot traffic, and a plant base or a new duct installed without thought interrupts the route. The result is a local low point with nowhere to send its water.
What can be done short of a new roof?
A good deal, and the order is worth following because it runs cheapest first. Clear and rod the outlets, and see what is left after a week of weather. Where pools persist, establish whether the low point is local or general: one dip beside an outlet is a different job from a whole bay that has dropped. Local dips can be built up and re-laid to a route that drains, and an additional outlet is sometimes the honest answer where a roof was only ever given one.
Rebuilding falls across a whole roof is real work and belongs with a covering that is due renewal anyway, which is the point at which the options guide takes over. What nobody should do is treat the pool as the problem and resurface over it. The water will be back in the same outline by autumn, over a covering that is now one layer newer and no better drained.