How to map downpipes for draining water from the roof?
roof:downpipe | Keys | OpenStreetMap Taginfo? Used 18000 times (looks like mainly in one inport) but never written up!
typically you just add a node to the horizontal gutter and add the tags, that’s it, you can ignore the downspout offsets if they are minor like in your picture.
Sometimes it not entirely clear which part is the downpipe and which count as horizontal
This tag is a bit odd. As I understand it, it’s a building characteristic. I’d like to mark individual vertical pipes. In my city, this will almost always be a point object, because the location of the horizontal part is not obvious. Moreover, in the case of flat roofs, the horizontal part will be the entire roof surface.
By the way, how are horizontal gutters mapped? A link to a map would be helpful.
Are far as I can see, they’re not! (Or at least not with a dedicated tag?)
Virtually all of the uses of downpipe are from HOT mapping in Kampala, Kenya, to say a building has gutter=yes/no & roof:downpipe=yes/no e,g, Way: 754861270 | OpenStreetMap & Way: 761163104 | OpenStreetMap
Gutter has a wiki Key:gutter - OpenStreetMap Wiki which seems to relate to street gutters? When you again take out Kampala, it’s uses ~10 times, mainly for street gutters.
I’d suggest write a wiki for roof:downpipe to say that it can be either mapped as a yes/no attribute to roof (& maybe building?), & also as a key node to show where the downpipe/s is located on the edge of a roof.
Similarly, roof:gutter as an attribute & also a way along the edge of the roof.
IMHO the tag should be documented as property if this is the usage, and it should be proposed a distinct feature tag for explicit mapping (which would surprise me if these were mapped more than a handful - if any)
Except for pure visible occurrence tagging (because it is there), I’ve no idea why downpipe mapping has been required by HOT or any other mapper, but I can’t currently think of a use without knowing where it drains to. Just over ground, direct to drain, intermediate tank or butt etc.
So, why map down pipes?
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for rainwater routing? 
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I agree with you that downpipes, just like horizontal gutters along the roof, are a level of detail that does not fit well with how we generally represent buildings, where the generalization suggests it is made for scales like 1:500 and less, while downpipes are something I would expect in drawings (specific to plumbing) of 1:50 and more. We represent nothing of buildings if not a generalized outline / volume with approximate roof shapes. Entrance doors are nodes, and here we usually stop, no protruding elements like balconies, no windows, no other doors, no decorations, no chimneys, etc. but downpipes? In theory it is possible to do it, we do not require mapping to be consistent in detail across the whole map or even within the same settlement, but I would suggest to not do it (in particular not the horizontal gutters), because it makes mapping for following mappers much more complex, and the gain seems minimal while the effort is enormous.
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a new main tag seems appropriate, precipitationway
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It’s unlikely that anyone will use OSM data about them, but I have a few thoughts.
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Another reference point for placing POIs. They’re often located right between windows, making it easier to locate entrances, POIs, and small details like surveillance cameras. I also spot missed building passages, and I often have to navigate by the opposite side of the street. They can also be used to check whether roadside objects, crosswalks, speed bumps, and fences are correctly positioned.
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I’ll add an example of a panorama of my city, which shows that this is a fairly noticeable detail of the buildings https://yandex.ru/maps/-/CTEOA6yf And in my opinion, it is much easier to map it, unlike, for example, surveillance cameras.
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And in winter these pipes turn the streets into something like this:
Not to mention the fact that they themselves can fall due to frozen ice. It quickly comes to mind that you can mark where the water drains and whether they have heating :) In our city, an activist once pressured the authorities to remake a kerbs to make it wheelchair accessible. As an additional argument, he provided links to the OSM node with the measured height. The same could potentially be done with dangerous pipes.
Downpipes are physical objects and you are free to map them, if this makes sense or not. Anyhow the arguments you posted sound a bit far fetched to me.
I have never needed a downpipe to locate the exact position of an entrance or a building passage.
Also I do not understand it as a PRO that they are easier to map than surveillance cameras. The point is that the position of surveillance cameras is of interest to many users whereas downpipes are not.
And the ice cover in winter is absolutely independant from the exact position of the downpipes.
Sure, but it is a hundred times more likely that a flower pot or a loose roofing tile will hit you - is that an argument to start mapping flowerpots and roofing tiles also?
That is why I agree with @Adrian_Shobrooke and @dieterdreist - mapping downpipes does not add value but just more complexity to urban areas which probably are already packed with nodes for all kinds of stuff.
During a thaw, water flows through these pipes onto the sidewalk and freezes there.Moreover, an icy slide forms near the pipes, which often becomes an unpleasant obstacle on narrow streets.
In any case, yes. Considering that no one has ever mapped them before, I might change my mind. But for now, the question remains: what tag can be used for this node object?
Surely this also depends on how the downpipe deposits the water? At the very least you’d also need to tag if it goes onto a paved area, an unpaved area, if it connects into a more horizontal pipe to carry the water away from walls, or connects straight into a sewer or storm sewer?
TBH I am more curious how you will be able to locate the downpipe on aerial imagery? Or do you plan to use relative positioning like recording that a downpipe is 2 metres west of an entrance door, or 5 metres from a corner of the building?
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As a Canadian it is my duty to point out that the beautiful English word “eavestrough” exists and unlike “gutter” does not require disambiguation or prefixing!
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please in case you decide to do it, keep it simple, e.g. add a diameter tag rather than representing the pipe as a circle 
Which is why knowing where it drains to matters. Most of which is unverifiable to many of us.
Yes, that’s roughly it. For example, using a photo like this, it’s easy to divide a building into sections of the desired proportions:
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Nice word for sure, but afaik it only stands for the horizontal part of the gutter discharging into a downpipe finally.
Yes, hence why I quoted the part mentioning roof:gutter! I have no objection to “downpipe”