Some roofs have “inverted” gable or round shapes, potentionally more shapes. Examples:
roofs on train station platforms, e.g. File:Pepsi CenterElitch Gardens.JPG - Wikimedia Commons When I did a picture search for “Bahnsteig”, around 50% of the first 18 pictures were such cases, i.e. it is not that uncommon.
roofs made of plastic sheets that are under tension, e.g.
In Key:roof:shape - OpenStreetMap Wiki I did not find how to map them in a KISS way, but only by slicing one phyisical feature into two OSM features just to tag them. In the forum, I only found the question how to map them ("Neue" Dachform - #18 by Elefant_aus_Wuppertal) but no answer - as I did not want to “hijack” the related thread, I opened a new one
I think buildings with roofs made of plastic or fabric under tension are usually considered temporary structures and shouldn’t be mapped in OSM. They should be standing in the same place for awhile.
The exact length of time is hotly debated, but I would bother if it was less than a year. In you do map a structure with a plastic roof refer to link below for the roof:material wiki page.
The reference I use with a view of 3D rendering is this wiki OSM-4D/Roof table - OpenStreetMap Wiki on which your inverted_gabled roof is not found leaving what you did if 3D is the objective of splitting the roof in 2 parts, adding sloped or skillion, roof:orientation=along/across, plus roof:direction of the slope.
For a name, no clue but butterfly or frown comes to mind and in fact found in TagInfo butterfly_roof (104 uses) and just butterfly (45 uses) which would have my vote without the _roof suffix, it’s a roof:shape of course.
There are numeric parameters that can be entered, as 3dr:height1= (N=1483). If they allow negative vals, it might be possible. Though this could make levels fail. Or an axis should be able to be defined including negative direction, which may further allow tilted buildings (with a tilted axis) more easily; and even multiple faces with multiple axis by linear, or polar pattern etc.
There is =gabled_row . Which may degenerate into this at 3dr:n=0.5i . The inversion of a repeating pattern has already been provided.
An eg roof:shape:inverted=yes would be more general than roof:shape=butterfly. It could allow inverting all roofs without needing an =inverted_* for each, including =round .
I see it differently. Many structures are built from such elements. One example is the canopy at Magdeburg’s main train station (west side). This was built in 2003 and will continue to be permanently installed there.
Fabric under tension, in particular, is a material that is used time and again in architecture for lightweight structures.
I think buildings with roofs made of plastic or fabric under tension are usually considered temporary structures and shouldn’t be mapped in OSM.
I agree with you that short term buildings, like for a festival oder emergency, shall not be mapped. While such buildings often have plastic roofs, plastic roofs are also used for “permanent” buildings, especially when larger areas shall be covered but without walls and as few columns as possible, see e.g. Membrane structure - Wikipedia and think e.g. about Radom Raisting – Wikipedia from 1963 and still standing
BTW, the picture in the initial post shows bike stand roofs that will probably stay for decades. Hence, I consider them worth to map.
Butterfly roofs are IMHO too common to require people reading themselfs into tilted axis or the like in a foreign language (the majority of the world does not have EN as mother tonge). I’d prefer a name.
I like the idea of “inverted”, as it could allow much more roof shapes in a user friendly way – thinking e.g. of StreetComplete, we’d just add one single toggle instead doubling the amount of shown options/tiles to select from [of course, “doubling” is just an approximation as not all shapes will exist inverted in relevant amounts]. Whether we’d use “inverted” in GUI and also tag “gabled” + “inverted”, or we use “inverted” only in GUI to toggle “gabled” to “butterly” and then tag “butterfly”, I tend to add few new well-defined values and avoid introducing a new tag that massively alters another tag and thus makes data usage more complex.
+1 for only “butterfly” because shorter is better, it’s more consistent with the other existing values (the wiki page lists 0 with *_roof), and the value’s meaning is clear without “roof” due to key’s name “roof:shape”. This is more important to me than that List of roof shapes - Wikipedia lists other roof shapes like gabled, hipped etc without “roof” while it lists “Butterfly roof”.
There was a recent discussion about canopy covers, the classic here 4 poles with a tensioned semi-pyramidal tent fabric like roof, they’re OK to map. Many car dealers use these as cheap permanent cover for their wares. https://taginfo.openstreetmap.org/tags/building=canopy#overview
From your description I am not sure you mean an “inverted” roof shape, i.e. the middle being the lowest point, or a “normal” roof shape i.e. the middle being the highest point – like in the two example pictures of Canopy (architecture) - Wikipedia
