From what I read, creating two separate polygons and tagging them with “building=semidetached_house”, plus the house number / address information, seems to be correct.
However, when tagging both halves with “roof=gabled”, the result is a zig-zag-shape with two separate roofs. (Unfortunately as a new user I cannot put a second picture in here, Discourse won’t let me.)
What would be the way to go: A single “building=detached”, with two address nodes? A single “building=semidetached_house” polygon, although it includes both halves? Or is there a way to tag a roof as “one half of a gabled roof”?
It depends on whether these are actually two buildings, i.e. they do not share one wall in the middle but each of the two parts has an own wall. This property is often somewhat impossible to survey, so unless you got land register data at hand which may reveal whether it is two buildings or not, it often comes down to preference or a best guess:
If you map it as one building, you tag the roof with roof:shape=gabled and ideally add the addresses at the entrance=yes vertices on the building outline.
If you map it as two buildings, you tag the roofs with roof:shape=skillion and the direction of the skillions with e.g. roof:direction=NE for a skillion roof that approximately faces north-east.
(For this particular case, my guess would be that it is an old farmhouse that has been repurposed to be a duplex, i.e. one building.)
However, when tagging both halves with “roof=gabled”, the result is a zig-zag-shape with two separate roofs. (Unfortunately as a new user I cannot put a second picture in here, Discourse won’t let me.)
if you tag the individual halves the roof shape should be skillion
In OSM, there is a consensus that buildings should ultimately be represented by the outermost line.
If two buildings really do share a roof, I think there is a way to draw the building and roof separately.
I think you could draw the building along the outside line of the building, draw the roof covering it, and tag it with ‘roof:shape=gabled’.
If split in 2 parts at least roof:orientation=along and roof:direction=n (2 parts each pointing at their lower side) are needed to get it simple 3D correct. Then roof:height or roof:angle to get the slope right.
(I’ve tried this splitting of roof with all the bells and whistles on a non-square church gabled roofed building but did not get it right in StreetGL, F4 did though tho.)
This would be something of the type 4 of the OSM-4D proposal. It would be nice to have a new discussion about the 3D tagging. With the increasingly detailed imagery and interest of mappers, some consesus completing simple 3D would certainly be welcome.
