Ah, this is downstream of some problems inherent to Dungeondraft. The Building tool's "floor" data only takes a specific kind of randomized tileset & it doesn't inherit from the pattern fill tool for some reason. The normal recommended workflow is to use the Wall tool with the Pattern Fill tool, but this is not as convenient as the Building tool honestly.
It is an oversight I think because I could probably add the textures to the Building tool's floor patterns. I will see about doing that soon.
Fast edit: Right. So the Building tool only takes images of 4×4 tiles where each tile is randomized in placement (like, try using the default building textures for a bit & you'll see what I mean). It'd totally screw up the hatching textures (since each tile would be randomly placed & it'd have seams on all the sides since the texture wasn't made for that) and it wouldn't be suitable for the transparent textures.
I could do an update with the solid color fills as options for the Building tool though since that's just a simple fuzzy texture that shouldn't be impacted too badly by the random tile placement.