Hi Arno,
Have a look here:
There are a couple of things to note…
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You don’t need to boolean everything… I use Cura to print and it does a fantastic job of printing a whole bunch of solids as a single print. This is much more robust than trying to union.
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Rooms can be extracted to ‘fill’ the voids to give you a ‘solid’ model.
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It isn’t necessary to have a solid model to print. For most buildings the 3D printer can print over empty space and create a floor / roof just resting on the supporting walls. The underside will be quite stringy, but this isn’t usually noticable and allows a skeleton of a building to be printed with actual rooms inside.
Hope that helps,
Mark