Hi Eugenio, is the whole house one single solid?

In order for the booleans to produce a new solid, they have to create a well formed intersection curve between the objects being booleaned, the intersection has to be closed and not have any self intersecting loops. If there is any difficulty in getting that type of intersection then that can be one way that you'd get a "Joined surface" result. There are various different situations that could cause that, like if some surfaces have any self-intersections where the surface folds back over itself or also if there are 2 surfaces that barely skim over each other with overlapping surface area that can make for a sort of meandering complex intersection curve. So if you have something like that it can help to make the pieces push through each other a little ways instead of having shared surface area, although if it is a very exact case with overlapping planes that's usually ok.

It's difficult to know exactly what your case is without seeing the objects, if it's just 2 smaller objects in your model you can select just those and use File > Export to make a smaller file with just those objects in it to upload, or otherwise you could use a file sharing service like Dropbox to host your file and then just post a link to it here rather than making a large attachment directly here in the forum.

- Michael