I don't know how to paste a link to this, but go to Home, have a look in the 'Community events' section and find the "Essen 2/3 01 2010 Playmo Lego Börse" thread and look down the questions.
I asked Macgayver how they did the roofs and this was the answer he gave
"The pieces that hold the roof together on the larger houses are from the last dollhouse
The small ones have been cut and pasted"
By that I think that he means that he cut took some of the roofholders, cut them and then glued them together to make the size he needed.
Hope this is clearer!
Elaine