"My houses have no roof."
Translation:Mes maisons n'ont pas de toit.
The real question is why this man can afford so many houses but does not purchase a roof.
I had many houses made of Lego when I was a kid, and many of them had no roof, for better manipulation with the toys inside :-)
EDIT: another option - I've heard that in Greece tax must be paid for buildings since they have a roof - so there are many houses without any roof. Especially those who have money for more houses can have saved them on taxes.
Interestingly it actually ended up becoming a styled feature. It's not uncommon to see modern houses with pseudo-windows put in, despite there being no actual window there. Perhaps it is because of that link to the richer neighbourhoods, that it is done.
It's interesting to see how such things affect culture though.
Why is "pas des toits" not accepted? Does the negative form necessarily need to be singular, or is this just duolingo being strict?
It is actually a very simple rule that is super easy to remember. When you have an indefinite article (un/une/des) or a partitive article (de la /de l' /du /des) in a negative construction, the indefinite/partitive article will change to simply "de", meaning "any".
"J'ai un velo" ---> "Je n'ai pas de velo" or "Je bois du vin" ---> "Je ne bois pas de vin"