This article is from the Neapolitan Mastiffs Breed FAQ, by firstname.lastname@example.org (Barry Reder).
Neapolitan Mastiffs love the outdoors and will enjoy it year round.
They will tolerate temperatures as low as 0 degrees F provided they
have an insulated dog house, and below 0 degrees if the dog house is
heated. Because they are a brachycephalic (short-nosed) breed, they
can have much more trouble with hot weather. They must be provided
with shade and have lots of clean fresh water available. In extremely
hot weather, it is advisable to move them into the basement or some
cooler place. Every year many otherwise healthy dogs die suddenly due
to heat intolerance. Owners must be extremely sensitive to this issue
and must be very cautious especially in hot, humid weather.