The gambrel roof is at its peak in the decades around 1900. As can be seen from the figure, the structure is almost equivalent to having an extra floor, and is constructed of timberwork above which an ordinary double-pitched roof is erected.
The extra floor typically has interior walls of timber to ensure stability.
Recommended literature in Danish:
Kaare Kristensen: Tømrerarbejde (1920)
Gjerløv-Knudsen og H. Bonnesen: Husbygningsteknik (1948)
Danskbyggeskik.dk: Bygningsdel/Tage og tagværker/Manzardtag