If you are not familiar with a gambrel roof then let us first explain its architecture in brief.
Gambrel and mansard roofs.
You can consider gambrel roof to be a modified version of a gable roof.
Mansard roof type of roof having two slopes on every side the lower slope being considerably steeper than the upper.
The only significant difference is that a gambrel roof consists of vertical gable ends and is not hipped at the four corners of the building structure.
A gambrel or barn roof is much like mansard in a sense that it has two different slopes.
A mansard or mansard roof also called a duch roof is a four sided gambrel style hip roof characterized by two slopes on each of its sides with the lower slope at a steeper angle and more vertical than the upper punctured by dormer windows to create additional habitable space such as a garret.
The difference between the two is that the gambrel only has two sides while the mansard has four.
A gambrel roof is positioned such that it overhangs the façade of the building.
Similar to mansard the lower side of the gambrel roof has an almost vertical steep slope while the upper slope is much lower.
A gambrel roof overhangs the façade whereas a mansard normally does not.
The cross section of a gambrel roof is very similar to that of a mansard roof.
The steep roof with windows creates an additional floor of habitable space a garret and reduces the overall height of the roof for a given number of habitable stories.
Simply put the mansard roof also known as the french roof or curb roof is a hybrid between a gambrel roof and a hip roof.
The cross section of a gambrel roof is similar to that of a mansard roof but a gambrel has vertical gable ends instead of being hipped at the four corners of the building.