Carpenters may build a number of different types of roofs depending on the homeowner's personal preference, the local climate and the architectural style of the building. Gambrel roofs are relatively common on agricultural buildings and in some regional building styles. Barns in the United States often sport the classic gambrel roof. However, some other styles, such as the French mansard roof, may also be considered gambrel relatives.
The classic gambrel roof often occurs on traditionally styled homes, barns and other agricultural buildings. The top of a classic gambrel is similar to that of a simple gable style, but the roof also includes another angle to create four surfaces instead of only two. The classic gambrel roof resembles half an octagon in cross section, and is framed using the purlin method. The angles of roofs supported this way contain ridge and purlin boards to help maintain their stability. This technique requires more exact calculation than the simpler wall supported style.
Wall supported gambrel roofs do not include purlin or ridge boards. Instead, the upper rafters rest on walls, while the lower rafters butt up against the wall studs. On the outside, this type of gambrel roof looks much like a purlin-framed gambrel, but it is simpler to calculate. One disadvantage of a wall supported gambrel roof is that the interior support walls reduce some of the available floor area under the roof. However, the affected area is located close to the roof's slanted sides, and is normally usable only for storage.
A close relative of the gambrel, mansard style roofs originated in France. Unlike a classic gambrel, mansard roofs have an angled lower roof on all four sides of the building, rather than the two seen in a standard gambrel. Mansard roofs may be used to disguise the height of a structure or to hide unsightly equipment on the rooftop, such as vents or heating and cooling machinery. They are also common in buildings constructed in a European style. Mansard roofs are normally wall supported, with the support resting on a rafter or joist.