Gambrel Roof
A variation of the gable roof with mansard roof characteristics: Each slope is broken, transitioning from a gable roof to a 2 sided mansard type roof at uniform points on the rakes. A gambrel is a two-sided roof with two slopes on each side.
The usual architectural term in eighteenth-century England and America was "Dutch roof," and is commonly known as a "barn roof" in North America. The term gambrel is of American origin.
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. A gambrel roof overhangs the façade, whereas a mansard normally does not. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maximizing headroom inside the building's upper level and shortening what would otherwise be a tall roof. This style is used on two-story construction. It permits more efficient use of the second floor level. Dormers may be included along the sides, with windows on the gable ends.
Installation of roofing materials on this type of roof design can be challenging. As with other non conforming, unique structural designs, such as a geodesic domes,
for example, it can be difficult to determine where code required products such as Ice and Water Shield should begin and end... the determinations can add up to a lot
of dollars spent or not spent wisely.
For example, if the soffit overhang on a conventional gable or hip roof is 12 inches or less, than we can usually get by with a single course of IWS 3 feet wide along the eaves. However, the steeper the pitch, the more roof exposure there is; on a gambrel the side pitches can be 20 / 12 or more, which then requires IWS on the entire slope, which adds up to hundreds if not thousands more to the cost of the roof. Ironically, the steeper pitch really does not physically need IWS, since the steepness of the slope naturally allows for quick drainage and prohibits the formation of ice dams, the very reason IWS is used.
The same irony applies to Geodesic dome roofs; they too have steep sides that require Ice and Water Shield, until about three quarters of the way up the sides, where the pitch starts rounding off. Felt or synthetic underlayment can be used on the balance of the structure until the low slope of the top of the dome is encountered, then the slope less than 2/12 will need IWS, or flat roof material, installed.
There is one critical aspect of installing modern asphalt shingles on very steep Mansard or Gambrel slopes that should be taken into consideration: Modern dimensional shingles have very good adhesive tabs that will stick to the underlapped shingle to resist wind uplift, but many roofers miss the "hand sealing" function that is recommended by most manufacturers if the slopes are extraordinarily steep. We have seen time and time again shingles "sagging" off mansards and gambrel roof sides because the roofers did not follow manufacturer instructions. Call to find out more about this critical installation step if you are looking at replacing this type of roof.
It is for these reasons that you need to hire true professionals in roofing and construction. Any commercial roofing contractor can install asphalt shingles, but we can help you overcome the confusion, and will interface with the local building department before work begins to ensure that your project is entirely code and local environment compliant regardless of the roofing product you prefer. Don't hesitate to call or email before you "break ground" on your dream project.
