A roof exists to protect a house from the elements. A roof is a system built in layers:
Plywood decking: Sheets of wood are placed over rafters. The most common type is Oriented Strand Board, or OSB. OSB is composed of compressed wood fibers that form a strong sheet of support for the upper layers of the roof.
Felt paper: This used to be called “tar paper”. It’s organic felt saturated with asphalt. It serves as a barrier so shingles don’t stick to decking under the heat of the sun, and it prevents a reaction between the chemicals in asphalt and the wood material in the decking.
Shingles: The final layer of protection against sun, rain and wind. Shingles come in a wide variety of materials (composition, or asphalt-saturated paper; wood; metal; slate or imitation slate made of cement fiber; clay; concrete; etc). Make sure you keep in mind the shingle material you plan to use when you design a roof on a new house. The roof must be built strong enough to bear the weight of the shingles that will sit on top of it. Clay or concrete will weigh four or five times as much as composition shingle.
The roof system does not provide insulation, but its shape can affect the amount of insulation you can have below the roofline. Flatter roofs don’t allow much room for an attic lined with insulation material.
Pitch: The angle or “steepness” of a roof is called the “pitch”. Pitch is measured in a ratio of drop over length. A “6/12” roof drops 6 inches for every 12 inches of roof length. A flat roof would be a “0/12” roof, while a 12/12 roof slopes at a 45 degree angle (12” of drop for every 12” of roof length). A roof with a higher pitch will shed water and snow better. So in the northern sections of the country, a high-pitched roof would make sense to avoid heavy snow buildup in the winter. A low-pitched roof must be built with sufficient strength to bear the extra weight from snow or moisture that would otherwise run off a higher-pitched roof.
The pitch of a roof plays a large part in determining the character and look of a house. A single-story house with a very high-pitched roof would look a little strange because there may be more roof than house. On a 2-story house, the 2nd floor could consist simply of the space under a high-pitched roof.
The pitch of a roof will also determine the amount of space available for insulation above a ceiling and under a roofline. A flat roof leaves little room for insulation, and little room for the air circulation necessary to insulate a house.
Variety of Roof Designs (offering a combination of functionality and appearance):
* Gable Roof: a ridged roof with 2 sloping sides over two vertical walls forming a triangular shape at each end.
* Hip Roof: a pitched roof with 4 (or more) sloping towards one another
* Flat Roof: Most appropriate in warm climates; it can cause problems in areas with a lot of rain or snowfall.
* Ridge: the uppermost, horizontal crest of a roof
* Valley: an inner angle where two planes of a roof meet
* Hip: an outer angle where two planes of a roof meet
Other Design Features:
* Eyebrows: a rounded roof section over an upper-story window
* Turrets: they are purely decorative, with no real functional purpose. They can be expensive to build because you’ll be building a rounded architectural shape with flat building materials.
* Dormers: these look like little doghouses that project from a roof. Small dormers may serve a purely cosmetic function when seen from the street. Large dormers can create additional space projecting out from a bedroom.