Roof slopes are determined by a factor of how far vertically a roof goes up for every 12” or feet it runs horizontally. If you can take a 12” level from the edge of the roof on a gable side and then take a 90 degree line running straight up to the roof, the number of inches to the roof will be your slope. If it is 4” it is a 4 in 12 slope, 6” is a 6 in 12 slope etc. A 4/12 slope is the minimum slope required for installing most sloped roof materials without using special “low slope” installation requirements at the membrane level under the roofing material. A 2/12 slope is the minimum slope that most sloped materials can be used with even with these special low slope treatments. Any slope under a 2/12 slope should have a “flat roof” system or membrane applied. Between a 4/12 and a 7/12 slope standard roofing practises are generally used. For an 8/12 or greater slope, special safety considerations need to be factored in which greatly increase the cost of the roof job.
Roofers generally talk about area of your roof as a “roofing square”. This is simply a 100 sq. ft. area – 10’ x 10’. When they talk about 3 bundle, 4 bundle, 5 bundle or 6 bundle per square asphalt shingles. They are referring to how many bundles of shingles it will take to cover this 100 sq. ft. area. Obviously if it takes 4 bundles rather than 3 bundles to cover the same number of sq. ft., the 4 bundle sq. product is thicker, weighs more and is of better quality than the 3 bundle sq. product. Another improvement in quality comes when “SBS” a kind of rubberized asphalt is added to the shingle.
Remember, roofing area is not the same as the square footage of you house. A roof area will include all of your overhangs and canopies, the “slope factor” multiplier of the actual roof area (for example on a 12/12 roof slope a 100 sq. ft. flat area will be 142 sq. ft. on the roof). It will also include a waste factor and the roof area over your garage. On a typical bungalow with an attached garage, your roof area will often be 1.5 to 2x larger than your house sq. ft.