Built up roofs are used when the pitch is no more than 3 inches rise to 12 inches run. This roof is installed by alternating layers of heavy roofing felt and hot asphalt or tar. The final product is coated with a mineral such as gravel. Built-up roofs are rated by how many layers are applied, usually between 3 and 5 layers are considered standard.
Pros:
Lower cost
Good weather resistance
Cons:
Can be messy to install
Not intended for high rain or snow areas
Modified bitumen roofing (click for glossary definition) Similar to the tar and gravel roof, this roof adds layers of polyester or fiberglass with bitumen. These additional layers add strength and resistance to a flat roof.
Pros:
Lower cost
Good weather resistance
Cons:
Can be messy to install
Not intended for high rain or snow areas
Roll roofing (click for glossary definition) This low cost roofing product is made from materials similar to asphalt shingles, but has a relatively short life expectancy-less than 10 years-because it is a single-ply roof. Roll roofing is commonly known as 90-pound felt, since one roll covering 100 square feet weighs around 90 pounds.
