When building a roof, roofers use layers of various building materials to provide protection from the elements. A vital layer in most roof construction is tar paper, hidden beneath the roof's shingles. Tar paper's primary function is to keep moisture or water, which makes its way under the shingles, from seeping into the building.
A roof normally begins with the roof rafters or trusses, which provide the framework for the roof construction. A common roof typically consists of three layers -- sheathing, tar paper and shingles. The roofer covers the rafters or trusses with the sheathing, which might be sheets of plywood or particle board. Over the sheathing goes the tar paper, and over the tar paper goes the shingles. In some roofs, the roofers add an additional layer between the sheathing and tar paper. This additional layer might be strips of self-adhering polymer bitumen tape affixed to the sheathing, and under the tar paper.
Tar paper, used in construction, is heavy duty paper treated with tar, to make it waterproof. Also called building felt, tar paper comes in rolls of varying lengths. It is slippery to walk on and tears easily. Not all building felt or tar paper is suitable for roof construction. Roofing felt is a type of tar paper, of a specific weight, typically used for roof construction. When affixed to the roof, the tar paper attaches in overlapping layers.
Each roof layer has a function. The sheathing gives structure to the roof, provides a barrier to keep out water and snow and holds a distributed load, like snow or the weight of a person walking on the roof. Without the sheathing, the tar paper would attach to the rafters and easily tear, exposing the building to the elements and those walking on the roof could fall through the rafters. The tar paper water proofs the sheathing. Shingles provide another layer of protection to the building and add to its architectural appeal. Yet some inexpensive roofs use rocks, instead of shingles, over the tar paper.
The primary purpose of the tar paper or roofing felt is to keep the water out of the building. While shingles also keep water and moisture from entering the structure, the tar paper ideally prevents the water, which makes it past the shingles, from soaking through the sheathing.
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