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  • Writer's pictureAlas & Bravo

Roof Underlayments and Sheathing

Updated: Aug 26, 2021

Synthetic & Tar Based Underlayment

There are two types of underlayment that are typically installed during roof replacements. There are asphalt based papers as well as synthetic based ones.

For a re-roof, a client typically chooses between a synthetic underlayment or a tar based one. Many roofing contractors typically try to sell the synthetic underlayment as an upgrade.

We at Alas & Bravo Roofing Company do not upsell the synthetic underlayment as the material cost to us, between a synthetic paper and an asphalt based one, is the same. By working with us, a client can choose synthetic if they prefer at no extra cost.

In the photo to the right our client chose the traditional tar based ASTM #30 underlayment. Many clients choose the tar based paper because of its proven track record. It is one of the longest used underlayment in the roofing industry. One advantage to traditional tar paper is that it is a breathable layer while synthetic is not.

Tear Off & Wood Work

Before the underlayment can be installed, all existing layers of roofing material must be removed. We advise against installing new roofing materials like asphalt based fiberglass shingles on top of older materials. Installing on top of older layers may reduce the expected lifespan of the new materials.

Any wood damage must be repaired prior to the installation of the new underlayment. There are two types of plywood typically used as sheathing on roofs. CDX 1/2 inch plywood is pictured to the right. An alternative plywood is a compressed board type known as OSB 1/2 inch plywood.

Wood work on a re-roof project is typically determined after roof removal. Any contract

with a client is typically modified via a change order to properly account for the wood work required. Wood work rates are typically approved and agreed upon before the project is started and are charged by the foot or sheet. This is the most transparent method to estimating the wood work involved in a project.

Other wood substrate areas the inspector may look at are shiplap areas. Homeowners may also choose to replace their fascia board at this time. After passing sheathing inspection by a city inspector, the project is at its midpoint.


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