"Experts in the science of quality roofing." What is Underlayment all about?
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Underlayment:
The covering over the roof deck, above the eave protection, prior to the application of new shingles.

Purpose:

  • gives that extra back-up protection from the [weather] elements, in the event of hard wind-driven rain.
  • provides a Class-"A" fire-rated roof system, when installed under fibreglass base-mat shingles.
Products:
  • GAF - ShingleMate or CERTAINTEED - RooferSelect > the best.
  • [high quality] #15 Felt/Tarpaper is acceptable, although not as good as the fibreglass reinforced products [above].
Note #1: When Organic or Paper Base-Mat shingles are used, you can only achieve a Class-"C" fire-rated system. [This is the case, whether Underlayment is applied, or not.]

Note #2: Be wary of those who claim that Underlayment is not good, due to Ventilation and Condensation issues. They do not understand Building Science [Physics]. The ROOF of most houses has nothing to do with the "building envelope". It is only an umbrella, to deflect water away and off the building-top. Standard tarpaper, however, can make the roof look bad, if there are poor attic conditions. When excessive condensation exists within the attic spaces, that moisture is absorbed by the tarpaper, which ripples. This is amplified to the shingles and they look bad. Correct the hidden attic space problems, and the condensation will not be there.

WARNING!
Many roofing contractors state, you don't need Underlayment or you should not consider it. It helps them keep their price cheaper, but they are not being of service to you.