A felt underlay provides a barrier between the decking of the roof and the shingles. It helps to repel water while still providing some air to pass through. This protects the wood from becoming damaged and essentially gives you a better looking and longer lasting roof.
Felt also provides a safer working environment for the crew by giving them better traction and helping to keep them from slipping.

#30 or #15 Weight Felt?

While #15 is a good option for most roofing jobs, #30 provides more protection and performance for steep roofs and flat roofs. If your roof does not have a steep pitch, you can use #15. This weight of felt is a good economic choice if you have a standard roof. But if your roof has a steep pitch, #30 is a better option because it is thicker and tears less during installation.
With #30 you get a thicker layer of underlayment and protection. Even if there is a lot of moisture it will have a harder time getting through the thickness of #30. The thickness of the #30 felt helps with better traction for the workers while on a steep pitched roof.
The best person to consult is a roofer you can trust, someone who knows the benefits of both weights of felt. Generally speaking, if your roof is above a certain pitch you will need the #30 pound felt and if your roof is under a certain pitch you can go with the #15 pound felt. Your felt is usually an automatic industry standard, most often the customer does not choose the felt they would prefer, roofers will follow code based on the specs of the roof.
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