What is the difference between 15 pound and 30 pound felt paer for roofing?

the best reason i can think of is the 30lb felt is denser and when applied over and uneven, older pine roof boards or plywood the density would be able to cover blemishes in the sub roof. Splinters from old nail removal, or other slight imperfections. Ice guard as used in freeze areas is also more durable to puncture...The need to lay the 30lb felt with little or no wrinkles also makes for smooth runoff. the use of correct material is important when the pitch of the roof is reduced hope this helps. jerome