Felt underlayment is a mix of fibers that have been saturated with asphalt. It's a cheap alternative to rubber underlayment. Comes in 3'x~100' foot rolls. Each roll is heavy.

The felt underlayment goes ontop of the roof clading. It's attach with roofing nails. The nails overlaped with the felt to prevent water from getting in.

References

Sauna Roof Underlayment

Sep 30, 2022

From Felt to Rubber

✒️ Posts,

🔥 Sauna

Then covered that with felt underlayment. But the felt kept being ripped and pulled off by strong winds.