Anyone who has seen a roof being applied to a house has seen the roofing felt. Because it comes in 36 inch-wide rolls that are up to 100 feet long, it can be applied very quickly to a roof to act as a moisture barrier. Some contractors for a time preferred not to use roofing felt under shingles because the technology of manufacturing shingles had improved the quality of the shingles a great deal.
Certain studies seemed to imply that in some instances having roofing felt beneath the shingles actually contributed to their life expectancy being reduced for a variety of reasons. This approach appears to have all but disappeared in recent years.
Begin applying roofing felt along the lowest edge of the roof.
The felt should be rolled out along the length of the roof at the edge of the eaves. If it is not too windy, you will be able to roll out enough felt to make one entire run before nailing. If a wind is blowing, you will want to nail the felt to the roof decking as you proceed across the roof. Cut the felt off evenly at the end of each run. This will allow you to have a clean start on the next row.
Be sure to keep the runs of roofing felt parallel with the line of the roof.
Because roofing felt is produced with lines running the length of the roll, it is important to keep the felt running in straight lines. These lines are intended to be markers for the roofers to follow that makes it easier for them to keep the shingles in proper alignment. If necessary, use a chalk line to mark the deck of the roof ahead of time. Use these lines to keep the roofing felt on track as it is applied.
Do not overuse nails in the roofing felt.
Nail the felt to the roof with a sparing number of nails. If the felt will be used as a temporary roof, you will want to use more nails to insure that it stays put until the shingles can be applied. The reason that you do not want to over nail the felt is that you will be putting a lot more nails through it and your roof as the shingles are laid. Every nail creates a hole and damage to the roof decking. When the roof is complete, there will be plenty of nails holding the felt in place.
In most applications, the decision to use 15# or 30# roofing felt is a matter of personal preference.
While 30# felt is going to give more protection to the wood and the shingles, 15# is enough. Some people simple prefer to go bigger and stronger. The results may not actually justify the additional cost and effort required to lay the heavier felt. Both will let moisture escape from the house and keep it away from the wood. Each will stop any pine resins or glue from reaching the underside of the shingles. The roofing felt will always outlast the shingles covering it by many years.
Other types of uses for roofing felt.
Any place that you want a good barrier under the roofing materials is a good place to use roofing felt. It is also generally the product that is used under the tar coating on flat surface roofs. Roofing felt is even sometimes used as a barrier in flower beds and septic fields.
