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Leave old felt on roof.
Use the hammer s claw and the stanley knife to rip away the bulk of the felt then the screwdriver and the claw to pull out the old clout nails.
The old felt should be stripped off and the boarding aswell so you have a clean service for rubber or you can primer for a fibreglass roof.
If you re going to add a roof on top of an old one it s a good idea to remove any loose shingles and apply a new layer of 30 pound roofing felt before laying the new shingles.
Also you will need to remove any old felt on the roof as well as any nails or roof staples.
This would impact the felt s ability to protect against moisture.
You must remove any rotted pieces of wood on the roof and replace them with your plywood planks.
The 20 30 40 50 yr designations on your shingle type only refers to the length of time ther manufacturer anticipates the shingle will resist the sunlight weather.
Photo courtesy galow homes.
The felt is the last line of defence.
Above is a peel and stick mineral surfaced roofing underlayment shown during installation of a metal standing seam roof.
Lay down felt and secure to roof.
But if the felt roof is done with good thickness high performance felt it should last a long time depending on when it was done.
This is done to ensure a flat surface for the roof felt.
It looks as if the installer of asphalt roof shingles still visible at below right but now worn out may have placed roofing felt on top of a still older wood shingle roof.
Find a pre screened local roofing contractor.
Hope all goes well.
There are several disadvantages to using felt underlayment on a roof.
Having a layer of roof felt will block the resin preventing it from staining or compromising your roof shingles.
My boys always try to sway me into leaving it no dice.
The material may dry out or leach oils in the heat.
And as someone else posted you need to apply the ice and water to the plywood directly.
When i do a roof as with a few other boys on here it is also in my contract that a complete strip down of all old roofing material down to the sheathing to ensure that my customer gets the best possible roof for their dollar meaning if there are any current or developing problems with the sheathing they are taken care of before they become a problem.
When to use roof underlayment no matter what type of asphalt shingle you are using whether traditional or more robust styles such as under premium designer architectural and performance shingles it is recommended to use a underlayment.
Sweep the roof clear of grit and dirt to avoid poking holes in the new felt.