If your roof is leaking it is not because of faulty under felt.
How to repair felt under roof tiles.
Put these nails at the bottom of the felt and leave a small portion available.
Bend the shingle over the ridge and nail down firmly.
Observe equal distance of each nails approximately 20 inches so they are properly attached to the roof.
It is because you have faulty tiles somewhere on the roof.
The roof felt is merely a secondary emergency waterproof membrane.
Take the next shingle and overlap it on top of the first on.
Cut off a piece of flashing strip to fit the length of the tile you need to repair.
Replace refit loose or broken tiles and use flashing tape to repair the roof felt underfelt.
Of course re felting the roof correctly will involve the removal and replacement of the tiles anyway.
If this leaks the roof tiles needs repairing.
Fasten the felt on your roof using the cap nails.
Put a coat of flashing strip primer onto the crack and the area around it.
Cut each strip into 3 individual tiles as shown.
Start by giving the area around the crack a good clean with a wire brush wiping away the dust and debris.
Image description step 2.
To replace a small section of roofing felt involves stripping the tiles from the roof and replacing felt between two joists.
Where wind driven rain is working under the roof and causing leaks then the felt will need to be replaced.
This will cost around 460 510 made up of.
When the first length is secured lay the next layer of sarking felt on top so that it overlaps by approximately 100mm.
It would be far cheaper to fix the faulty tiles than to re felt the roof.