Permits are required for most new roofs, re-roofs and major roof repair jobs, but many cities don't require them for minor repairs or replacements of less than 100 square feet (one "square") of roofing area.
If any areas of the roof have become severely damaged, it's usually better to remove the old shingles and start from scratch. ... 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.Dec 7, 2019
Replacing Felt on a Flat RoofIf your shed has a pent roof (flat, with no ridge) you can apply the felt in exactly the same way as you would for one side of a pitched roof.Apr 9, 2015