I am about to start to re-roof my house using the same fiberglass
standard three tab shingles. If I am able to remove just the shingles
and leave the 23 year old felt paper on the roof should I. Assuming it
is laying flat would adding a new layer of 15 or 30 lbs. (?). over it
plus the new shingles possibly cause any problems? I.E. moisture /
condensation or heaving? My thinking is during the project that it
might protect the interior from any rain even if I temporarily have the
construction area tarpped. Also even though it has nail holes it would
add a bit more waterproof coverage over the sheathing after the job is
finished not to mention less to go into the landfill.