Tearing off less than all of the old shingles

So the dumpster was picked up yesterday, I have a one and a half story bungalow with dormers and a front porch and in the back a 10x10 mud room. At this point, I've torn off old shingles from all of the roof except for the main part of the roof on the southern side, I chose to skip tear-off on this one face because the shingle condition was acceptable for putting on a second layer of shingles - the rest of the house I tore off shingles because of extensive tab deterioration and/or green-ness ranging from extensive algae to moss an inch thick.

Beyond that, the roofing over the dormers was laid over rotted structure, I tore out rafters and top parts of the wall, and I chose to change the shed roof style to gable roofing over the dormers.

So in order to do this, on the south face I've had to tear off part of the face to put in the flashing for the valleys, so here-in I have my question.

When I lay down the felt on the exposed portion of the southern face, where it meets the roofing that hasn't been torn up what do I do, do I lift up shingles and tuck the underlayment under - and if so by how far, 12" or more? Or do I go over the old shingles and if so by how much?

 

You aren't going to be able to get anything 12" under the old shingles. I guarantee that they will break.

Multiple layers of shingles is getting to be a thing of the past, if it isn't already. It's not considered best practice anymore.

 

I agree with Pulpo. Total tear off is the normal procedure. With that you can inspect your decking, fastening, flashing, etc all at once, then have a good roof afterwards with peace of mind knowing you did it right all the way down. Don't rely on what someone else did 20 years ago.

 

Well... I have to start out by saying that I appreciate a lot of the good advice I get here, but in this case while I know it's not best practice, I have to say that I really would rather put on the second layer of shingles on the southern face.

I'm working through the northern face, and it's a 9/12 pitch and with a mild fear of heights (and a less mild fear of falling off a slippery roof) I just don't want to spend any more time doing this roof than I have to, and the 9/12 pitch section of this roof is slower work for me as I do the edge from a ladder then put in roof brackets and move them up as I go.

That and the fact that if I tear off the roof, it means that area is going to be exposed longer and it tends to rain in the fall, so by leaving the old shingles on the area is protected during the job.

I'd be more inclined to do the extra work if not for the fact that in 5 years we're planning to have the whole second floor torn off and rebuilt as a full second floor. I know plans can change, but even if I was to lose my job my wife's income is enough to keep up the payments and the house would be paid off in 4 years.


So with all of that said, I'm reading between the lines that what I would need to be able to do is lay down the underlayment so that it overlaps the existing underlayment under the old shingles by at least 12".

And I am comfortable with the condition of the sheathing, inside the house I have the ceiling torn off and I can see the sheathing from underneath and this particular area shows no problems.

 

Hire two day workers for two days & you'll be able to do all of the work the way we described.