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Location: Nebraska City
Is there a reason you feel obligated to do it? <shrug>
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Location: Royersford Pa.
It isn't hard. Most "How To" books will tell you how. Or just copy what was already there.
Make sure the OSB is solid and not spongy. It may be worth while to cut it out and replace it while the wall is open.
Twinn
Will post for food.
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Yes check the condition of the underlayment. If it needs replacing now is the best time. Cover with some felt paper. Yes you need a starter course on the bottom layer. They usually use rough shingles for this. Save the premium ones for the top layers. They are not cheap. Use good quality galvanized nails. Remember the spacing and chalk lines help. It is fairly easy job.
John T.
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Steve, I don't think you will have any problem installing the shingles. The challenge is removing the existing shingles that are fastened behind the courses you plan to keep. When I have time, I grind a hacksaw blade to fit my Sawzall, a long thin blade is required. You need to cut both in front and behind of the courses you're taking out; essentially cutting the fasteners into three pieces. After the staples are cut, I've found that tapping channel lock pliers downward to be the best way to pull the shingles out.
If the shingles were nailed vs. stapled, as some purists insist, then you could work them out, and pull the nails with a shingle spade. I've tried this tool many time on wood shingles but it never seems to work as well as it does with slate shingles.
Now for the "cheat". When, for whatever reason(s) neither of the above options is appropriate, slide aluminum flashing directly behind the course to remain, then cut the lower shingles off at the bottom of that course. Now drive the remaining portions up behind the remaining course 3/4" or so, apply a generous bead of caulk, and install the remaining course(s) cut to the appropriate length.
Whichever method you use, there will be the question of what fasteners to use for that last (or top) course. All the other fasteners will be covered with a subsequent course, and nail heads, even if painted, will always stand out on shingles. For this course I almost always revert to my finish nail gun. They're small enough to add a few extra, and thin enough to bend so I don't have to worry about the shingles splitting due to expansion/contraction.
Sign at N.E. Vocational School Cabinetmaking Shop 1976, "Free knowledge given daily... Bring your own container"