A metal chimney, power ventilator, skylight, brick chimney, dormer, etc., will cut the courses off from the original base and offset lines. The courses below the obstruction continue across the roof uninterrupted to the far rake. You want the keyways of the courses broken by the obstruction to line up exactly with the keyways of the continuous courses. Let me show you two obstruction situations and how to establish new verticals in each case.
Your baseline and offset line are to the left side. The size of the unit breaks the continuity of six courses. Lay the six courses up the left side of the unit and cut and trim them as shown. Carry the seventh course on across the top of the unit, nesting the tops of the new shingles to old shingles. Once you get to the top of the unit, nail the shingles in course 7 high. (“Nail the course high” by driving the nails at the top of the shingle, keeping the nails in a line directly above the keys.) Carry course 7 on beyond the power ventilator two (or three or four) shingles and stop laying in that course. When you stop laying in that course “leave the end loose.” (“Leaving the end loose” means you don’t drive the fourth nail at the end of the shingle.)
Now bring the eighth, ninth, tenth, and eleventh courses on across, stopping them when you have laid the same number of shingles you laid in the seventh course. Nail courses 8,9,10 and 11 in their normal positions above the keys, but leave the ends loose on each of these courses.
Drive a nail directly above the end of the last shingle on course 11. Hook a chalk line to the nail and run the line down across the gap the power ventilator caused in the courses. Hold the chalk line down several courses into the courses that continued uninterrupted below the power ventilator. Make sure it lines up with the ends of courses 9 and 7 above the power ventilator. Pull the line tight and pop it. You have the new baseline. Now lay the same number of shingles for course 12 as you did for the others coming across the top of the power ventilator. Nail directly above the end of the last shingle on course 12. Hook the chalk line, centering the line on the appropriate keyways on the uninterrupted courses, and pop a new offset line across the gap. Lay the shingles back toward the power ventilator and trim them in.
You “nailed high” on course 7, so you can slide the shingles for course 6 directly under the shingles of course 7 and set them on the offset line to trim them into the power ventilator. Nail the shingles in course 6 normally. As you continue courses 7, 8, 9, 10,11and 12, you can butt the next shingle into its proper position because you “left the ends loose” on these courses.
Of course, if you are overlaying the old roof, you nest the new shingles to the old shingles to carry the horizontal courses. If a tear-off has a big obstruction, just measure and pop lines for 5-inch courses up both sides of the obstruction.
Going up as many courses as possible above the power ventilator before you nail above the end of the shingle, and going down several courses on the uninterrupted shingles (to hold on the keyways) gives you more accurate lines than just holding the line across one or two shingles at the top of the gap and one or two shingles at the bottom of the gap.
When you use this method of redoing the verticals, the keyways will turn out straight—as if there had never been a power ventilator in the way at all.
OBSTRUCTIONS AT THE RIDGE OF THE ROOF
An obstruction such as a brick chimney can reach the ridge of the roof. In a case like that, lay uninterrupted courses across the bottom of the roof. Go several courses down from the last uninterrupted course you laid and find the end of a shingle in one course down from a baseline course. Carefully set the nail hook under the full tab, making sure it is centered exactly below the joint where the two shingles in the base course above are butted together. Pull the chalk line gently straight up the roof and lay the line down. Now weigh the hook down under the tab with a bundle of shingles and pull the chalk line directly up the roof. Pull it snugs, centering it over the keyway of the several courses above where you have it hooked. Hold it up at the ridge and pop the new baseline.
Set the nail to hook up or down one tab, centering it directly below the butted joint of shingles in an offset course. Repeat the process and pop the offset line all the way to the ridge. Lay the courses back from the new verticals and tie the shingles into the obstruction. Then continue on across the roof.
If the bottom edge of a tab gets nicked when you pull the chalk line against it, push it down smooth with your finger or tap it smooth with a hammer. (Someone else holding the chalk line makes all this easier: he can hold the chalk line directly over the butted joint. I have described the method for setting the nail hook to pop a chalk line when you are working singlehanded. You will probably have to set and weigh the nail hook more than once to get the chalk line to pull right over the joints.)
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