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If I snap lines for a architectural shingle roof and it shows that the roof is 1" out of square at the ridge, what is the best procedure for coming out with a straight row of shingles along the top of that roof (e.g., how can I adjust my snapped lines to account for that 1" out of square)? My goal is to come out with every single row being straight without having to (visibly) fudge any rows.
 

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Firstly, I have never been on a roof that is not out of square. That being said, you will have to adjust the opposite corner (gable) to accomodate. I.E...your shingles would be trapezoidal. If the top right of the roof is 1" higher then the top left when measured off the starter, then move the bottom left down an inch to make up for it, and trim the shingles so as not to droop, and then install G-TAPE and snap lines as directed. G-Tape will make your roof perfectly straight if you use it properly.
 

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What aaron said will definetly work and personally if I had to choose the out of square being at the top or bottom I'd choose the bottom where it's less likley to be seen.

Though another method would be to adjust a few row's exposure at the center of the roof so the top and bottom rows are even but 4 rows spaced apart every few rows are 1/4" out of center. They eye may catch 1", but from the gound is not likley to catch 1/4"
 

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what's G-tape?
 

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All roof decks are out of square!! I make my adjustments somewhere in the middle. If too close to the ridge or eave, the adjustment course (courses) can be noticed. I never take more than 1/2" / course.
Jim
 

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I like to go 1/4" per course if I am squeezing or add a course insteas of spreading. Overexposure is BAAAAAAAAAD (in a sheep voice) :eek: :cheesygri
 

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Taking for granted, for instance, a 1" shingle overhang at the eave, - - at the larger end I would start at the 11" point, and mark 5" increments up to the 51" point, - - then at the lesser end I would again start at the 11" point, but this time mark 4 7/8" increments up to the 50" point, - - then snap the guide lines, and once these 9 courses are run, just shingle as normal. No overexposure involved.
 

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Here's what i'll do ; Prep the roof for shingles including the starter and first course. Determine which side of the roof is the tallest as measured to the headlap of the first course (tall side 120", short side 118" for example). Now go down to the short end and hook your tape on the headlap, pull to ridge and angle your tape toward the middle of roof until 120" lines up with ridge. Now back down the roof marking the fives as you go. Walaa, assuming five inches is your exposure.
 
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