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I just started pulling up shingles to reroof a 1989 house only to discover the plywood sheathing was installed without gaps. As a result, there is a lot of 1/4 inch buckling, especially on the unsupported seams.

Can I run a circular saw on all the seams set to the depth of the plywood to add spacing? Will the buckles ever lay flat or do I need to replace all the panels with buckling? can I put 2x4 pieces underneath in the attic and screw through the sheathing to try to level it out?

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    Is the joist spacing 16 or 24", just curious. If it is a 24" span, it should have plywood clips mid span of the joists, a code requirement. These are to aid in keeping the edges aligned. – Jack Sep 2 at 0:21
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    I have been in the trade since 1972, I have always seen them used on plywood decking. – Jack Sep 2 at 15:33
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    @Jack Ply clips are not required by Code, provided the proper thickness of plywood is used: 1/2” for 16” joist spacing and 5/8” for 24” joists spacing. (The plywood should have a grade stamp.) (See ICC 2304.7.2 and Table 2304.6) – Lee Sam Sep 2 at 17:47
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    @LeeSam, I should have been more specific on my description, I was referring to 1/2" plywood being used at 24" centers. But I did think 5/8" required clips at 24" centers as well. Thanks for the clarification. – Jack Sep 3 at 0:02
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    Well, this is 1/2" ply on 24" OC rafters. I guess if you don't know to gap the decking, then clips are out of the equation too. I could still read the plywood where it said 1/8 gap required on roofs! Adding clips in where I can and blocking elsewhere. Plus replacing sheets as needed. Ripping the seams with the circular saw seems to work well too. Now I am notching the blocking along the eave wall to allow air gaps when I pull the sheets up. This is a big roof job! Also, the decking was out down with Staples, which are all loose, so I am having the nail everything down. – Frank f Sep 3 at 2:08
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For your situation, I would cut the gaps like you suggest and add blocking to force the alignment of the edges. Plywood clips should have been used to keep the edges in alignment, but the gaps should have been there too. Over cut the gaps enough to make sure the butt joints are open totally

You will only need to use scraps to block the edges, you could even use 3/4" plywood for the blocking using 2 screws on either side of the gap. I would do the same with 2X if you used that as well, that way one screw will help the other draw the blocking tight to the plywood. The blocks do not need to go over the gap parallel to it, it can go perpendicular to the gap to bridge the two. That is how I described the screw install.

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