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If building a 12x10 flat-top roof with 10ft joists spanning the 10-ft dimension, what is the proper way to cut the plywood and position the blocking?

I'm assuming I should cut the 4x8 sheets to be 4x5, and then put the blocking in the middle of the 10-ft joists. The blocking would be alternating like blocking normally is (i.e. every other blocking slightly closer to one side or the other to be able to get the nail gun in there). That way at least some of the middle edge of the plywood can be screwed into the blocking and not just the joists, but it's less than I'd think is optimal.

I was trying to think of some way that the entire middle edge of the plywood could be screwed into something, but the only thing I could think of would be to perfectly center all blocking and toe nail it, but this is not something I've done before and wasn't sure about if I should go that route.

Or perhaps I should get some ties to attach the blocking and center it that way?

As a side note, in case there are concerns about this type of roof, assume proper sealing will be applied after that can handle sustained ponding - i.e. Henry's Tropi-cool silicone roof sealer with appropriate underlayment.

g491
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1 Answers1

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That's a huge waste of plywood, and orienting the plywood the wrong way for the primary strength direction. 6 sheets wasting 2-1/4 sheets.

You cut one sheet in half the long way, one sheet in half the short way, and it takes 4 sheets total, only wasting 1/4 sheet. Long direction across the joists, put the 2 foot strip in the middle with full sheets at both edges. The joints are offset by 4 feet, like bricks. Or you can offset the joint in the middle strip 2 feet and get two 2x2 foot squares for waste rather than one 2x4 foot rectangle.

The panel edges are tied with H-clips, or you can stick in 2x4 blocking at the top of the framing if you really want to screw them down.

H-clip picture from nachi.org

H-clip picture from nachi.org

H-clips are considered to be equal to or better than blocking for edge support. Better than because they ensure proper panel spacing. They also cost less in time and money than installing blocking to screw to. Images from nachi.org

If you have rim joists you don't need any blocking for this short span.

As for "sustained ponding" flat roofs should not be flat. They just have low slope, on the order of 1 or 2% (1/8 to 1/4 inch per foot. 1 to 2 cm per m.) Water should run off. They'll still be plenty "flat" to walk on, if that's intended.

Ecnerwal
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  • Thanks - good point about the strength direction. I posted a follow-up question about avoiding rake walls yet still adding the slope you recommended at https://diy.stackexchange.com/q/262344/135898. Just to clarify, when you say adding blocking at the top of the framing, by "top" do you mean to use smaller boards like 2x4s instead of bothering with 2x8s for the blocking since not needed anyways? – g491 Dec 12 '22 at 14:39
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    2x4 (as stated, even) and might as well put them flat so you have 3-1/2" to hit with screws. – Ecnerwal Dec 12 '22 at 14:54
  • agreed. Who blocks roofs? Put the rafters at the proper spacing. – Tiger Guy Dec 12 '22 at 19:07
  • @TigerGuy since it's a flat top deck, the thought is being able to screw the plywood around the entire perimeter of each piece to keep things level where they meet in case there is warping or anything. Definitely considering H-clips though like Ecnerwal mentioned – g491 Dec 12 '22 at 22:39