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We have a gabled-roof single-story detached garage with horizontal 2x4 beams spaced four feet apart and a 20' span. There are two vertical 2x4 beams connecting the two horizontal center beams to the roof. There are also 1x6 diagonal braces connecting the roof to every joist.

I'd like to install plywood flooring above most of it for storage. Nothing that we put up there will be too heavy, but I'm certain I'd need to provide additional support. I would plan to walk on the floor to store and retrieve things.

What size and spacing of supplemental joists would be reasonable to support a floor? How should they be safely secured to the existing structure? And what thickness should the plywood floor be for reasonable support? Should I consult with a structural engineer?

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The following may be duplicates.

  • What's the span? Nobody can use span tables to look up the joist size, without knowing what the span is. – Tester101 Nov 10 '15 at 16:01
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    I'll go take some pictures and make some measurements and update the question. It's a smallish two car garage and the joists run perpendicular to the cars. – glenviewjeff Nov 10 '15 at 16:03
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    Are you sure those are joists and not rafter ties? Often, they are one in the same, but if the intent was for them to primarily be rafter ties, they're likely not 'engineered' to also act as floor joists. – DA01 Nov 10 '15 at 16:26
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    @glenviewjeff Those are not joists/rafters. They're there to prevent the roof from flattening out, and are not designed to support anything on top of them. – Tester101 Nov 10 '15 at 16:31
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    ah, yes, the photo helps a lot. Those are indeed just rafter ties. You can certainly stick a piece of plywood up there and use it to store things like that ladder, but if you want an actual floor to walk on and store heavy objects, then you need to install actual floor joists. Doable, but might just be easier to build a shed somewhere else in the yard. – DA01 Nov 10 '15 at 16:37

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