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I have seen quasi knee walls trying to act like purlins on older construction but not on new like that around my area.

Under the IRC only a properly built purlin could span multiple rafters. If you do a knee wall you would need to construct the same as a wall. I have seen lots of quasi knee walls on undersized ceiling joists over loading them and putting a nice sag in the ceiling.

What code is in place there?

Of course anything could be engineered over the prescriptive requirements of the IRC. That is one of the excuses I get from the muni's for that crazy fill in framing with truss systems.

If I saw what you are showing in the pics here I would explain what might be considered the deficiencies of doing an incomplete knee wall like those.

What were those knee walls sitting on? Tops of bearing walls or just in the field of the unsupported ceiling joists?

Chris, Oregon

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That's not a knee wall.

Those are purlins and wind bracing and absolutely not constructed properly.

The purlins should be the same size as the rafters (2X6) and installed vertical to the rafter (not flat). The wind braces look like they are t0o long for a single 2X4 - they need to be doubled.

You should also check to see the wind bracing is supported by a load bearing wall and that the studs are doubled beneath each wind brace.

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You say tomaitoe I say tomahto.

I know that a lot of guys call anything supporting rafters a midspan purlins.

Call it what you want.

In my opinion the design elements as they appeared to be me more closely act as a knee wall as I call it. Some will argue becuase of the lack of a support at every other rafter that the 2"x in flat use is acting as a beam and therefore should be considered a purlin.

Its neither, the way its constructed. To complete its design as a knee wall one would fill in the wall with studs under every rafter. To complete it as a purlin one would have to nail on a 2"x6" with is narrow end supporting the rafters. All this assumes that the bottom plate is bearing on top of a bearing wall etc.

I would prefer to see the 2"x6" directly bearing on its supports rather then side nailed into them. Doing so in this case would require more design elements so economically its cheaper to make this into a knee wall then a purlin in my opinion.

If you want to call it wind bracing thats fine. I am not familiar with what you mean by that here where I am as we don't see hurricaines and very rarely a twister in these parts.

Chris, Oregon

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