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Community and Q&A

Metal roof on uneven strapping / purlins

etting | Posted in General Questions on

My simple gable roof is built with trusses covered by OSB then underlayment then 2×4 strapping/purlins. The top chords of some of the trusses seem to be slightly bowed, because if I lay a straightedge from peak to eave, along some of those lines one or more of the purlins is up to 1/2″ higher than the others. The roof has a 4:12 pitch and covers a 20′ width and 48′ length.

To install Classic Rib exposed-fastener panels, how much variation in the purlin height is tolerable?

If I need to shave off the high areas on some purlins, would an electric planer be the best tool? (I screwed the purlins down, so I can remove the screws before planing.)

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  1. Expert Member


    If you can notice the difference in height it is too uneven for a metal roof.

    Shim the low ones, and take the high ones off after marking where they sit on the trusses and notch them (the purlins, not the truss chords) with a circular saw and chisel . A plane will take ages.

  2. etting | | #2

    Thank you, Malcolm.

    I don't know whether I would have noticed the height differences if I hadn't checked with a straightedge, which would have revealed differences as small as 1/8". Do builders typically use a straightedge to check, or do they eyeball it? If the former, how much variation do they accept?

    If I end up needing to shim 1/2", would a piece of OSB do as a shim? I'm in Arizona, so it wouldn't be subjected to a lot of moisture.

  3. GBA Editor
    Martin Holladay | | #3

    Yes, an experienced roofer checks to see whether the rafters, trusses, or purlins are co-planar before beginning the job. It's hard to get a good 20-foot straightedge, so most roofers use string for this job.

    OSB is an acceptable shim, although I like to make shims on a tablesaw, using 2x4 scraps that are knot-free. You can also use cedar shingles for shimming.

  4. seabornman | | #4

    With an exposed fastener roof, you are unlikely to notice a 1/2" difference. All those ribs hide a lot of sins.

  5. Expert Member

    in a sense there are two questions here.

    Does the 1/2" variation between trusses make any difference to the way the roofing will function? No.

    Will it look funny? It depends on what level of workmanship you are comfortable with.

    I find cleaning up these sorts of variations make a big difference in how a job looks when it is finished. Like un-plumb walls or out of level trim, you don't once what the problem is, it just looks - out.

  6. etting | | #6

    Thank you, Martin, Joel, and Malcolm. To be clear, I observed up to 1/2" variations going from peak to eave along single trusses, not between two different trusses. A panel will bend easily across its width, but the ribs may kink if bent too far along its length.

    Martin, When experienced roofers check flatness with a string, how much variation from that flatness do they/you bother to correct? Surely not 1/8", but how about 1/4", 3/8", 1/2"?

  7. Expert Member

    "I observed up to 1/2" variations going from peak to eave along single trusses, not between two different trusses"

    Ah - that's a different kettle of fish. It's most likely seasonal truss lifting, and may go away. I wouldn't worry about a 1/2" bow in a 20 ft span. The ribs take that easily.

  8. etting | | #8

    Thank you, Malcolm. I wish it were a gradual 1/2" variation over 20', but it's one purlin 1/2" higher than the ones immediately (2') above and below it.

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