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Rafter concerns

Brian Adams | Posted in General Questions on

Code allows 24 inch o.c. rafters with 1/2 inch OSB with H clips.

My framer isn’t loving the advanced framing and wants to do 16 inches o.c.

The problem being that every third board would land dead between wall studs and even with the extra grace from a double top plate, that won’t fly with inspectors. They are planning on 2×8 rafters.

I plan on metal roofing on battens, could even bump those up to 2×4 if that would be the easiest way to solve things.

I know there are a number of ways to address this. Thoughts on the best way to proceed?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what you mean by "every third board." (A board is one inch thick, or 3/4 inch thick when planed.) From context, I think you mean "every third rafter."

    If you have double top plates, rafters don't have to be located directly above a stud.

    Perhaps I'm misunderstanding your question. Can you clarify?

  2. Brian Adams | | #2

    Yes, I mean every third rafter.

    I thought that the double top plate would fix the issue but the inspector differs. I'll dig through the code to see if I might be able to argue my case. They go by 2012 IRC here.

  3. Expert Member
    Michael Maines | | #3

    R602.3.3: restricts alignment of stud and rafter to within 5". There is an exception for a double 2x6 top plate. If you're using a single top plate, you have to align studs with rafters.

  4. Brian Adams | | #4

    Indeed, thank you! I found the same wording in the 2012 code as well.

  5. Expert Member
    Malcolm Taylor | | #5

    Are you sure you need H-clips with 1/2" OSB? We don't here in BC and a quick search shows they don't appear to be required down south either.

    "The International Residential Code (IRC) and the Residential Edition of the Florida Building Code (FBC) both refer to Table R503.2.1.1(1) of the APA Engineered Wood Construction Guide for specs.

    For the most common roof sheathing in our area, which is 1/2” nominal (7/16” actual) sheathing over trusses or rafters at 24” on center, there is no requirement for H-clips. The APA table has a span rating for the sheathing installed with, or without, H-clips, and the span rating is the same both ways. "

    1. Brian Adams | | #6

      I was repeating what the inspector told my, I had assumed he was correct.

      You said 1/2" sheathing over 24"oc rafters. Is there a specification of ply vs OSB on the sheathing? And what dimension are the rafters?

  6. Expert Member
    Malcolm Taylor | | #7

    My understanding is both 7/16" OSB and 1/2' plywood are rated for a 28" span without H-clips, on roofs above a 2/12 slope. However I don't know what code you are under, so it's best to get someone familiar with it to confirm that.
    Is there a reason you have both sheathing and battens under your metal roofing?

  7. Brian Adams | | #8

    The primary reason for the battens is for airflow under the metal and to give condensation and future leaks somewhere to go other than through the deck.

  8. Expert Member
    Malcolm Taylor | | #9

    There have been a few discussions here recently about the benefits of an airspace between the decking and metal roofing. I won't pretend there was a consensus, but my own view is that the gap isn't helpful.
    Unless there is an inlet and an outlet for the air, at each end of the cavities, and a change in elevation, there is no mechanism to exhaust any moisture that may accumulate.
    It is also an open question whether the space may become a reservoir for moisture. Without the gap there is nowhere for moisture to accumulate.
    I can see the appeal of a second layer to stop possible leaks (although metal roofs, properly installed should never leak until they eventually need replacement), but battens, which will have to run horizontally across the roof, don't provide a drainage plane that leads anywhere.
    My own advice would be to install the roof panels directly on an impermeable underlay fastened to the sheathing.

  9. Brian Adams | | #10

    I understand the concern, I think maybe my choice on the metal will make a difference. I'm planning on a "Tuff Rib" design over standing seam, there will be several ridges on each sheet that will allow a good bit of air to travel up to the roof ridge. I'm also thinking of spacing the battens a bit so any accumulated water will eventually seep down and not pool indefinitely.
    That's my thought anyway, I could easy be sorely missing the mark and I appreciate all the feedback!

  10. Expert Member
    Malcolm Taylor | | #11

    If you do go that way remember to detail your eaves so that the water you anticipate draining down through the battens ends up where you want it. Typically metal roofs either have nothing, or a starter strip at the eaves, with the fascia below that. Your plan needs some way to get the water from behind the fascia.

  11. Expert Member
    Armando Cobo | | #12

    1. If a framer tells me he's not comfortable using my preferred framing methods, I'll get a new framer I can trust.
    2. I would double check your roof framing with an Engineer or a Code Official. At the very least, I think you need to visit 2012 IRC Tables R802.5.1(1) through R802.5.1(8). Allowable rafters depend on spacing, length, lumber spices, ceiling attachment and roof loads. One size does not fit all.

  12. Tim R | | #13

    The framer wants 16"oc so the roof feels stiff. It will also keep the sheathing from sagging over time if you did comp shingle. If you change to 5/8" sheathing the 24" oc, It should feel stiffer like the 16"oc when walked on. If every thing is up to code spec.

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