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Thread: Ridge Pole and Purlin Loads

  1. #11
    Quote Originally Posted by BigD View Post
    My faulty logic tells me that if adding purlins decreased the load on the ridge and plate logs, then all one would have to do is keep adding enough purlins before their log home started to hover. i.e. adding purlins won't change the load. What the purlins would do is to help with the loads, mid span.....but the same weight is going to be on the ridge and plate logs.
    This is what I remember from my engineering classes (simple Free Body Diagrams).....but I could be off base, entirely.
    Yes, that would be my goal, to support the load at mid-span. Still would plan a ridge log large enough to carry half the load.

  2. #12
    Quote Originally Posted by rreidnauer View Post
    It isn't split equally exactly. It's the area of tributary width, with tributary width being half the span between two supporting members. So:
    Ridge pole carries half span to each purlin.
    Each purlin carries half span to Ridge pole and to cap log.
    Each cap log carries half span to purlin.

    A standard ridge pole/cap log arrangement is 25%-50%-25%.
    With purlins, it's 12.5%-25%-25%-25%-12.5%.
    As you can see, the cap logs always carry less load being they only have a single side of tributary width. (in theory, as they still need to address the weight on the cantilevered eves)
    That's great information, thanks! Make sense now that I think about it.

  3. #13
    Quote Originally Posted by Arrowman View Post
    Depending on what you are trying to use for a ridge pole, purlins are primarily used to span longer lengths without having to go to extremely large rafter sizes. I'm not sure I would design a house using purlins to meet a load requirement on a smaller ridge pole. Put another way, I would two piece a ridge pole before I butchered my floor plan by dropping purlin supports into it. You may be able to design a floor plan that integrates the purlin supports, but good house design is tough enough dropping three RPSLs into the mix, not to mention designing around an addition 2-6 purlin support point loads.
    Quote Originally Posted by Arrowman View Post
    Depending on what you are trying to use for a ridge pole, purlins are primarily used to span longer lengths without having to go to extremely large rafter sizes. I'm not sure I would design a house using purlins to meet a load requirement on a smaller ridge pole. Put another way, I would two piece a ridge pole before I butchered my floor plan by dropping purlin supports into it. You may be able to design a floor plan that integrates the purlin supports, but good house design is tough enough dropping three RPSLs into the mix, not to mention designing around an addition 2-6 purlin support point loads.
    Yes, that's what we're struggling with in the design - posts all over the place. Wife says she wants as few as possible, and I agree. We're going for the 'open floor plan' style. It's one thing having three RPSLs (four in my current design) to plan around. At least they are all in a line through the center of the house, pretty easy to work with. But start throwing in purlin 2-4 purlin support posts which are halfway between the eaves and the centerline, and it gets really messy.

    The only reason I'm even considering adding purlins is that I surmise that if done properly, we can do away with the rafters entirely, using SIPs to span between the plate and purlin and purlin and ridge. My thinking is this would save a boatload of time getting the roof on - faster is much better according to my wife - and perhaps a little bit of money as well, since 2x12 rafters 16" O.C. wouldn't be exactly cheap. Even if we stick with a traditional rafter design, we'd still be using the same SIPs on top. But of course, there are benefits to both ways. Rafters give you more headroom on a 1/2 story if you attach the ceiling planks to the top of the rafters per usual, with no pesky PSLs or purlins to intrude upon you....

  4. #14
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    Quote Originally Posted by aaroncgi View Post
    Yes, that's what we're struggling with in the design - posts all over the place. Wife says she wants as few as possible, and I agree. We're going for the 'open floor plan' style. It's one thing having three RPSLs (four in my current design) to plan around. At least they are all in a line through the center of the house, pretty easy to work with. But start throwing in purlin 2-4 purlin support posts which are halfway between the eaves and the centerline, and it gets really messy.

    The only reason I'm even considering adding purlins is that I surmise that if done properly, we can do away with the rafters entirely, using SIPs to span between the plate and purlin and purlin and ridge. My thinking is this would save a boatload of time getting the roof on - faster is much better according to my wife - and perhaps a little bit of money as well, since 2x12 rafters 16" O.C. wouldn't be exactly cheap. Even if we stick with a traditional rafter design, we'd still be using the same SIPs on top. But of course, there are benefits to both ways. Rafters give you more headroom on a 1/2 story if you attach the ceiling planks to the top of the rafters per usual, with no pesky PSLs or purlins to intrude upon you....
    I haven't looked at different SIPs, but I'm not sure how they would make one to span 12' cheaper than you could do it. Typical SIP is 2xX framing (depending on how much insulation you want) with a piece of EPS sandwiched by some 7/16 OSB. I have seen some with 2x6 T&G preattched to the inside but again it costs more. SIPs will just hide the lumber used as rafters. They will make the roof go faster, but will be more expensive. Putting OSB over EPS won't lessen the amount of rafters needed regardless of whether the 2x12's are imbedded in the SIP or exposed.


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  5. #15
    LHBA Member rckclmbr428's Avatar
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    Two things, first, I can put an entire house worth of rafters on before lunch, and level and pin them after lunch. I've also built with purl ins, and trying to get everything lined up just right is a giant pain. You would come out ahead on time and frustration by using rafters. Second, you can put the rpsls on the outside of house if you don't want to see them all the time.

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  6. #16
    LHBA Member rreidnauer's Avatar
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    I will have purlins, and want to avoid additional purlin support logs. The way I'm handing this is two step. First, my gables (on a 12:12 pitch roof) will be log gables, securely fastened to RPSL and to a set of aligned rafters to RPSL, so just like girders, purlins will be directly supported by the log gables. Second, in the middle of the structure, support will be done with vertically oriented 3-1/2" 8" paralams buried directly in the wall structures. No visible support, nor any obstruction/protrusion into floorplan.
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  7. #17
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    It's good if you can get the supports in the walls. Depending on how you design that house that is easy or nigh-on-impossible. For me, it was going to butcher my floor plan.

  8. #18
    Quote Originally Posted by Arrowman View Post
    I haven't looked at different SIPs, but I'm not sure how they would make one to span 12' cheaper than you could do it. Typical SIP is 2xX framing (depending on how much insulation you want) with a piece of EPS sandwiched by some 7/16 OSB. I have seen some with 2x6 T&G preattched to the inside but again it costs more. SIPs will just hide the lumber used as rafters. They will make the roof go faster, but will be more expensive. Putting OSB over EPS won't lessen the amount of rafters needed regardless of whether the 2x12's are imbedded in the SIP or exposed.


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    I am no expert on SIPs either, but have looked at a few. There can be a huge difference in price between different types of SIPs, for the same insulation value. Some are actually cheap enough that indeed, they are less expensive than 'rolling your own' using rafters and separately installed rigid foam - not as cheap as fiberglass or cellulose, though. The one's I'm most considering omit the OSB entirely - we'd install plywood instead - and use polyurethane foam, so about half the thickness of EPS. We looked into the SIPs with pre-attached tongue and groove boards on the bottom, but price was in the stratosphere - like 2+ times as much.

  9. #19
    Quote Originally Posted by rckclmbr428 View Post
    Two things, first, I can put an entire house worth of rafters on before lunch, and level and pin them after lunch. I've also built with purl ins, and trying to get everything lined up just right is a giant pain. You would come out ahead on time and frustration by using rafters. Second, you can put the rpsls on the outside of house if you don't want to see them all the time.
    Talked it over with the wife, and with myself in my head, and the more we think about it, the more we agree with you. Good to know rafters installation can go so quickly. There are just so many advantages to using rafters with very few drawbacks, and lots of complications and complexities using purlins. I think I was just enamored by the elegance and simplicity of being able to support the entire roof in huge panels (SIPs) with just the walls and three other support logs. I also liked the idea of a 6-8" thick roof instead of 18-24" thick, but meh, bigger looks better on a log home, right? Love that second house picture! For me, I think a rafter-less ceiling with a couple purlins would be more attractive inside, but lots of work and obstacles to go through just for that.

    No need to put the gable RPSLs outside, though we considered it for a few milliseconds. Our plan has them mostly hidden on the first floor and a design element on the second floor. Who wouldn't want a big tree inside their house, anyway?!

  10. #20
    LHBA Member rreidnauer's Avatar
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    You need to pay close attention to what you are considering buying. (and likely the reason for widely varying prices) There are SIPs (STRUCTURAL insulated panels) and there are just insulated panels. (often referred to incorrectly as SIPs) The former is truly structurally engineered to carry span loads on its own. The latter is not, and requires separate load supporting members.

    The price difference may not be that different when you account for the additional rafters required for plain insulated panels, and the additional labor of installing those rafters.
    Last edited by rreidnauer; 03-15-2017 at 01:44 PM.
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