Ridge Pole and Purlin Loads

[aaroncgi]

Hi All,


I have some hopefully quick questions about loads on roof members.

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

What happens if you start adding intermediate purlins between the ridge pole and plate logs?

For example, say you add a single purlin on each side, halfway between the plate and ridge. Does your ridge pole still support half the roof, while the remaining half is split equally between the purlins and plate logs (aka 12.5% each). Or, is the load split evenly between all five members - 20% to each? Or something more convoluted? :)


Cheers!

[rocklock]

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

I don't think its that simple...If I remember some of my engineering courses about forces and stuff, but then again its been 40 years... Which is why a wet stamp is required on our plans...

[mudflap]

You have to do some vector calculations, along with figuring the weight of each purlin. The angle of the forces butting against the ridge pole vs the forces of a relatively non-loaded center purlin add some complexity as well.

My old math professor, when asked "what could you do with a Phd in math besides teach?" said he had a friend in California that worked for the highway department making $400/hr "thinking" about highway bridge design. Now THERE'S a government job for ya... :)

blog: https://loghomejourney.wordpress.com/

[BigD]

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.

[Mountain Lion]

Let's say you add 2 purlins on each side. If you set them low such that the rafters don't touch, clearly the ridge and cap continue to carry all the load. If you set them high enough, you can get them to take all the rafter load off the ridge and cap. If you do the normal thing and put tops of ridge, cap and purlins on a plane, the load will distribute. Math left as exercise for the reader :)

Purlin load is carried by gable walls and purlin support vertical logs.


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[rreidnauer]

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)

[Arrowman]

Hi All,


I have some hopefully quick questions about loads on roof members.

I understand that if you have a structural ridge pole and your rafters clear span between the ridge pole and the plate logs on the walls, then your ridge pole is supporting 50% of the roof, and your plate logs support the other 50% (or 25% the weight on each wall). That seems clear enough.

What happens if you start adding intermediate purlins between the ridge pole and plate logs?

For example, say you add a single purlin on each side, halfway between the plate and ridge. Does your ridge pole still support half the roof, while the remaining half is split equally between the purlins and plate logs (aka 12.5% each). Or, is the load split evenly between all five members - 20% to each? Or something more convoluted? :)


Cheers!
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.

[Mountain Lion]

Intuitively thinking, the cantilevered weight hanging over the cap log (eaves) will first unload the purlin and then eventually the ridgepole. This depending on the rigidity of the rafters and the rest of the roof system. The unlikely case of a perfectly rigid roof is an exception where the ridgepole would unload at twice the rate of the purlin. I suspect that any math done on this will make a lot of assumptions (which will be close enough, given our propensity to overbuild).


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)

[rreidnauer]

You fail to take into account the area of load on the interior side of the cap logs far exceeds that of the eaves. There cannot be the uplift you describe. Any forces associated to uplift end when your interior side tributary width exceeds the eve cantilever distance.

However eve overhangs do, as I previously mentioned, increase bearing to cap logs, that the span and live load calculations do not take into account when designing a roof.

[Mountain Lion]

I'll be the first to admit that I could be thinking this wrong.

However, I mentioned "unload", not "uplift". I meant that the forces on the non-cantilevered portion of the roof are reduced by the cantilever weight. You can think of this in the limit. There is a force that you can apply on the lever of the cantilevered roof that will lift the rest of the roof. When you apply that force, there will be no force on the ridgepole or purlin. When you apply no force on the cantilever, the ridgepole and purlin will be fully loaded. At all intermediate cantilever force levels, the ridgepole and purlin will be partially loaded and there should be a continuous function that describes the relationship. This is the case for a perfectly rigid roof structure. Flexibility of the roof structure does not negate the effect of the fulcrum, only where the loads ultimately fall.

This is how I thought of it, but like I said, I could be all wrong.

[aaroncgi]

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.

[aaroncgi]

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.

[aaroncgi]

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.


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.... :)

[Arrowman]

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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[rckclmbr428]

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.
http://i279.photobucket.com/albums/kk122/rckclmbr428/20161013_162218_zpsthsl266m.jpg (http://s279.photobucket.com/user/rckclmbr428/media/20161013_162218_zpsthsl266m.jpg.html)
http://i279.photobucket.com/albums/kk122/rckclmbr428/20160508_101826_zpsa9y4gcw9.jpg (http://s279.photobucket.com/user/rckclmbr428/media/20160508_101826_zpsa9y4gcw9.jpg.html)

[rreidnauer]

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.

[Arrowman]

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.

[aaroncgi]

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.

[aaroncgi]

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?!

[rreidnauer]

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.

[aaroncgi]

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.


That's a great way to go, and I thought of using glulams for the purlins - can even put a bevel on the top to match the roof pitch. Our issue is that our second floor plan is almost completely open, so no walls in which to support the purlins. :)

I actually thought of a kind of crazy way to support PSLs, using a log girder spanning across the center of the house, on top of and perpendicular to our second floor girder (ie the same plane and direction as the second floor joists. The PSLs would then rest of top of the these logs, halfway between the eaves and center. Naturally, the second floor girder would need to be significantly enlarged, and the corresponding posts... No idea if this would have worked or not, but it looked and sounded cool in my head!

[aaroncgi]

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.

Yes, I am aware of that, but thanks for the reminder. I'm only looking at true SIPs, with structural members inside and capable of spanning the same distance as rafters of the same dimensions and species.

[loghousenut]

I say put up rafters, then purlins on top of the rafters, then more rafters on top of the purlins.

The rafters are supporting the purlins, and the purlins are supporting the rafters. Use Allthread and the upper rafters are pulling up as the lower rafters are pushing up and then the purlins are probably not doing much of anything except hanging around as eye candy.

On second thought, if your ridgepole is large enough you can probably just build it like Skip said to do it in the first place.

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