Ridge Pole and Purlin Loads

[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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