Agreed

Meh rather do it on the roof or it’s way harder to straiten out the gable truss

😊

I think the main condensation risk is from warm/moist interior air getting to the metal. So proper vapor barriers and venting will prevent that. However, most metal installers still want some sort of barrier between OSB and metal panels, usually an adhesive membrane/shield product (there are many nowadays, most common is the blue stuff). That will prevent any sort of environmental moisture that condenses under the metal from causing rot/damage to the OSB. No matter what you do, metal surfaces can sweat under environmental conditions. I occasionally see it on my open pole barns with metal roof and purlins. It's rare but happens. And think about all the other metal objects that can sweat under particular conditions. I've even seen my sawmill sweat when we get a sudden increase in temperature/humidity after it's been cold. It's under a roof and well ventilated, but if the metal is cold and the air is moist, it will sweat.

@@Lumber_Jack Yep, even firearms will sweat laying in the glove box or on a gunrack.

Different scenarios to the two methods you cite, and they generally are not comparable choices. The plastic you saw is probably a bubble wrap radiant barrier to reflect heat out and provide a minimal insulated vapor barrier to ward off condensation problems. This is done when the metal shares airspace with a closed area that generates moisture and/or has limited use climate control. In stick-frame consruction (trusses or rafters but no purlins) a metal roof is often installed over OSB sheathing with a roof membrane. Under the OSB would be a traditional attic or ceiling construction with appropriate insulation.

Can't give a good answer without knowing your local roof load requirements, but I'd suspect not if laid flat. Even with no snow load a 2x4 can sag under it's own weight over time when laid flat on a 4' span. For 4' span most people put the 2x4 on edge, but that adds other complications.

I use pressure treated for roof framing on boat house roofs that are over water, but it's not necessary on land. If you use pressure treated, be sure to put a membrane material between the roof panels and purlins to prevent corrosion of the roof, and use hot dipped galvanized roofing screws.

Wood does not cup toward the bark. So bark side up.

Cupping toward the bark side is a general rule, but 80% of my rough hewn pine purlins cupped toward the bark side. When I contacted the local sawmill the old man there said it all has to do with specie and drying process. He said smaller pine boards will cup towards the bark as the tension and compression is released. @@edwardbowles8613

I'd pick 16D for sure. 60D seems insane.

I “think” that’s for if you are doing purlins one their sides.

60d are for purlins on edge. They have to go through 3 1/2" of wood before getting to the rafter. I just finished the on-edge purlins on my building yesterday - one hell of a job!

Nominally 2' here but it really depends on the wind loads and fastener requirements. Some locations might need more purlins.

@@Smoothjaswi I usually do 2' on center from eaves going up, and then the last spacing near the ridge might have to be less since the rafters are rarely a multiple of 2. But it really doesn't have to be exact or specific unless you are needing to calculate for fasteners and uplift resistance.

For this project the purlins meet on the rafter, joint centered on the rafter, and screw in (screws angled so they aren't too close to the ends of the purlins). However this is not the best way to do it because it puts a lot of screws in one spot on the rafter and won't have good pullout resistance against uplift. Need to make sure to stagger the purlin joints when doing it this way so that they alternately fall on different rafters. If uplift is a concern, the best way is to have the purlins meet halfway between two rafters and put a 2' section of 2x4 underneath them that you can screw into with spread out screws. That keeps the joints off the rafters and avoids having too many screws in one spot.

Thanks for the idea of matching purlins between the rafters, I'm dealing with eastern red cedar, thats why I'm wringing my hands over so many screws in the edge of 2 of my rafters, I'd been toying with attaching a nailer 2x4 to both rafters ending one purlin on the rafter and the other on the nailer but this is a pergola, and appearance is a consideration. Thanks again.

What if you're building a 24x 35 rv cover how many do i need

It can be anything up to about 18-20" but the longer you go with overhang the more support you will need to keep the gable ends from drooping.

Joints should be staggered but no need to have long purlins. This roof is 24' long, and I've built pole barns up to 80' long, so inevitably it involves stitching the roof together with shorter pieces of lumber.

@@Lumber_Jack alrighty.. appreciate the quick reply

This is all rough sawn yellow pine, not commercial lumber. It has no treatment.

Only at the gable overhang if needed, but typically there is some other type of gable framing use to brace or box-in the gable ends.

Alright! Thanks for clarifying that!

Not in this application.

@Lumber_Jack I'm building a living space with a gambrel roof. Would you in that case?

@@bandmasterjf If it's a conditioned space then you need a plan to deal with condensation. That will be dependent on the type of insulation, the roof structure, the local climate, etc. There is a lot to it. The current state of the art is sheathing, roof underlayment, then the metal. It has to be a system that works together.

If using screws, it's important to use structural screws, like these GRK screws I use for framing. Do not use deck screws, as they will crack as you noted -- they are only meant to hold wood down and don't do very well with shear loads or bending loads. I have also used glue-coated ring shank nails shot in with a nail gun which hold roof purlins very well.

@@Lumber_Jack Thanks for clearing that up for me. I been out the game for awhile. Excellent information. I will be purchasing some of those screws soon.

Your foundation would have to be very poor for the building to move enough to break screws. I’ve seen many roofs blown off because the nails just pull out though

This way is fairly standard practice, but your neighbor is 100% right. When there is a higher risk of wind uplift loads (say for an open structure, or a high-risk location), I instead join my purlins in between rafters, because I do not like concentrating fasteners from two purlins into the a rafter if there is a higher load. When doing that, an additional short block goes between the rafters, under the purlin joint. You just need to think about fastener placement to keep them spread out.