Cabin Build/Engineering Question

TaperPin

WKR
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Jul 12, 2023
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Finished with OSB sheeting, tar paper, purlins and metal roof panels
I have a nephew with a similar situation - when we run the numbers on modifying the existing space, it’s hard to get things to pencil out. Rather than spend a lot to remove and rebuild what you have, it might be nice to add on a great room with a higher vaulted ceiling. The additional sqft also adds to the appraised value, whereas vaulting an existing ceiling won’t change the valuation at all.
 

Boonie327

FNG
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Dec 27, 2019
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Location
WV
Finished with OSB sheeting, tar paper, purlins and metal roof panels
I don’t think the juice is worth the squeeze in this case when you look at it objectively. There are way better ways to spend the money that this would cost and the fact that none of the 4 contractors you spoke to didn’t say that is scary. There’s been some good guidance on here so far, but it’s not just the best way to spend your money.
 
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Tater86

Lil-Rokslider
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Aug 3, 2017
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239
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Pennsylvania
Just curious. You said you’re tired of seeing open trusses. There’s no ceiling right now?
Nope, just looks like this. We never finished it off because we weren't sure how we wanted to do the ceiling. Yeah, I figured there were better ways to spend the money. Stayed in one too many cabins that had cathedral ceilings and kind of pushed me to look into getting it done on my place. Estimate I have now to do the work and finish it with Tongue and Groove is $20K.
 

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eshunt

Lil-Rokslider
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May 23, 2012
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NV
I'm a PE registered in CA, so my information is different than PA. I work mostly on single family residential structural projects in mountainous regions with snow loads up to 500 psf.

What are your goals with removing the trusses?

I don't design trusses, I leave that up to the truss proffesionals. So if I was to change even 1 small part of a truss I would consult a truss engineer.

Based on what you have written above, in order to remove the trusses (this isn't just an alteration, but a complete removal) you would need to have an analysis done on the complete roof system. A ridge beam spanning 26' with even a small snow load of 67psf isn't a small beam as many people on here have stated. Is this 67 psf the ground snow load? Or the roof design load? You have to add in the roof component dead load as well for design.

Once the ridge beam size is determined, you would also have to adust the size of the members supporting the roof member, being either within the wall or intermediate members. With trussess, you don't have columns in your exterior wall currently, and there would not be any sort of spread footing. So, those columns would need to have appropiately sized footings as well. What is your allowable soil bearing pressure? You would have to underpin the exterior continuos footing to allow for a spread footing under each column supporting the ridge beam. And also have the appropriate hold downs installed as well. As a retrofit, this can get very expensive.

You would also have to install either collar ties or some form of mechanical resistance to horizontal forces. Otherwise the walls would push outward and the roof could collapse. With collar ties/rafter ties you really would only gain a couple feet of ceiling height in the center of the room at most. Is this worth the expense??

Based on design of what you are requiring there could be many other issues come up as well. It would be determined based upon a complete review of the property/building. I would not trust a "contractor" that is not a structural engineer to give you advice.

Just as a reference, if you are looking to span all 26', you would need GLB 5-1/2"x24" to make it work. And then also include the added footing at either end. And the collar ties to keep your roof from spreading. So, not a small project by any means at all.

My advice would be to enclose the existing ceiling with trusses unless you have a need for a vaulted ceiling.
 
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Tater86

Lil-Rokslider
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Aug 3, 2017
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239
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Pennsylvania
So, I just got off the phone with a local structural engineer. He stated I would need a 3PLY 1.75"x18" LVL @ 2.1. The weight need to be support on each end of the beam would be 6207LBs.

I'm working on sending him blueprint plans and additional information for him to look at it a little further and provide stamped drawings, but the above information he provided.

Question, if I go the rafter tie pathway, would that still gain me a little head space, based on what you all said I think it would give me 9FT ceilings in the center. Do rafter ties need to be every 24" OC or can they be 48"+ OC? I'm assuming putting in rafter ties would be cheaper?

I appreciate everyone's help. Personally I just like to gather as much info as I can regardless of who I talk to, engineer or contractor, before I step into any major project. I just like to talk intelligently with these folks, so thank you all!
 

woods89

WKR
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Sep 3, 2014
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Southern MO Ozarks
There’s really no need for OSB and felt paper if you’re installing metal roofing over the purlins.
Does this not cause condensation issues in your area? I know Colorado is a lot different than where I'm from in Southern MO (my hunting partner runs a spray foam business in the Fort Collins area), but here metal panels over purlins will give you a lot of dripping in certain conditions.

Standard minimum practice here for us in anything more than an open air building ( not enclosed on all 4 sides) is some sort of thin rolled insulation (double bubble or similar) between panels and purlins. This generally takes care of the problem. If it's going to be conditioned space I personally won't hardly do purlins, I will push hard for solid sheeting. I am in the land of unconditioned attics, though, which is a different story than when the insulation is generally closed cell foam on the roof deck. I'd still rather have foam sprayed against solid sheeting than the panels themselves.

I'm guessing PA where the op is from is a more humid climate, similar to ours, which makes me think they probably have to mitigate for condensation as well.

Also worth noting, although probably not an issue with the OP's cabin, is that every concealed fastener metal panel that I am familiar with requires solid sheeting if you want any warranty coverage at all.
 

cnelk

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Colorado
Does this not cause condensation?

Standard minimum practice here for us in anything more than an open air building ( not enclosed on all 4 sides) is some sort of thin rolled insulation (double bubble or similar) between panels and purlins.

Not having insulation under the metal roof panels will definitely have condensation.
And there are several ways to do install insulation like you mentioned.

But there’s still no need for OSB and felt paper under the metal roofing. Those aren’t very good insulating material - if at all
 

woods89

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Southern MO Ozarks
Not having insulation under the metal roof panels will definitely have condensation.
And there are several ways to do install insulation like you mentioned.

But there’s still no need for OSB and felt paper under the metal roofing. Those aren’t very good insulating material - if at all
Agreed that they are not insulative, however, if we have to mess with a rolled insulation I'd just as soon sheet it with Zip and be dried in. It seems like having that barrier generally is just enough to isolate the panel tempwise. I also like solid sheeting when there are complicated flashing details or valleys involved, as appropriate blocking for trim can get pretty extensive. And it should go without saying that sheeting or rolled insulation is only supplementing proper attic insulation, whether loose fill above the ceiling or foam encapsulating the attic.

I will agree that if all factors are considered purlins with exposed fastener metal directly installed can work fine, but I've seen it done improperly a lot. Perhaps that sours my opinion.

More than one way to skin a cat.....
 
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Tater86

Lil-Rokslider
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239
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Pennsylvania
I have a nephew with a similar situation - when we run the numbers on modifying the existing space, it’s hard to get things to pencil out. Rather than spend a lot to remove and rebuild what you have, it might be nice to add on a great room with a higher vaulted ceiling. The additional sqft also adds to the appraised value, whereas vaulting an existing ceiling won’t change the valuation at all.
Curious, just another option. Would engineered Timber Trusses every so feet OC be another solution instead of going with a ridge beam?
 

ben h

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Jun 17, 2012
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SLC, UT
But there’s still no need for OSB and felt paper under the metal roofing. Those aren’t very good insulating material - if at all
Roof sheathing with OSB usually serves a few purposes: 1) something to walk on and secure roofing 2) it is what provides the resistance from horizontal forces (wind/seismic). If you eliminate it, you need another way to handle the horizontal forces, which probably could be done with standing seam metal but you'd need careful detailing because the cladding would become a structural element.
 

cnelk

WKR
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Mar 1, 2012
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Colorado
Roof sheathing with OSB usually serves a few purposes: 1) something to walk on and secure roofing

Thats funny.
45 years ago I started walking on purlins dragging up roof metal sheets up to 18' long, sometimes longer.
Did it that way for many years without any OSB.
Sometimes we'd install 4' wide insulation on the purlins before bringing up the 3' wide sheets

I guess times have changed
 
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