Cabin Build/Engineering Question

[Yellowknife]

I just pulled up the 7-16 code and it says 30psf. I'm in south central, pa. If the builder uses collar ties 1/3 of the way down from the peak, does that eliminate the need for the solid stud down to grade?

Taperpins last posts are on track. You can't use a collar tie, as that serves a different purpose. Rafter Ties are what you would need. More on Collar Ties v. Rafter Ties.

Collar Ties vs. Rafter Ties

Learn about roof collar ties, rafter ties, ridge beams, bending moments, tension and compression.

www.nachi.org

If you want to get rid of both the beam span issues AND the load path issues, a rafter tie every 48" will do it. They will be exposed below the finished ceiling and need to finished appropriately.

 

[TaperPin]

Another option are scissor trusses. The inside pitch is about half the exterior pitch, but you don’t need a load bearing ridge. They can be built with a raised heal that gives you more room for insulation, which adds to the cost, but makes insulating a lot easier unless you’re going to spray foam everything, which is the best practice.

 

[Rbehrens26]

Scissor trusses are likely your best option.

Rafter or collar ties would eliminate the need for a column in the middle of the floor by restraining the roof framing from pushing "out" on the wall when loaded. However, rafter ties are generally pretty tough to get the numbers to run with dimensional lumber.

The rafters themselves generally fail in flexure due to a large point load that is observed from the tie. Also, adequately transferring the force from the rafter to the tie is generally difficult to accomplish with wood and screws. The entire assembly is subject to deflect much more than a truss as well due to the reduced depth of the overall assembly.

 

[Yellowknife]

Switching to scissor trusses would basically require demoing the whole existing roof system. And really need to be at least 6:12 outside pitch to make any sense, as a 4:12 doesn't leave room from much interior pitch. It's certainly the preferred way of getting a cathedral ceiling though.

 

[cnelk]

Ante up and hire a structural engineering and get stamped truss drawings.

Protect your investment

 

[woods89]

I do construction and remodeling for a living, with just under 20 years of experience.

No way this is going to work with dimensional lumber without ties in the bottom third, which in my opinion kind of defeats the purpose. This will take a substantial laminated or steel beam for a ridge. And that's only the start, both ends of the beam need direct load paths to foundation walls or footings.

Either there is a miscommunication going on here or the caliber of contractor that you are talking to is not very high. Turning a truss system into a cathedral ceiling with a structural ridge is a project you need the A team for.

 

[Rotnguns]

Excellent comments and advice here. To add, wind loading should be considered as well. Suggest hiring a structural engineer, who can properly assess loads and apply pertinent building codes with design factors. The structural engineer will have access to current analysis and design software applications, including truss designs that you or the contractor may not have considered. One thing more expensive than hiring a structural engineer is not hiring one.

 

[Mtndawger]

Do it right. Spend the money and consult with a structural engineer. That's what I would recommend if someone came to me with a project like this. If I was going to do it I would insist on that or I wouldn't do it. You are going to need a substantially bigger solid beam to carry that 67psf snow load through posts to substantial footings and who knows what your soils are like. Sistering solid lumber onto the top cords along with collar ties could get you there but you will be losing a substantial amount of head room to get it.

 

[ben h]

I'm pretty impressed with the comments here. This isn't a little beam and you should definitely enlist an engineer to help size this. I did not think I'd learn on Rokslide that the ASCE 7 changed load combinations in the 2022 edition. Here's what I'm coming up with for estimating purposes using ASCE 7-16, 30psf snow, 15psf dead load. The end reactions are about 9,000lbs so you need columns and footings too.

I agree with others that collar ties aren't a good idea here and scissor trusses won't get you much of a "cathedral" ceiling in this application.

 

[Tater86]

Appreciate everyone's feedback and advice in my situation. I've sent off a message to my contractor and the other ones to get some feedback and to provide a detail mock up of the design. I'll also reach out to a local structural engineer as well to evaluate the design.

I did have one contractor who wanted to take the roof off and build from scratch. I might have to reach back out to him and discuss as I didn't want to undertake that path because of how we build additions onto the cabin. That's a whole different issue, haha. We just tucked those roof lines under the existing room and used a 1:12 pitch on those two.

 

[Rotnguns]

Appreciate everyone's feedback and advice in my situation. I've sent off a message to my contractor and the other ones to get some feedback and to provide a detail mock up of the design. I'll also reach out to a local structural engineer as well to evaluate the design.

I did have one contractor who wanted to take the roof off and build from scratch. I might have to reach back out to him and discuss as I didn't want to undertake that path because of how we build additions onto the cabin. That's a whole different issue, haha. We just tucked those roof lines under the existing room and used a 1:12 pitch on those two.

Good luck on your adventure! Perhaps more complexity and detail than first planned and envisioned, but a great learning experience and you'll end up with a fine cabin that you can be proud of!

 

[Tater86]

Good luck on your adventure! Perhaps more complexity and detail than first planned and envisioned, but a great learning experience and you'll end up with a fine cabin that you can be proud of!

Thanks! I built the cabin from the ground up with my father. He has now passed and was always the go to build guy for it so I'm trying to do right by him for the build. I've been getting tired of looking at open trusses for the last 7 years so I wanted to switch it up

 

[Rotnguns]

I'm pretty impressed with the comments here. This isn't a little beam and you should definitely enlist an engineer to help size this. I did not think I'd learn on Rokslide that the ASCE 7 changed load combinations in the 2022 edition. Here's what I'm coming up with for estimating purposes using ASCE 7-16, 30psf snow, 15psf dead load. The end reactions are about 9,000lbs so you need columns and footings too.

I agree with others that collar ties aren't a good idea here and scissor trusses won't get you much of a "cathedral" ceiling in this application.


View attachment 830614

Thanks for this interesting post! I note a maximum deflection (presumably at midspan) of about 1.2 inches, and that the design is judged to be "ok" based upon design stresses, but I note that the beam weight was not included in the calculation. Also, curious about the cross-section used to calculate the area moment of inertia. Did you use the cross-sectional area of two beams joined together, as the OP mentioned? I see a "section used" number but not sure what it is or what the units are. I'd like to use the elastic modullii and density provided on my MD SOLIDS app to see if I get similar values.

 

[Weldor]

I'm pretty sure here where I live , you have to use a engineered truss. Saves on bad construction and safety. When we did our addition years back, all the scissor trusses had to be engineered and come from a truss manufacturer. No home made ridge and rafters. County won't allow it.

 

[Tater86]

Just curious as I was looking at truss designs this morning. Would something like this work? Not sure how much height that would provide, but just looking.

 

[Mtndawger]

Appreciate everyone's feedback and advice in my situation. I've sent off a message to my contractor and the other ones to get some feedback and to provide a detail mock up of the design. I'll also reach out to a local structural engineer as well to evaluate the design.

I did have one contractor who wanted to take the roof off and build from scratch. I might have to reach back out to him and discuss as I didn't want to undertake that path because of how we build additions onto the cabin. That's a whole different issue, haha. We just tucked those roof lines under the existing room and used a 1:12 pitch on those two.

Taking the roof off and rebuilding from scratch would be the preferred option IME. It will result in higher overall cost but maybe not as much as you would think on first take. It would minimize the compromises with the retrofit approach, which typically end up costing more over time rather than just biting the bullet and doing it "right" from the get go. You would get to do some modifications like raise the plate height to accommodate those additions with the 1/12 pitch or steepen the pitch to get more cathedral effect. This also would allow more Rvalue in your insulation, particularly on your rafter/truss heel area where the current minimal heel height will likely result in ice damming. Wouldn't be a bad idea to discuss with that contractor who suggested it as he may have similar experience/other ideas to share. Experience is invaluable.

 

[TaperPin]

Just curious as I was looking at truss designs this morning. Would something like this work? Not sure how much height that would provide, but just looking.

View attachment 830742

It’s not a recognized framing style in the code books, so an engineer would have to approve it, which would only happen if it were way over built and more expensive than scissor trusses.

As something guys throw together in areas not requiring building inspections, the methods guys in the field would assemble it with would never last - simply nailing it together is very very weak compared to the plates used in a truss plant, and the roof will sag guaranteed.

 

[TaperPin]

Is the roof finished with shingles and all?

 

[Tater86]

Is the roof finished with shingles and all?

Finished with OSB sheeting, tar paper, purlins and metal roof panels

 

[Geewhiz]

If it were me I'd put a post (or 2) in the middle, set a ridge beam, run rafters to the ridge, and call it good.

Actually if it were me I'd just leave it as is.