Cabin Build/Engineering Question

[Tater86]

Question, I'm having my roof truss system in my cabin converted to a vaulted ceiling and want to make sure my contractor is modifying it correctly. I'm no engineer and I'm completely loss on how to calculate how the ridge beam should be. As stated, I currently have a truss system with a 4:12 pitch. Dimensions of the area are 8' walls, 20' wide cabin, 26' length and current truss system is 24" OC. The plan is to install a ridge beam using (3) 2x10s, nailed, glued and seam staggered and then sister 2x8's along the current 2x4's running from the top of the walls to the peak (about 12'). Is that sufficient to prevent sagging and the walls pushing out? Located in south central PA, snow load is typically no worse than 4-8 inches at most with an occasional once in a blue moon blizzard at 16"

 

[ben h]

Go get your snow load here: ASCE Hazard Tool
draw a plan view showing the beam and posts and attach some photos (inside and out).

 

[Tater86]

Per that report the snow load is 67LB/SF. Not sure how to draw a plan except on paper, but total cabin is 20x36 and 20x10 of it will still have the trust system. From the door you see in the outside picture to the left with the guy on the ladder will still have trusses set up.

 

[TaperPin]

Question, I'm having my roof truss system in my cabin converted to a vaulted ceiling and want to make sure my contractor is modifying it correctly. I'm no engineer and I'm completely loss on how to calculate how the ridge beam should be. As stated, I currently have a truss system with a 4:12 pitch. Dimensions of the area are 8' walls, 20' wide cabin, 26' length and current truss system is 24" OC. The plan is to install a ridge beam using (3) 2x10s, nailed, glued and seam staggered and then sister 2x8's along the current 2x4's running from the top of the walls to the peak (about 12'). Is that sufficient to prevent sagging and the walls pushing out? Located in south central PA, snow load is typically no worse than 4-8 inches at most with an occasional once in a blue moon blizzard at 16"

No way are three 2x10s with staggered joints going to span 26’ as a load bearing ridge beam - they will sag under their own weight with no snow load. The 2x8s will span 10’ just fine.

Who ever you hired doesn’t know what they’re doing, or they are planning on having horizontal collar ties 1/3 up from the plate, which isn’t much of a vault for that much work

 

[displacedtexan]

No way are three 2x10s with staggered joints going to span 26’ as a load bearing ridge beam - they will sag under their own weight with no snow load. The 2x8s will span 10’ just fine.

Who ever you hired doesn’t know what they’re doing, or they are planning on having horizontal collar ties 1/3 down from the ridge, which isn’t much of a vault for that much work

Exactly.

Especially the last part.

Trusses are an engineered product, I don't like modifying them without a drawing from an engineer.

To start with, a solid engineered lumber beam...

 

[TaperPin]

This chart doesn’t go to 26’, but shows how thick ridge beams get quickly - and these are one piece - laminated multi piece with staggered ends anrent even a thing.

Ridge Beam Sizing Calculator

Use the tools on this page to select your loading, house width, and material to calculate the appropriate size of Weyerhaeuser TimberStrand or Parallam ridge beam.

www.weyerhaeuser.com

 

[Tater86]

No way are three 2x10s with staggered joints going to span 26’ as a load bearing ridge beam - they will sag under their own weight with no snow load. The 2x8s will span 10’ just fine.

Who ever you hired doesn’t know what they’re doing, or they are planning on having horizontal collar ties 1/3 down from the ridge, which isn’t much of a vault for that much work

Ok, now I'm scared to go with any contractor. All of them were planning to place a ridge beam and add 2x8 rafters along side the current truss system. That was at least 4 different contractors that quoted this way.

 

[Tater86]

This chart doesn’t go to 26’, but shows how thick ridge beams get quickly - and these are one piece - laminated multi piece with staggered ends anrent even a thing.

Ridge Beam Sizing Calculator

Use the tools on this page to select your loading, house width, and material to calculate the appropriate size of Weyerhaeuser TimberStrand or Parallam ridge beam.

www.weyerhaeuser.com

I tried using that calculator but it doesn't go down to a 20' width calculator either.

 

[Yellowknife]

Also, I'm confused how a ridge beam goes into place without totally demo'ing the trusses. Lots of questions with this plan.

Usually if using a structural ridge beam, you need to have supports at regular intervals. Add a couple of mid-span posts, and you could have it, but they would need to have a load path all the way to the ground. If you have center posts under that floor, it could work.

If you use a non-structural ridge BOARD, you can do this plan, but you need to tie the walls together with rafter ties every 4' or so to prevent spread and sag. Or maybe they are planning on leaving the bottom cord of the trusses in place?

Either way, a 2x10 laid up beam is not going to carry any weight on those spans. Either shorten the span with posts, or make it a non-structural ridge board and include exposed rafter ties.



I've spanned 26' before, but I used a massive 36"+ log to do it and did some engineering analysis to make sure it would work (it did). Had to use a crane to get it up there. We also have a small cabin with a 20' free span, but it also uses a single log and only supports 8' of rafter on each side.

 

[Tater86]

Also, I'm confused how a ridge beam goes into place without totally demo'ing the trusses. Lots of questions with this plan.

Usually if using a structural ridge beam, you need to have supports at regular intervals. Add a couple of mid-span posts, and you could have it, but they would need to have a load path all the way to the ground. If you have center posts under that floor, it could work.

If you use a non-structural ridge BOARD, you can do this plan, but you need to tie the walls together with rafter ties every 4' or so to prevent spread and sag. Or maybe they are planning on leaving the bottom cord of the trusses in place?

Either way, a 2x10 laid up beam is not going to carry any weight on those spans. Either shorten the span with posts, or make it a non-structural ridge board and include exposed rafter ties.

View attachment 830378

I've spanned 26' before, but I used a massive 36"+ log to do it and did some engineering analysis to make sure it would work (it did). Had to use a crane to get it up there. We also have a small cabin with a 20' free span, but it also uses a single log and only supports 8' of rafter on each side.

All of the trusses would be removed minus the ones running along the OSB sheeting on the top running from the top wall plate to the peak.

 

[TaperPin]

Ok, now I'm scared to go with any contractor. All of them were planning to place a ridge beam and add 2x8 rafters along side the current truss system. That was at least 4 different contractors that quoted this way.

A ridge beam is fine, but it has to be sized correctly.

 

[displacedtexan]

All of the trusses would be removed minus the ones running along the OSB sheeting on the top running from the top wall plate to the peak.

So he's going to remove trusses and stick frame the vaulted portion?

 

[TaperPin]

I tried using that calculator but it doesn't go down to a 20' width calculator either.

You are correct, it’s just to show you how large structural ridge beams are with something close to your situation.

 

[Tater86]

So he's going to remove trusses and stick frame the vaulted portion?

Yes, he was going to place the ridge beam and then install 2x8 rafter along each truss. Connecting top of wall plate to the peak and then as he was going down, he was going to remove the lower truss cross members. Only portion of the truss that would stay in tack in the one running along the bottom of the sheeting to the peak.

 

[Yellowknife]

Assuming your 67 lb/sq ft snow load is right (note, this is a very high snow load), you calculate your ridge beam loading as follows:

Each of the two walls carries 1/4 of the roof load, and the ridge beam would carry 1/2. That means that each linear foot of ridge beam span is loaded with 10 sq ft. 10 sq ft x 670 lb/sq ft = 670 lbs. Then add the dead load of the sheathing and roof materials Call that 15 lbs/sq ft, or an additional 150 lbs per ft of beam. A span table will then tell you what can carry that combined load of 820 lbs

About the only option to carry a load 26 ft is a deep (20"+) glu-lam. But there are some options if you can put a post or two in the middle. For instance, an off the shelf 3.5x14" glu-lam is good for that loading at a 14 ft span.

https://rosboro.com/wp-content/uploads/2017/09/X450-18.pdf

An example only, because glu-lam load capacity will differ by manufacture.

You should be able to get a more accurate local snow load for your area. Most towns have it listed in their construction codes, or you might be able to look it up in the paperwork that came with the trusses. And once you figure your loading, the lumber yard can tell you what glu-lams would work at a given span.

As another note, if using a ridge beam you need a clear load path to the ground at both ends as well. That means you need to add framing to the gable wall to carry it to the floor, and then there needs to be a post under the floor to the ground. The same for the interior wall. It needs to be supported by framing down to the ground.

 

[Rbehrens26]

A portion of Pennsylvania requires a case study to determine the snow loading criteria. I assume the 67 psf snow load you were able to find was based on ASCE 7-22, which has not been implemented into many codes and is considerably more complicated than previous versions. Without knowing exactly where the cabin is and assuming it is not within a case study area of Pennsylvania, I ran the beam quickly (and with a few assumptions) using ASCE 7-16. Assumed ground snow load = 35 psf, sloped roof snow load = 30 psf (conservative)

The 26' ridge beam would be a 3 Ply - 1.75x16 LVL 2.0E. Each end of the ridge beam needs to be supported by solid stud bearing to grade. The solid studs should be braced at the truss bearing elevation in both directions to prevent buckeling. Good practice would ensure that a footing is placed under solid studs to prevent settlement or concrete cracking (most notably at the interior location where the column would otherwise likely land on the slab). That said, a reinforced 4" concrete slab will likely be enough to prevent settlement/bearing issues. No promises on cracking.

 

[Tater86]

A portion of Pennsylvania requires a case study to determine the snow loading criteria. I assume the 67 psf snow load you were able to find was based on ASCE 7-22, which has not been implemented into many codes and is considerably more complicated than previous versions. Without knowing exactly where the cabin is and assuming it is not within a case study area of Pennsylvania, I ran the beam quickly (and with a few assumptions) using ASCE 7-16. Assumed ground snow load = 35 psf, sloped roof snow load = 30 psf (conservative)

The 26' ridge beam would be a 3 Ply - 1.75x16 LVL 2.0E. Each end of the ridge beam needs to be supported by solid stud bearing to grade. The solid studs should be braced at the truss bearing elevation in both directions to prevent buckeling. Good practice would ensure that a footing is placed under solid studs to prevent settlement or concrete cracking (most notably at the interior location where the column would otherwise likely land on the slab). That said, a reinforced 4" concrete slab will likely be enough to prevent settlement/bearing issues. No promises on cracking.

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?

 

[TaperPin]

Per that report the snow load is 67LB/SF.

I felt bad leaving you without a better answer so here’s a better solution.

Other beam tables show spans based on weight per linear foot that the beam will hold.

On a roof that’s 20’ wide, half of the roof weight is supported by the walls, so a section 1’ X 10’ is supported by every foot of the beam.

With a snow load of 67 lbs per sqft, the beam has to support 670 lbs per linear feet. In addition, the weight of roofing, plywood, 2x8s, insulation, and Sheetrock are 15 to 20 lbs per sqft. Adding that 200 lbs per linear foot to the 670 lbs per linear foot, is 870lbs per linear foot.

The longest glulam beam chart I can find only goes to 24’, but lumber supplier in your area probably has a longer chart. The chart at 24’ shows 6-3/4”x16-1/2” will support 970 lb per linear foot, or a 5-1/8”x19-1/2” supports 1000 lbs per linear foot.

The first weighs over 600 lbs and the second is over 700 lbs, so you need a boom truck or all terrain forklift. A typical 6,000 lb forklift will have a chart similar to the one below - extended out and up far enough to set the beam the capacity is in the 1,400 lb range so it would lift it ok IF there’s room for it to maneuver.

 

[TaperPin]

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?

I misspoke about collar ties 1/3 down. Collar ties do go in the top 1/3, but for use to raise the ceiling, the proper term is rafter tie and it can only be 1/3 UP from plate height.

 

[TaperPin]

With 30 lb snow load the beam will have 300 lbs of snow plus 200 lbs of roof weight per linear foot on the beam.

We’re off the chart 2’ so going up a notch past 500 lb minimum it looks like you would be 5-1/8 x 16–1/2” or 6-3/4” X
15”

This chart is for Doug fir - if your supplier uses a different wood species, the chart will be different.