Ceiling joist support

[Gordon Gould]

ceiling joists and collar ties are two different structural members and are subject to differnt types of loads and resistances.

collar ties are generally placed 1/3 of the total height* below the ridge and help prevent the walls/beams from being forced outward.

ceiling joists are basically non structural and are for the most part deadwood to support the ceiling although their tensil strength does add to the overall strength of the structure.* heigth from top of wall/beam to top of ridge.

I would strongly disagree with this statement for a building in VT where you need a 40 or 50 lb/sq ft roof system. A building with standard rafters and no joists to hold the plates together would split right apart. Your right that collar ties help but they won't do the job. The purpose of a collar tie is to hold the tops of the rafters together in high winds. Needed on steeper roofs because you get an airplane wing lift effect on the roof which tries to pull the tops of the rafters away from the ridge pole.

 

[Fixemall]

Do yourself a big favor and frame the walls in 2x6 studs and use a triple LVL to span the entire length then you can use shorter dimensional lumber for ceiling joists, I would also consider LVL for your ridge board as well.

 

[massey990]

I would strongly disagree with this statement for a building in VT where you need a 40 or 50 lb/sq ft roof system. A building with standard rafters and no joists to hold the plates together would split right apart. Your right that collar ties help but they won't do the job. The purpose of a collar tie is to hold the tops of the rafters together in high winds. Needed on steeper roofs because you get an airplane wing lift effect on the roof which tries to pull the tops of the rafters away from the ridge pole.

I understand the live load reqiuirements in heavy snow zones, I was responding to the OP's original questions and terminology.

Unless you are applying some tornadic activity where drastic pressure differences and shear are common the idea that there is any type of "airplane wing lift" on a roof structure is ludicrous. Wind does indeed create lift on any exposed structure but it's not the same as air passing over a wing.

Not sure what you mean by a ridge pole as the OP stated there would be no interior supports. As far as a ridge board, they have very little structural effect, they do add some longitudinal tensile strength, but they are mostly used to make common rafter framing easier, there are many "stuck" roofs that have no ridge boards, the decking/sheathing/purlins serve the same purpose structurally.

edit:
One other purpose of a ridge board is to extend the ridge for the gable etc. eaves, allows a barge type rafter to fastened by something other than just hung from the sheathing/decking.

 

[KennyG]

Oh boy, these threads are fun. I think I'll throw in my 2 cents, get a beer and sit back.

My recollection of the building code is that collar ties, rafter ties and ceiling joists are three distinctly different things. Collar ties, as noted are in the top 1/3 of the peak, prevent uplift splitting of the roof, and are not intended to prevent wall spreading. rafter ties are usually about 1/2 way up or less and are qualified to prevent spreading of the walls. Ceiling joists can function as rafter ties if required. Depending on the loading conditions, rafter ties can be allowed as much as 4 foot spacing.

If proper ties are used, the ridge pole sizing is immaterial, since it only stabilizes the rafter spacing. Using a large beam on the ridge will allow you to do away with ties completely if you can prevent wall spreading. I know a structural engineer who put a peaked ceiling on a 30 x 30 room by designing a steel belt to wrap the top of the walls.

Given the problem originally stated, I still think rafter ties and no ceiling joists are way to go. Maybe collar ties are now required by some codes, but they didn't used to be. I've built some good sized roofs with just rafter ties on every other rafter and they passed inspection.

 

[massey990]

Collar ties, as noted are in the top 1/3 of the peak, prevent uplift splitting of the roof

What type of loads or forces create uplift at that point on a roof? Other than an overhang at a gable end and at the eaves, where, how can there be lift?

edit:

should have said "enough" lift to cause damage.

 

[KennyG]

Wind loading. High winds will take a roof off intact unless it's secured with tie down straps. I've seen a properly braced roof (but not strapped down to walls) carried 100 feet and left in one piece. However, without ties of some sort, the wind will lift it and snap it in two along the ridge.

If you see a house with a vaulted ceiling that has a flat section at the top, it has collar ties. If it goes to a peak, it should have straps holding the rafters to the walls (not just nailed) and a larger ridge beam to allow a stronger attachment of the rafters at the ridge.

 

[massey990]

Not a reply to any particular previous post but it has been shown in hurricane zones that full hip roofs with little or no overhang are practically impervious to the same force winds that can decimate gable or similar roof structures.

acknowledge previous mention of hip roof strength by another member.

 

[mjncad]

My first choice for such a building is pre-engineered trusses. TJI's can be used as rafters per their web site; but they aren't rated for direct weather exposure as I recall. Any exposed to the elements portion of the TJI needs to be enclosed somehow.

Would a clear-span steel building work for your application?

 

[CurlyDave]

Another vote for trusses.

Every time I have compared stick-built vs. trusses, the trusses are less expensive than just buying the lumber to stick build. Not to mention the labor savings.

As has been mentioned, a scissors truss system will give you a large clear span and a vaulted ceiling.

 

[johnrex62]

My dad told me a story about two homebuilders back in the 60's that were in disagreement over the stregnth of trusses vs stick built roof systems. They settled it with a bet. They would each build a house with their preferred system (both were big volume builders) and they would test the roof by setting a car on the top of each house to see if it would hold.

The truss system survived while the stick built house required some repairs and some auto bodywork after a few hours.

A volume buyer can get materials and a good crew can definitely build a stick roof cheaper than pre-built trusses, but an engineered truss is proven stronger.