Vaulted ceiling without a ridge

[XR250]

I have an open structure that is 10 ft. wide and is to be a vaulted gable roof. There is no place to support a ridge at each end. The pitch is 3:12. It looks like I could cut the roof shape out of a 24" LVL for each rafter pair.
This may not be the cheapest solution but it is fast and easy. Does anyone have any objections to this?
In the past, I have designed 4 ft. wide, 3/4" plywood gussets for the rafters - which is also an option.

 

[phamENG]

I've done that before. LVLs aren't dependent on grain and knots, so there's no issue with the rip.

I prefer it to the gussets.

 

[Pmtottawa]

Would a collar tie Hugh up be an option

 

[XR250]

The customer does not want that look.

 

[Pmtottawa]

Gotcha...
Plywood gusset an option.
Loads shouldn't be that high with the small span.
Or custom metal plate.

 

[JStructsteel]

How much thrust is there at 10' wide? Can you design a collar beam at the top of the wall?

 

[HouseBoy]

Might not matter but... How long are the sidewalls? Could you resist the thrust with the top plates (and tie the ends together)?

Otherwise, 24" LVL x 20 ft long cost $432 at Menards. 24 ft long costs $520 (if you need the extra length for the bearing).
(I use their website as a price check when I want to have some idea of the cost impact of my choices.)

I would not object to your ideas of either LVL or big plywood gussets.

 

[GeorgeTheCivilEngineer]

Would you be able to post a sketch? Esp of the gussets?

The wall plate could be oversized and (if available) supported at the ends. That would take the horizontal thrust rather than the wall below.

 

[phamENG]

HouseBoy, George - how often does that actually work for you? I've never been able to get a top plate to span more than about 4 feet, and over sizing it has never been an acceptable option.

 

[HouseBoy]

pham,
Not very common but sometimes the soffit will provide space for a "horizontal beam".
Naturally it also depends on the length of the wall. (That's why I asked about that.)
Still, one must be able to tie the ends together....

 

[XR250]

The ends are completely open up to the ridge so there is nothing for the top plates to react against.

 

[XR250]

I use their website as a price check when I want to have some idea of the cost impact of my choices

I try to be mindful of that as well. Too many engineers around here will throw LVL's at everything, even when dimensional lumber will do. On the flip side, I have contractors throwing LVL's at everything even though
I am showing dimensional lumber. WTF?

 

[gte447f]

XR250, regarding your idea of cutting the relatively shallow roof profile out of a 24" LVL to form continuous rafters, would there be any issues related to the principal stresses in these continuous rafters being inclined relative to the grain of the wood in the veneer laminations? It sounds like a small structure, so probably low stresses, but this would be something I might have some concern about.

 

[XR250]

@gte,

I'm at around 800 psi so I am not going to worry about it. The roof pitch is only 14 drg. so the angle ain't that much.

 

[gte447f]

XR250, sounds reasonable to me. Probably no big deal. I do wonder if the LVL manufacturers have any opinions or guidance on this issue (i.e., inclined stresses).

 

[phamENG]

FWIW, when I've done this I've kept the bottom of the LVL flat. The angled rip is just to get the roof slope.

 

[KootK]

I'm at around 800 psi so I am not going to worry about it. The roof pitch is only 14 drg. so the angle ain't that much.

I share gte447f's concern on this. It sets up a situation where a component of your flexural stress is, effectively, perpendicular to grain. Granted, perpendicular to grain stresses in LVL are surely less critical than they would be in lumber. At the least, it's an evaluation hassle as I'd be astonished if the manufacturers would be willing to stick their neck out to comment officially on something like this.

All that said, yeah, this probably would work at this scale. And mechanically, I suspect that it would perform better -- and more reliably -- than the gusset alternative. So if it's down to those two options and costs are comparable, I vote LVL.

My default for this situation is steel moment frame rafter things at the ends -- or intermittently -- with conventical ridge beam and rafters in between. Granted, I've never looked at one quite this small before.

 

[phamENG]

My default for this situation is steel moment frame rafter things at the ends

This was my first thought, but figured there wouldn't be much point in bringing it up considering XR originated the thread.

 

[KootK]

This was my first thought, but figured there wouldn't be much point in bringing it up considering XR originated the thread.

Yeah, I'd considered the XR factor a well. That said, I feel that the more conservative view deserves some air time for the sake of posterity if nothing else. I also didn't bring up parallel chord trusses as I know that XR and his market favor lumber and there surely isn't the available heel height for it.

 

[XR250]

I considered the steel thingy but this structure is not very long so it is likely more cost effective overall to do the LVL's