Structural Question From a Mechanical Guy

[vc66]

Hi All-

This is probably a simple question for you structural guys, but I want to make sure I'm correct in my thinking.

I have a house, and I'd like to vault the ceilings. It's a one-story, with only attic space on the second floor. Attached is a rough schematic of the layout. It's an older house, wood-frame construction (pine). All the joists are 2x8's, 16" on center. Based on the layout attached, I basically just want to take out the ceiling. Foundation is good. No cracks in any of the joists that I can see. Based on the static load calcs I did, it looks like it should be fine, but I wasn't sure if there was something I was missing.

I'm going to hire a Structural Engineer to help me, but I want to make sure it's feasible (without much extra work) in the first place.

If I'm missing any important info, please let me know.

Thanks for all the help, in advance.

V

 

[vc66]

So they're both mounted the same way?

The cross-section of the ridge beam is just thicker?

V

 

[jike]

It is assumed that ridge board doesn't really do anything structurally. It is only for construction. It is too difficult to fasten the tail ends of two rafters together end to end, therefore, if you provide a (ridge) board perpendicular to the rafters, each rafter can be fastened to it.

It is assumed that the rafter forces pass completely thru the ridge board.

 

[vc66]

Ahhhh, I see. It's getting clearer now.

The ridge board still does see loads though, doesn't it? It has to. It's just not MADE to take any loads. It's just for ease of construction. Am I correct in my line of thinking?

V

 

[Lion06]

The only load it sees is compression perpendicular to the grain - no bending.

 

[COEngineeer]

the ridge board is just there as blocking/nailer.. Your house is old school construction. the tie, the rafter, and the ceiling joist are forming a truss.

What you want is feasible but you will need to add ridge beam. Hire a structural engineer so they can size that ridge beam and he will figure out how to transfer the load to the foundation. If you dont have a window in the attic, they may have to cut your exterior wall to bring the beam into the attic. Good luck...

Never, but never question engineer's judgement

 

[BAretired]

I don't think the project is feasible without a fair bit of work, but the attachment indicates one possibility using scissor trusses at 4'-0" centers.

BA

 

[msquared48]

The purpose of collar ties is to avoid having to use a long, deep ridge beam. At four feet, considering 2X12 rafters for insulation, I would expect that a 2X10 or 12 "ridge beam" would work very well. At eight feet, probably a 4X12. Have the structural engineer check it out.

BA's solution is good if you want to see a sloped ceiling. Otherwise, placing the collar tie at the intersection of the scissors gives another ceilinmg look.

Mike McCann
MMC Engineering

 

[TheParp]

Hi,
New to forum.
I know a little about the subject and I would definitely like to join to your conversation.
The only thing Is that I'm french and dpn't really understand all the english terms your using. Can you help me?

For what I understand, BA got a big point with his drawing, the only real expressing way of an engineer And his suggestion is, I think, the only way to prevent your walls from going out from the horizontal forces created by the roof and in the same time, having a "cathedral" ceiling.

Ok now for translation, what is:
- rafter
- collar ties

Nice project vc66
Thanks to those who help me with the words

 

[SteelPE]

Another thing to keep in mind is the required insulation for the ceiling and ventilation requirements for the roof.

I’m not sure what the requirements are on long island but I needed an R30 in my ceiling in New England.

Air ventilation is provided by soffit and ridge vents. These vents are connected to each other by Styrofoam spacers, which keep the insulation away from the underside of the sheathing. I’m sure there are other ways to provide this ventilation, but this is the way that I am most familiar with.

I suspect that your 2x8 may not be deep enough to provide the required insulation/ventilation requirements for your area.

TheParp, hopefully the attached picture can help you out.

 

[vc66]

OK, so if I need R30, I'd need *at least* 2x10's to accommodate the insulation and venting. I'm _almost_ positive that there are no ridge vents, or soffit vents, as it's a very old house. I will definitely have to install these.

I like this structural thing. Maybe I need to go back to school.

V

 

[SteelPE]

I don’t remember how deep the vent is…. But I believe R30 fiberglass insulation is 9.5 inches thick meaning you would require the depth of a 2x12 in order to get proper ventilation and insulation. Basically in order to achieve this depth you would have to pad out your roof rafters (not hard to do… just more work you may have not thought of).

Again, there may be other ways I am not familiar with but other members of this forum may be.

 

[vc66]

Gotcha. I looked online, and I'm finding R30 to be 8" thick. So with a 2" ventilation space, I'm looking at 2 x 10's. I get your point though.

Thanks very much for everyone's help, and if there are any more opinions, please feel free to share them.

Thanks again!

V

 

[rb1957]

the scissor trusses are a good idea, and you'd probably want to reinforce the roof trusses (for the new load being added). if you needed to increase the connection between the walls (the problem is out-thrust), you could add a steel tierod ... wouldn't look really that nice (nor that ugly), and you'd still get the effect of an open ceiling. just a thought ...

 

[vandede427]

you can remove the ceiling joists and replace them with ones higher up, thereby increasing your ceiling height. but you definitely can't rely on just the collar ties at the ridge. you're walls will deflect outward (roof will sag bad, then you've got big issues).

 

[csd72]

vc66,

I hope you realise that the above is only very general advice and is no substitute for consulting an engineer who has actually seen the building.

 

[vc66]

csd72-

Are you serious?

I'm going to hire a Structural Engineer to help me, but I want to make sure it's feasible (without much extra work) in the first place.

I'm not just some hillbilly trying to jack up my ceiling. I wanted to understand everything that went into this, so I have some underlying knowledge, so that I can speak to the Engineer intelligently instead of like a damned fool. That's why I figured, what better place to ask for general advice.

V

 

[vc66]

Wow, I just read that back after I posted it, and I sound like a baby.

Sorry, no offense, csd72. Need more coffee.

V

 

[vc66]

vandede427-

Don't ceiling joists basically become collar ties once you go high enough?

I guess I'm planning more of a tray ceiling than a full vault. I wasn't planning on going straight up to the crown. Maybe 2-3 feet lower.

V

 

[BAretired]

The simple span moment from end to end is wL^2/8 where w is the load per lineal foot and L is the overall span. A ceiling joist will have a tension of wL^2/8h where h is the height from ceiling joist to ridge.

If the ceiling joist is moved upward, it becomes a collar tie. Height h decreases and the tension in the collar tie increases.

The main change is that the moment in each rafter increases substantially. The vertical reaction at each support, R = wL/2. If the horizontal distance from support to the start of the collar tie is x, then moment in the rafter becomes R*x. It is unlikely that the existing rafters are adequate to carry a very large moment.



BA

 

[BAretired]

Sorry, I should have said the moment in the rafter becomes R*x - wx^2/8.

BA