Structural Question From a Mechanical Guy
Structural Question From a Mechanical Guy
(OP)
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.
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






RE: Structural Question From a Mechanical Guy
how's the "bearing wall" in the middle of the room going to be connected to any sort of load ??
there are ways to resolve these questions, i'm sure; just as i'm sure it's not a matter of just cutting out the ceiling joists (not that i think that's what you're planning on doing).
RE: Structural Question From a Mechanical Guy
What I would be more concerned about is the performance of the roof when you remove the joists. They are likely acting as ties to keep horizontal loads off of the exterior walls. Once you remove them, all of the horizontal thrust is going to go to the tops of the exterior walls. If you have a true ridge beam (one that is adequately designed and has proper supports on each end), then this will help to minimize the thrust, but won't eliminate it.
RE: Structural Question From a Mechanical Guy
It does not look feasible unless you can come up with a way of resisting the horizontal thrust at the top of the exterior walls. Note that the weight of the roof joists causes an outward horizontal thrust. Also, wind load may cause an inward thrust (direct pressure) or outward thrust (wind suction) depending upon which way it is blowing.
RE: Structural Question From a Mechanical Guy
There's a horizontal beam across the center of the house between the living room, and the dining room/kitchen. That "bearing" wall supports that beam, so I assumed it was load bearing.
My concern was, as a couple of people said, the horizontal loads on the top of the walls.
I guess my question should be, what kind of supports are used in this situation? Obviously there are many houses with vaulted ceilings... so how do they achieve this?
V
RE: Structural Question From a Mechanical Guy
The existing beam and interior load bearing walls (Likely only support the ceiling joist load) will have to be investigated onsite to determine their purpose and design for their removal accordingly.
Depending on your location (i.e. climate)and local building codes, you will need to carefully consider insulation, vapour retarder and roof ventilation details for the new cathedral ceiling.
RE: Structural Question From a Mechanical Guy
I'm on Long Island in NY, USA so I don't believe climate is too much of an issue.
There's no HVAC that goes through this section of the roof. No wiring, or anything else like that.
Would it be possible (generally) to use big enough collar ties to react the horizontal loads?
Thanks for all the opinions so far.
V
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
Mike McCann
MMC Engineering
RE: Structural Question From a Mechanical Guy
I'm pretty sure it's a ridge beam as it's a pretty thick board, but I'll double check. Thanks.
Mike-
I was going to double up the collar ties anyway, but you make a good point about the bending of the rafters themselves.
V
RE: Structural Question From a Mechanical Guy
Man, I think I need to buy myself a AISC manual.
V
RE: Structural Question From a Mechanical Guy
Mike McCann
MMC Engineering
RE: Structural Question From a Mechanical Guy
I need an acronym manual as well, it seems.
V
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
Mike McCann
MMC Engineering
RE: Structural Question From a Mechanical Guy
V
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
I am assuming that all the thrust from the intermediate rafters goes into the top wall plate and the wall plate then spans horizontally between the collar tied trusses. You are assuming that the intermediate rafters are supported on the ridge beam spanning between each collar tied truss.
It is probably a combination of both of these things!
RE: Structural Question From a Mechanical Guy
I know that the beam supports a structural load, and a ridge board doesn't.
But how are the two mounted differently to explain the difference in function?
V
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
The cross-section of the ridge beam is just thicker?
V
RE: Structural Question From a Mechanical Guy
It is assumed that the rafter forces pass completely thru the ridge board.
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
BA
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
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.
RE: Structural Question From a Mechanical Guy
I like this structural thing. Maybe I need to go back to school.
V
RE: Structural Question From a Mechanical Guy
Again, there may be other ways I am not familiar with but other members of this forum may be.
RE: Structural Question From a Mechanical Guy
Thanks very much for everyone's help, and if there are any more opinions, please feel free to share them.
Thanks again!
V
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
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.
RE: Structural Question From a Mechanical Guy
Are you serious?
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
RE: Structural Question From a Mechanical Guy
Sorry, no offense, csd72. Need more coffee.
V
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
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
RE: Structural Question From a Mechanical Guy
BA
RE: Structural Question From a Mechanical Guy
Mike McCann
MMC Engineering
RE: Structural Question From a Mechanical Guy
BA
RE: Structural Question From a Mechanical Guy
seriously though, you could consider sprayfoam insulation to get the required R value in a thinner space. I think it's about an R value of about 6 for 1" of closed-cell foam (depending on the supplier etc).
RE: Structural Question From a Mechanical Guy
RE: Structural Question From a Mechanical Guy
Very usefull the picture! :)
RE: Structural Question From a Mechanical Guy
Will let you all know how it goes (if you care
V