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Structural Question From a Mechanical Guy

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.

V

RE: Structural Question From a Mechanical Guy

if you tske out the ceiling joists, what's going to complete the roof rafters ?

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

I'm not sure I'm buying that the wall is actually a load bearing wall?  If it were, I would expect it to go all the way across the house.  As it stands, some joists have to span the entire house while others don't.  That doesn't quite make sense.  The only option I see is if one of the bearing walls is "extended", so to speak, by a beam that heads off the joists.

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

The ceiling joists are not only supporting the weight of the ceiling but are also keeping the exterior walls from spreading at the top.

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

(OP)
I shouldn't say the the house is one floor. To get to bedroom 1, 2, and the master, you need to go up a half flight of stairs--so it's really more of a split.

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

If you would like to remove the ceiling joists above the living room, dining room and kitchen ceiling, you will need to install a structural ridge beam. Each end of the structural ridge beam will have to be supported by a column. The location of these columns will need to be reviewed to ensure the load from the roof is properly transferred to the foundation walls/footings. A structural ridge beam will significantly reduce the potential for horizontal thrust at the top of the exterior walls due to gravity loads.

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

(OP)
There is a ridge beam already installed (sorry for not detailing it). I was hoping to use big enough collar ties to handle the horizontal loads.

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

Make sure it is a ridge beam and not a ridge board! These function completely different!

RE: Structural Question From a Mechanical Guy

The lower the collar tie is p[laced, the lower the tension forces and bolting requirements, and the less the horizontal spreading.  It is possible to do what you mention but the structural will also have to check out the bending of the existing rafters with the collar ties installed.  You may need to have double collar ties at each rafter, but that will get you where you want to go.

Mike McCann
MMC Engineering

RE: Structural Question From a Mechanical Guy

(OP)
The slope is definitely greater than 4 in 12.

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

(OP)
The ridge beam is currently supported by collar ties at looks to be 8 foot intervals. I was under the impression that they had to be supported at 4' spans, or is that just a local code thing?

Man, I think I need to buy myself a AISC manual.

V

RE: Structural Question From a Mechanical Guy

Think Timber, not Steel.  You need an AITC manual...

Mike McCann
MMC Engineering

RE: Structural Question From a Mechanical Guy

(OP)
Ah, you're correct.

I need an acronym manual as well, it seems.

V

RE: Structural Question From a Mechanical Guy

Collar ties do nothing to support a ridge beam. Sounds more like a ridge board.

RE: Structural Question From a Mechanical Guy

On the contrary Jike, ridge beams can be spanned bvetween collar tied trusses and are all the time.  The collar tie trusses are just much bigger due to more load seen.

Mike McCann
MMC Engineering

RE: Structural Question From a Mechanical Guy

(OP)
If it is, in fact, a ridge board, I'm assuming I'd have to put up scaffolding, jack up the rafters, remove the ridge board, and install a ridge beam, correct?

V

RE: Structural Question From a Mechanical Guy

There are probably ways around doing all of that work.  A ridge beam will have to be installed, but if done right, the scaffolding and existing ridge board could possibly remain.

RE: Structural Question From a Mechanical Guy

Mike:

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

(OP)
I'm not sure I understand the difference between a ridge beam and a ridge board.

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

A ridge beam is sized to take the vertical reactions from the sloped roof beams and has it's own supports.  A ridge board doesn't, it's just there for something to nail to really.  The ridge beam won't completely eliminate thrust, though.  That only happens if it's infinitely stiff and doesn't deflect, but we know that's not true.

RE: Structural Question From a Mechanical Guy

(OP)
So they're both mounted the same way?

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

V

RE: Structural Question From a Mechanical Guy

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.

RE: Structural Question From a Mechanical Guy

(OP)
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

RE: Structural Question From a Mechanical Guy

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

RE: Structural Question From a Mechanical Guy

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

RE: Structural Question From a Mechanical Guy

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

RE: Structural Question From a Mechanical Guy

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

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

(OP)
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. wink

V

RE: Structural Question From a Mechanical Guy

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.
 

RE: Structural Question From a Mechanical Guy

(OP)
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

RE: Structural Question From a Mechanical Guy

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 ...

RE: Structural Question From a Mechanical Guy

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).

RE: Structural Question From a Mechanical Guy

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.

RE: Structural Question From a Mechanical Guy

(OP)
csd72-

Are you serious?

Quote (Me, first post):

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

RE: Structural Question From a Mechanical Guy

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

Sorry, no offense, csd72. Need more coffee.

V

RE: Structural Question From a Mechanical Guy

(OP)
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

RE: Structural Question From a Mechanical Guy

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

RE: Structural Question From a Mechanical Guy

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

BA

RE: Structural Question From a Mechanical Guy

If they are connected preoperly to the rafters, ceiling joists that rest on the wall top plates can also be considered collar ties as they limit lateral spreading of the walls.

Mike McCann
MMC Engineering

RE: Structural Question From a Mechanical Guy

I still got it wrong.  Should be R*x - wL^2/2.  I know...I should think before writing.

BA

RE: Structural Question From a Mechanical Guy

buttresses!

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

vc66, it is possible to detail this out. Go ahead and hire a competent structural engineer.

RE: Structural Question From a Mechanical Guy

Thanks SteelIPE,

Very usefull the picture! :)

RE: Structural Question From a Mechanical Guy

(OP)
Got the structural coming in on Saturday.

Will let you all know how it goes (if you care wink).

V

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