Question for framers on open ceiling

[DWinTX]

Hi, I am about to start construction on a new detached garage. It will be 20' x 28' with 10' walls. I want to do a combination loft and open (cathedral style) ceiling. The open area will be 20' of the 28' length, and the remaining 8' will be the loft.

My goal here is to have enough height in the 20' span for a lift without worrying about ceiling height. The loft area will be for storage. I have 4 classic cars I am restoring and I have a LOT of parts.

I'm meeting with the contractor tomorrow to discuss how the garage will be built. I want to make sure that the open area is constructed with sufficient height.

I've read that there are two ways to frame without the normal ceiling joists, with a structural ridge beam on posts or with collar ties, which I guess is more a vaulted ceiling that a cathedral. I have one question on each:

If using a structural ridge, do the posts have to go straight down to the slab? I'll have a 16' garage door on one end and 6' double doors on the other. Can you run a post down to the door header and increase the strength of the header and it's supports sufficiently to support the roof?

If going the collar tie route, can anyone tell me how to calculate the ceiling height I would get? I've read that the collar ties can only go one third up the length of the rafters to still supply the torsional strength to prevent the ridge from sagging. My walls are 10', the span of the gable end is 20' and the roof pitch is 8/12. How high off the slab will the collar ties be if they one third up the length of the rafters?

Any help you can give me will be much appreciated. This is the first time I've contracted to build a building of any type, so I want to make sure I have the contractor do it the way that will work the best for my needs.

 

[Rock knocker]

If using a structural ridge, do the posts have to go straight down to the slab? I'll have a 16' garage door on one end and 6' double doors on the other. Can you run a post down to the door header and increase the strength of the header and it's supports sufficiently to support the roof?

An archy or engineer can design a header system around just about any opening, the only limitations is you wallet

If going the collar tie route, can anyone tell me how to calculate the ceiling height I would get? I've read that the collar ties can only go one third up the length of the rafters to still supply the torsional strength to prevent the ridge from sagging. My walls are 10', the span of the gable end is 20' and the roof pitch is 8/12. How high off the slab will the collar ties be if they one third up the length of the rafters?

It will depend on the pitch of your roof
http://www.nachi.org/collar-rafter-ties.htm

Scissor trusses are another alternative

 

[NUTTSGT]

Just thinking off the top of my head with the layout of the trusses, you may need to go with 12' walls for clearance. I'm not sure if you will have enough room on the side (near the wall) even if you use scissor trusses.

 

[TommyK]

Hi, I am about to start construction on a new detached garage. It will be 20' x 28' with 10' walls. I want to do a combination loft and open (cathedral style) ceiling. The open area will be 20' of the 28' length, and the remaining 8' will be the loft.

My goal here is to have enough height in the 20' span for a lift without worrying about ceiling height. The loft area will be for storage. I have 4 classic cars I am restoring and I have a LOT of parts.

I'm meeting with the contractor tomorrow to discuss how the garage will be built. I want to make sure that the open area is constructed with sufficient height.

I've read that there are two ways to frame without the normal ceiling joists, with a structural ridge beam on posts or with collar ties, which I guess is more a vaulted ceiling that a cathedral. I have one question on each:

If using a structural ridge, do the posts have to go straight down to the slab? I'll have a 16' garage door on one end and 6' double doors on the other. Can you run a post down to the door header and increase the strength of the header and it's supports sufficiently to support the roof?

If going the collar tie route, can anyone tell me how to calculate the ceiling height I would get? I've read that the collar ties can only go one third up the length of the rafters to still supply the torsional strength to prevent the ridge from sagging. My walls are 10', the span of the gable end is 20' and the roof pitch is 8/12. How high off the slab will the collar ties be if they one third up the length of the rafters?

Any help you can give me will be much appreciated. This is the first time I've contracted to build a building of any type, so I want to make sure I have the contractor do it the way that will work the best for my needs.

A header can be sized to carry the load from the structural ridge over the garage door. It may involve a piece of steel.

Using the dimensions provided and the 1/3 rule for the collar tie height makes your ceiling height approximately (Edit) 12'-3".

 

[DWinTX]

I thought about scissor trusses too, but I'm guessing they are more expensive, I'll have to discuss it with the contractor tomorrow. I could go with 12' walls, I guess. I'll have to check on the additional expense.

If the collar ties will give me 12' 3", I think that would work. I know I'd have to position the lift in the center, or maybe just off-center, but that will be OK. I leaning towards the BendPak XPR -10, which is 145" (12' 1"), so it would just fit.

 

[kbs2244]

I am a big fan of ridge beam construction.
I just like the open “cathedral’ type space it gives you.
But you do have to support the beam at each end.
The beam will run 90 degrees to your gable end, and any door opening you have in that wall will to have a post centered in the gable end, or a header strong enough to take the load over any opening.
It can be done.
(Research a “king post” truss.)
But in my experience it is easier to have a centered post and two smaller doors.

You collar joist question is a local code question.
It can be anywhere from 1/3 to 2/3 of the joists.
It depends on the local snow load code calculations..

 

[DWinTX]

Thanks, I'll discuss the king post truss with the contractor and see what he thinks.

I can't go with two smaller doors, it would just be too difficult to maneuver my cars around in front of the garage to get them in and out. A couple of them are shells on body carts and have to be pushed. Unfortunately, I'm very limited space-wise. I would be building a bigger garage but my lot size and the required setbacks just won't allow it.

 

[fountain]

You can also go with a parallel chord scissor truss. The only issue is your heel height will be more than a standard energy heel which may be a hindrance if you have any height restrictions. Good luck and congrats on your new garage.