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Collar tie vs Ridge beam theoretical question 4

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DoubleStud

DoubleStud

Structural
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Let's say you have a gable roof with collar tie & ceiling joists. If you install a new ridge beam just below the ridge board and you also keep the existing collar ties, am I right to assume you can remove the ceiling joists and use the existing rafters?
 
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Yes, this is done all the time for people who want to vault their ceilings. Normally I prefer to remove the existing ridge board/ nailer and place the new beam all the way up, but putting the new ridge beam below is fine as well.

Depending on the height of the new ridge beam, on some occasions with deep LVL's I'll specify some sort of lateral bracing back to the existing rafters (would look like discontinuous collar ties that get interrupted by the new ridge beam.
 
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But if I remove the existing collar ties, I more than likely need bigger rafters?
 
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Collar ties don't really do anything except for uplift or unbalanced loads. I don't know anyone who is changing the rafter sizes because collar ties are or are not installed unless it was designed as a truss.
 
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Ok I just want to make sure the collar tie didn't make the design of the rafters smaller back then.
 
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Wait, you want to remove both the collar ties and ceiling rafters?
 
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I'm assuming the ceiling joists are at the bottom of the rafters and the collar ties are most of the way up, maybe 2/3 or so.

I think it's possible that the collar ties are acting as supports for the rafters. That is often the case where there is both a ceiling joist and collar tie attached to a rafter pair. With a ridge beam, I think the answer might be more complicated, due to the deflection of the ridge beam, but I wouldn't necessarily assume that they don't contribute to the overall strength of the roof. If you have some frame software it may be beneficial to run a quick analysis. If the collar ties are only little pieces of strapping barely connected to the rafters, in that case I doubt they're doing much.
 
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I was going to keep the collar tie. But would definitely analyze to see if the rafter is good spanning from ridge to exterior wall. Everything is covered by drywall right now and I was assuming things to tell the owner.
 
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Always check the rafters first as simply supported members because it's not rare to find overspanned 2x's. Then when you tell the homeowner that their entire roof needs strengthening it can change the scope of the project quickly. Once the rafters are settled then move onto the ridge beam.
 
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Also, contact ceilings are designed to L/240 instead of L/180 for non-contact so you have to check that your rafter actually can be used in this situation.
Additionally, if one of the gable walls is an exterior wall, you may have to sister the studs up to make it balloon framed to remove the hinge at ceiling level.
 
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Thanks XR250! Very good point about the gable end. Do you think this type of framing never balloon frame the gable end?
 
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I second that! Very good point by XR250.

In my experience, with a ceiling in place, the gable end walls are often framed with a lower wall of the same height as at the eaves. Another wall is framed on top up to the roof. The ceiling braces the hinge point where the upper and lower walls meet. I've even seen a lot of (non-engineered) walls incorrectly framed this way in large open rooms (without a ceiling), often with drywall crack/damage at the hinge point.

I doubt the gable end wall is balloon framed. That would likely be more work for the framers and isn't great in terms of fire resistance, with a fire being able to easily spread up the wall into the attic/roof.
 
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Here in California, we need 2x10 rafters to have enough depth for R-30 insulation, so the strength issue of the rafters is a moot point. We must add 2x10s. If your project doesn't have this (R-30) requirement, remember that, in most cases for conventional framing, the span of the roof rafter is reduced by purlin braces which will be removed. It is unlikely the typical 2x6 will span from ridge to bearing wall.

IMO, collar ties are Viagra for framers in that they assist in the erection of the roof framing before the roof sheathing is applied. If you modeled roof rafters with collar ties up near the ridge board, you would find the connection of the collar ties to the roof rafters to be impractical to detail or build.

Personally, I install the ridge beam below the existing ridge board. I face nail the existing rafters to the new 2x10s and connect the 2x10s to the new ridge beam.

Depending on the geometry, I have found the stiffness of the ridge beam is more important that its strength because, when the ridge beam deflects, the walls attached to the roof rafter bow outwards.
 
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Good call XR. Usually I see gable ends framed the same as the rest of the house so anytime I have someone vaulting a ceiling I just run a quick wind calc and have them add additional 2x4's at whatever spacing is needed that run full height.
 
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DblStud said:
Thanks XR250! Very good point about the gable end. Do you think this type of framing never balloon frame the gable end?
Click to expand...

Never seen one that way.
 
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