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Tray ceilings and rafter ties. Hmmmm.....

jar546

jar546

CBO
Joined
Oct 16, 2009
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I am going to continue since I am on a framing kick right now to help build good content for this forum and so that we can all learn something and some of you will teach us. So what do you fine folks do in this situation?

Let's take a typical 28' wide house with a simple gable roof. On one end of the house will be a master bedroom along with a large master bath. That takes up the last, oh let's say 20' of the house. The owner's want a tray ceiling that bumps up about 18" and the bathroom area will be typical flat ceiling.

As you can see, rafter ties are becoming a problem here and I would like to know what you folks approve for this situation. There are some options out there but without involving engineering, what can you do prescriptively?

Since the ceiling joists will be at different elevations at the tray ceiling, how do you handle the connection?

This is everyday real life for residential inspectors. No trusses by the way.
 
28 views and no replies. I guess no one ever sees tray ceilings or everyone just approves them without thinking about lateral restraint. Come on folks, show us your brainpower!!!
 
I have used the rule of thumb with some engineering study a local PE gave me, (yes I know that is dangerous) that if the ceiling joist is more the 1/3 the way to the peak from the plate it no longer operates as the tie at the bottom of the roof triangle.

Since the code assumes that ceiling joist is at the bottom of the triangle (see note “a” below) if the design present violate that condition then I ask for a bearing ridge calculation which usually is a miroclam design for the ridge to support the roof structure.

If the tray ceiling does not exceed the first third presupposition then I have no problem with tray ceilings. Disclaimer unless there is some other condition that warrants additional support.

2009 IRC

TABLE R802.5.1(1)—continued

RAFTER SPANS FOR COMMON LUMBER SPECIES

a. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters

on the bearing walls, such as rafter ties, is provided at that location. When ceiling joists or rafter ties are located higher in the attic space, the rafter spans shall be

multiplied by the factors given below:

HC/HR Rafter Span Adjustment Factor

1/3 0.67

1/4 0.76

1/5 0.83

1/6 0.90

1/7.5 or less 1.00

where:

HC = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls.

HR = Height of roof ridge measured vertically above the top of the rafter support walls.

b. Span exceeds 26 feet in length.
 
Flying buttresses? :)Here is a visual example of one I finished last month. The collar ties are high up on th rafter, but the pitch is 16-12, so I imagine you can go up further with a high pitched roof. While I built this I of course had line braces, but they tried to spead and I had to pull the walls together with a couple of long pipe clamps to keep them plumb until I tied it all together and got the sheathing on. The roof diaphram actually helps mitigate the spreading issue too.All engineneered and approved.

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Um...specs. That sounds like paper stuff. I don't write 'em, I just sing 'em :)

Meaning; I'm not sure, but I will look at the plans and find out. If it helps the job is in Sacramento, so no snow loading or anything like that.

And pick away compadre. There is always room for improvement.

Brent.
 
Clean framing job! Could not be approved without the engineered design, non-prescriptive. As stated the rafter ties appear to be at or past the 1/2 way point, which in turn brings up the question of the "non-continuous tie" where the ceiling is at different levels. One negative comment, the ledger on the left where the rafters are joist hangered should be wide enough to cover the complete rafter cut to pass code prescriptively.

Otherwise, Nice work!
 
hb077QA01-03_lg.jpg
 
MASSDRIVER,

Nice pictures and a good, clean looking Framing job. In picture # 3, it appears as though your

rafters are larger than your ridge board, or am I seeing it incorrectly (RE: Section R802.3, 2006

Edition of the IRC: "Ridge board shall be 1-inch (25mm) nominal thickness and not less than

the cut end of the rafter.").

.
 
STB said:
Clean framing job! Could not be approved without the engineered design, non-prescriptive. As stated the rafter ties appear to be at or past the 1/2 way point, which in turn brings up the question of the "non-continuous tie" where the ceiling is at different levels. One negative comment, the ledger on the left where the rafters are joist hangered should be wide enough to cover the complete rafter cut to pass code prescriptively.Otherwise, Nice work!
Click to expand...
Thank you. And you are correct. Convo with the enginenerd was the rafters are sized for 2x6 so as long as I have that section on a hanger I'm cool. The rafters are that size to cover insulation requirement.

Brent.
 
globe trekker said:
MASSDRIVER,Nice pictures and a good, clean looking Framing job. In picture # 3, it appears as though your

rafters are larger than your ridge board, or am I seeing it incorrectly (RE: Section R802.3, 2006

Edition of the IRC: "Ridge board shall be 1-inch (25mm) nominal thickness and not less than

the cut end of the rafter.").

.
Click to expand...
Thanks again, you are correct. Ridge would have to been 16" LVL to cover nailing. Enginerd instead called for those small ties directly under the ridge, tight against it, mitigating the oversized ridge.

The funny part is the ENTIRE rest of the roof in this 1936 house is 2x4. Rafters, hips, valleys...allof it except the ridge which is 1x6. And TALL. I will try to get pics of existing as I think I'm headed back there on Friday. I want to measure some things and have JAR look at it through struct calc just to see the result.

Brent.
 
MASSDRIVER said:
Thanks again, you are correct. Ridge would have to been 16" LVL to cover nailing. Enginerd instead called for those small ties directly under the ridge, tight against it, mitigating the oversized ridge.The funny part is the ENTIRE rest of the roof in this 1936 house is 2x4. Rafters, hips, valleys...allof it except the ridge which is 1x6. And TALL. I will try to get pics of existing as I think I'm headed back there on Friday. I want to measure some things and have JAR look at it through struct calc just to see the result.

Brent.
Click to expand...
Cool Brent, I'm game. I am sure STB would love to run the numbers too. He is my framing guru
 
We, my husband and I, are traying a ceiliing in our 1954 home.

Disclaimer: we are trades but plumbing and finish isn't framing.

We added 2x4 collar ties in the top 1/3 of the rafter roof. We are in the midsts of building the soffit on the edges. While we are doing this, we are discussing the rest of the needs of this roof structure. The rafter spans 22' and is currently 2x10 rafter ties and 2x6 ridge beam and rafters. I'm new here, so I don't know if I should tell you what we think we should do ( this might be comedic and add entertainment ) or if it is preferred for those experienced to just tell us what to do next. Thank you!
 
I am happy to see this thread resurrected again as it is an important topic.

In addition, it is also a great example as to why everyone should upload a photo as a file rather than provide a link. None of the pictures posted in this thread are available to us now that the links are no longer any good.

Let's discuss this, especially for those that want a tray ceiling more than 1/3 of the way up.
 
Thanks for the quick response! We do not want to go more than 1/3 of the way up. Current ceiling height is 8'. 55" is the height from the top of the existing ceiling rafter to the ridge board. We would like to move the ceiling joist up roughly 15"-18". The new ceiling joist pan area would be 14'. We would like to do this to 8 consecutive ceiling joists.
 
If instead of a ridge board you install a ridge beam supported at the ends the collar ties and ceiling joists become irrelevant in terms of building stability. This assumes you do not have a steep roof as is common in regions with significant snow.

If the ceiling joists or collar ties are needed for stability of the system and you do not want to rest ceiling joists on the top plates of the wall you should have an engineer look at it. This system should be treated as a truss.
 
I agree with Mark.....Either calculate a ridge beam to take the load, or enlist a design professional to help you out. Good luck!
 
As you raise the ties you also increase the stress on the rafters at the connection and rafters might need to be upsized.....Or the structural ridge...
 
Structurally recommended by who? A contractor or an engineer? A lot of people express an opinion but many do not have a basis for deciding what is needed. What worked on a previous project might not be appropriate since there may be significant differences.
 
For what its worth and depending on spacing and specie, 1934 2 x4's net dimensions are larger then todays and maybe stronger. My 1905 barn with 2 x 4's (net) at 36" o.c. space sheathed is still standing sag free.
 
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