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Dormer Ridge Board or Beam

i think you have a bigger problem

Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.
 
Shed dormer?

I agree that it is a beam or girder requiring engineering, per §R802.3.1 "...where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice
 
Jobsaver....what is controlling roof thrust? I have seen alot of these...structural ridge required or other engineering..(diaphragm for the roof system?)
 
The original roof line looks like a 6/12 pitch. The shed dormer looks to be about 1/3 of that (2/12 pitch). I see a minimum 12 ft span on the ridge board. Beam required
 
Francis:

This is my first post here, so I hope I’m not stepping on anyone’s toes, but my 8 cents worth.... I think you need engineering help on this roof job, and I don’t mean the loose talk you get on forum where no one knows what you’re really doing, want or need. Get a local Structural Engineer involved, so you don’t end up in trouble with one of the regular members on this forum, on down the line, or worse yet make a real mess of someone’s house.

You’re way short of providing sufficient meaningful info. to have an intelligent discussion on your roof design problem. Your thumbnails show three different things: the std. symmetrical gable dormer in the left sketch has identical pitch on both sides of the ridge board and ceiling joist as ties to take the rafter thrust at the top of the side walls; the shed dormer in the right sketch has an exterior wall and a significant header beam up near the main roof ridge to take the reactions of the low slope rafters which act much more like horiz. spanning fl. joists; the photo is a real bastardization which needs a ridge beam and its reaction posts and then, special engineering attention because the thrust from the two roof slopes is different and they don’t cancel each other out at the ridge, thus the ridge has a net lateral load, plus the vert. loads. Each of these requires special attention to different engineering details by someone who can look at the building your starting with and properly advise you what needs to be done. And, however complex and detailed and unintelligible we have made our building codes, of late, they just don’t cover all of these conditions in separate subsections, one at a time.
 
From this simple carpenter's perspective here's what I see.

The one by ridge has been there since before Methuselah's time. Someone embarked on an addition of a shed dormer and added a few recycled 2X4's as braces to support the ridge, in addition, of course, of the new rafters.

Solution: My bet is that the 2X4 braces are bearing on a bearing wall below. How about require that each rafter be so braced and add some A35's to the ancient rafter to ridge connection. Cornsiderin that the ancient ridge is a one by, some power blocking ( Oregon definition ) would provide the necessary purchase for the hanger nails. This design would depend on a cantilevered support of the new roof, no problem from my perspective.

Bill
 
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Shed dormer. Reference 802.3, Rafter ties/ ceiling joists in the lower third of roof height. If it has them, on the shed side they are at the bottom, on the right side they are in the upper third. I've built this with ties and a ridgeboard many times and in regions with a much higher snow load with no problem, we also used deeper rafters than structurally necessary. It works but it is not to current prescriptive code. If I were to do another it would be with a ridgebeam sized by the lvl supplier.

Just saw dhengr's response, and since i can't delete mine... yup, what he said.
 
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KZQuixote said:
From this simple carpenter's perspective here's what I see. The one by ridge has been there since before Methuselah's time. Someone embarked on an addition of a shed dormer and added a few recycled 2X4's as braces to support the ridge, in addition, of course, of the new rafters.

Solution: My bet is that the 2X4 braces are bearing on a bearing wall below. How about require that each rafter be so braced and add some A35's to the ancient rafter to ridge connection. Cornsiderin that the ancient ridge is a one by, some power blocking ( Oregon definition ) would provide the necessary purchase for the hanger nails. This design would depend on a cantilevered support of the new roof, no problem from my perspective.

Bill
Click to expand...
...and that's why there is a code which often requires a licensed design professional.
 
I will agree that pictures of different areas would help. The plans would also help.

There might be an argument made that the original roof used the floor joists as ties - allowing a ridge board. Might even be able to continue the argument to say that the new rafters allow for a ridge board.

But without more knowledge of the construction, who is to say.
 
This was drawn by an engineer, built in 20 psf snow country... although he also specced the let in bracing, we beefed that with ply corners later and didn't tell;

yokes001.jpg


The missing rafter bays have logs sitting on the plates and around the steep rafters at the 8' level as ties. The shed rafters sit on flattend section on the log ties' tops and framing angles secured them. Ties were bolted w/ 1" bolts and the connection from tie to wall was a heavy angle bolted down. 2x ridgeboard. The one above was built around '95 and we spoke with the owner a couple of nights ago. The house is still doing fine. Wind on the steep roof of that one is probably much higher loading than the vertical loads. I have done plenty with a conventional stick framed ceiling though. I believe those were all drawn by designers. I can think of two with lvl ridgebeams. One had tji rafters, the other hung trusses from the ridge. Neither of those had ties. I suspect the shed roof diaphragm is sufficient to take care of the lateral loads.
 
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As long as the rafters are directly opposing I don't think the board is the issue. As suggested by others, the ridge needs to be supported as a beam. I don't think 2x would suffice as a beam any more than 1x.

Hangers don't look to be skew hangers.. fail.

No comment on the sidewall framing for the shed dormer..
 
DRP said:
Shed dormer. Reference 802.3, Rafter ties/ ceiling joists in the lower third of roof height. If it has them, on the shed side they are at the bottom, on the right side they are in the upper third. I've built this with ties and a ridgeboard many times and in regions with a much higher snow load with no problem, we also used deeper rafters than structurally necessary. It works but it is not to current prescriptive code. If I were to do another it would be with a ridgebeam sized by the lvl supplier.Just saw dhengr's response, and since i can't delete mine... yup, what he said.
Click to expand...
I don't see the bolded wording in your quote in the code. Please instruct.
 
I agree ties required or ridge beam. Are the ceiling joists adequate as floor joists, or were they replaced?
 
Yankee said:
I don't see the bolded wording in your quote in the code. Please instruct.
Click to expand...
Prescriptively speaking, Tables R802.5.1 footnote a limits the tables to the rafter ties/joists being in the bottom 1/3 of the span.

Not to say you can't do it some other way, but you are outside the prescriptive design. I'm looking at our code based on the 2006 IRC.

Pitch is still a problem.
 
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DRP said:
This was drawn by an engineer, built in 20 psf snow country... although he also specced the let in bracing, we beefed that with ply corners later and didn't tell;
yokes001.jpg


The missing rafter bays have logs sitting on the plates and around the steep rafters at the 8' level as ties. The shed rafters sit on flattend section on the log ties' tops and framing angles secured them. Ties were bolted w/ 1" bolts and the connection from tie to wall was a heavy angle bolted down. 2x ridgeboard. The one above was built around '95 and we spoke with the owner a couple of nights ago. The house is still doing fine. Wind on the steep roof of that one is probably much higher loading than the vertical loads. I have done plenty with a conventional stick framed ceiling though. I believe those were all drawn by designers. I can think of two with lvl ridgebeams. One had tji rafters, the other hung trusses from the ridge. Neither of those had ties. I suspect the shed roof diaphragm is sufficient to take care of the lateral loads.
Click to expand...
"We've been doing it that way for years, and nobody has been killed."
 
dhengr said:
Francis:This is my first post here,
Click to expand...
Welcome to the forum!

Please continue to give us your input. A fresh set of brains is always welcome. And don't worry about toes.......we are used to people stepping on more than our toes! :)
 
TimNY said:
Prescriptively speaking, Tables R802.5.1 footnote a limits the tables to the rafter ties/joists being in the bottom 1/3 of the span.Not to say you can't do it some other way, but you are outside the prescriptive design. I'm looking at our code based on the 2006 IRC.

Pitch is still a problem.
Click to expand...
Just so it is clear, all ceiling joists acting as rafter ties attached anywhere above the rafter support wall need their span shortened by the amount in the table referenced at the bottom of table R802.5.1, , , and that table allows the adjustment a tie to a maximum of 1/3rd the height to the ridge.

Contractors use that "1/3rd" statement a lot without making the span adjustment required.
 
Francis Vineyard said:
Have plans reflecting this photo (taken from web) office discussing what's required here a 1x or 2x ridge board?We have 25 psf snow load
Click to expand...
Hi Francis,

"...Photo ( taken from web )..." Am I to understand that this is an academic exercise or is the photo of a job you're actually dealing with?

Thanks

Bill
 
Apologize for not posting drawings from the plans; I’m not computer savvy enough yet to reduce resolution to the download limit. I kicked back the plans for not giving the existing roof detail and ceiling floor joist size and wanted to be prepared for this next step.

The 8 ft. wide dormer is for headroom of back to back tub/shower in the center of a 64’ x 40’ house.

R802.3 in part says “Where the roof pitch is less than three units vertical in 12 units horizontal, structural member that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.

However;

R802.3.1 “Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice.

I was in agreement with the majority posted here but talking with a couple of local hammerheads (a new word I learned for the trade dictionary) they routinely attach to the 1x ridge board in this manner which is telling me other jurisdictions have been approving this installation. I need proof not just because that's how we’ve always done it!

Reference: Southern Pine Maximum Spans guide;

"Ridge beams must be installed at roof peaks with rafters bearing directly on the ridge beam or supported by hangers or framing anchors. Ceiling joists are not required when properly designed ridge beams are used.

A ridge board may be substituted for a ridge beam when the roof slope equals or exceeds 3 in 12, except that ridge beams are required for cathedral ceilings. Rafters must be placed directly opposite each other, and ceiling joists must be installed parallel with rafters to provide a continuous tie between exterior walls".

Guess really what I was asking if this type of dormer would be framed similar to floor openings as what is shown in the R/H drawing sourced from “Building Construction Illustrated; Francis D. K. Ching and Cassandra Adams? Where a header is sized as a beam supported by the double rafter trimmers. But is drawing is not showing this.

Welcome dehngr, ditto what Mule said. I must be a glutton for punishment since I cannot stop learning my lessons here.
 
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