Dormer Ridge Board or Beam

[GHRoberts]

DRP ---

No ridge is needed. The usual reason for a ridge board is to provide some stability during construction and a surface to hold the nails.

The problem with giving an answer is that I don't do designs like that and I have no prepared engineering that would indicate what issues are important. I would look for people who have engineered salt-box designs (New England).

 

[Yankee]

Lets get back on track, I'm more interested in structure.This is a 12/12 main roof and a 3/12 shed dormer. The building is 24' wide, the peak is centered. The tie is well connected.

Why does this require a ridge at all?

I plan on sheathing the dormer support wall and main roof but plan to skip sheath the shed roof. I'll let you plug in your own variables, please show me why this wouldn't work or what you would do differently.

If there is no ridge board then each set of rafters will be considered a (separate) truss, and engineering is required for trusses.

 

[DRP]

Lets take care of structure first. I was establishing whether there needs to be a structural ridgebeam. A conventional stick framed rafter set is a truss. A ridgeboard, as George pointed out, is a framing aid. A ridgeboard is not structural. I have almost always used a ridgeboard to ease assembly, but not always. R802.3, first sentence, a ridgeboard is not required.

I don't want to get too far off track, but a little history. Light timberframe "common rafter" construction often used an open mortise and tennon or half lap joint at the peak, obviously with no ridgeboard. This later became a butt joint with a nailed gussett. Since they were using board sheathing they didn't need a ridgeboard, one or two rows of sheathing held things in place as they assembled each pair. From observation I know these were sometimes tipped up as an assembly. Our code will still allow me to gusset a butt jointed rafter pair and board sheath it.

Francis was correct, the detail I drew is also in the WFCM and AWC's "Details for Conventional Wood Frame Construction"... it is industry standard construction. I've been rather suprised that it seems to have fallen off of everyone's radar.

 

[dhengr]

I think Yankee has it just about right. And, I think DRP is the one who is getting off track here because he is not taking care of or understanding the structure first. Despite the history lesson there is very little acceptable engineering thinking going on above. All George said was he doesn’t think a ridge board is needed, and that he probably wouldn’t be inclined to do a design like that.

Francis.... I would say DRP is taking things much further than one small step beyond a std. shed dormer. Why are rafters to be treated differently when the pitch is below 3/12, and is 3.08/12 really any different than 2.92/12 in this respect? Show me the code sec. on that issue, and explain the problems associated with low pitch rafters. Should the ridge line brg. on this rafter be plumb cut or have a horiz. seat cut, and what’s the diff.? Jobsaver was just asking about the need for more rules of thumb, to expand the limits of the IRC a little or to cover things the IRC doesn’t explicitly cover. DPR’s stretch here is a fine example of why poorly explained rules of thumb without some very specific limitations are dangerous, in the wrong hands. The idea that this roof geometry works in a 10,12 or 14' wide shed dormer, and then still works without any changes in thinking when you extend the dormer width to 30 or 40' and then only partially sheath the dormer roof, so it may not be a particularly good diaphragm, takes a real leap of faith in the nonexistence of gravity.

DRP..... Yes, That’ll work just fine, if done right. I’m sorta paraphrasing here, but you probably want to look in the mirror and see what engineering knowledge and experience is looking back at you, then you want to sit down and actually engage your thinking cap, and apply all of your best accepted engineering practices to the problem. Answer the questions below, I’d have to if I were doing the design, and you can save some engineering costs by gathering this kind of info. for me. Your answers will help us determine how acceptable your accepted engineering practices are, and then we’ll talk about whether you need a licenced engineer or not, before you go for a permit, because it’s a stretch for the IRC to cover this, I suspect. By the way, I kinda think you took it on the chin from both the engineer and the BO on that deck beam, but in their defense, decks have been such a liability of late that everyone is skittish. If we had worked together long enough, so I knew what you know, and I worked in that area so I knew the BO, I would think we could have worked that out with a phone call or a letter from me. But, you also have to understand that you are asking me to take on a potential million dollar liability for a few hundred dollar fee, and you grumble at that, and my insur. co. wants all the fee as premiums. The questions:

1. What are your roof LL’s (snow and drifting, I assume) and what does your code say about snow drifting and load reductions as a function of roof. slope? Wind and EQ loads?

2. What is the makeup of the roof system, starting with the sht.rk. and working outward in order, and what is the DL in #/sf in the plane of the roof? Any other roof or clg. loads?

3. What species and grades of lumber are you using?

4. What sizes and spacing are the various members in your bldg. section, per your accepted engineering practices? And, show me how you arrived at those sizes. The 17' rt. rafter looks too shallow in comparison to the shed rafter.

5. What does your code say about rafter ties higher than 1/3 the rafter length, and why, show me the code sec.?

6. What’s the elevation of the top of the fl. deck, top of top pl. on left wall, top of ridge? I assume 2' overhang on the left and 1' on the right?

7. Why are you skip sheathing the shed roof and what exactly does that mean? Show me a roof plan with the size and location of the roof openings and the framing around them.

8. This is similar to your earlier photo, but a new bldg., I assume, without the center wall and logs, right. How are you going to frame the gable end walls and the left brg. wall?

9. What are your horiz. and vert. reactions at each end of each rafter, and what’s the tension in the tension tie?

10. What is you connection detail at each end for each rafter brg., and at each end of the tension tie? You may be will connected, but I’m not sure your tension ties or the rafters are. Is there someplace in the IRC which defines ‘well connected’ or do you actually design those connections on a crazy roof like this?

11. Assuming the rt. rafter is well connected to the fl. deck and that its deflection is zero, what are the deflections of the left rafter at the top of the wall?

12. Are you a builder, where?

There is a little needling mixed in here fellows, but not real much. The idea that engineers mostly aren’t needed, and then you’re not real sure when they are, and then for a low fee they should, in effect, sign-off on it and act as the insurer of last resort, doesn’t appeal to me. It’s a heck of a way to make a living, since I am not your insurance co. But that is kinda the way it shakes out when you and the BO get us involved at the last minute, because you two have chickened out or won’t take the responsibility. If you stick to std. framing and the IRC you don’t need us, but if you and the Arch. want to go crazy, then you probably need an engineer’s help to keep it stable and standing. This is not std. framing even though you are talking rafters and ridge boards.

So far I haven’t seen anything that looks like acceptable engineering practice. What’s really frightening is that these darn drafting programs will let you draw anything, irrespective of the fact that it may or may not be workable, and then by some wild interpolation from a completely different condition, pretend it’s O.K., and that it’ll work. I would much rather discuss and critique these kinds of things one on one, rather than in front of the whole world. And, I would really enjoy meeting 10 or 12 of you at a time, with a blackboard, a few six packs, and one or two items on an agenda to get the ball rolling, then just let it be open to the next question. And, it’s entirely possible that I wouldn’t be able to answer every question, on the spot, but I’d find a reasonable explanation for the next meeting.

 

[steveray]

The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...

 

[GHRoberts]

Looking at the comments by dhengr and steveray reminds me of the importance of employing an engineer who can base an answer on previous experience rather than one who has little experience. Employing the later will result in a much higher bill. (Not to imply that either poster is deficient in any manner. It is just that there are a lot of issues that a person with experience would throw out. I claim no experience in structures like this and would submit a large bill.)

---

Since steveray mentioned the rafter on the right. Not knowing if bending is imprtant ...

Tip the rafter so it is level and looks like a beam. The beam is simply supported at the floor joist and the "tie". The end of the beam is cantilevered. The loading is 25# snow load adjusted for the angle and spacing. There is also a point load on the end due to the ridge board connection. 25# uniform load must be less than 53#/ft and allow a 15' span (2x10") between supports. I don't know what the load on the end of the cantilever is but it does subtract from the 25# uniform load so I suppose we are allowed about 7.5' span there.

There is compressive load on the beam so the numbers are wrong but close enough for me. I guess working those numbers out were a waste of time ($$$). Just look up the span from the rafter tables.

 

[Rio]

The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...

Collar Ties vs. Rafter Ties

by Nick Gromicko, Rob London and Kenton Shepard

Collar ties and rafter ties are both horizontal roof-framing members, each with different purposes and requirements.

Collar Tiescollar rafter tie

Collar ties are designed to tie together the tops of opposing rafters. This helps brace the roof framing against uplift caused by wind. Collar ties must be placed in the upper third of the roof.

Other facts about collar ties:

* They may or may not be required, depending on jurisdiction. InterNACHI inspectors should not call out lack of collar ties as a defect unless they know that collar ties were required in the jurisdiction in which the home is located at the time the home was built.

* Collar ties are probably not needed if metal connectors were used to fasten the rafters to the ridge.

* Where they are required, they should be installed on every other rafter where rafters are on 24-inch centers.

* The 2006 International Residential Code (IRC) requires they have a minimum nominal dimension of 1-inch x 4-inch.

* Collar ties, contrary to popular belief, do not prevent walls from spreading.

Rafter Ties

Rafter ties are designed to tie together the bottoms of opposing rafters. This helps keep walls from spreading due to the weight of the roof. When the walls spread, the ridge will sag. A sagging ridge is one clue that the home may lack adequate rafter ties. Rafter ties form the bottom chord of a simple triangular roof truss. They should be placed as low as possible in the roof framing.

Other facts about rafter ties:

* Rafter ties are always required unless the roof has a structural (self-supporting) ridge, or is built using engineered trusses. A lack of rafter ties is a serious structural issue in a conventionally framed roof.

* In most homes, the ceiling joists also serve as the rafter ties.

* Where rafters are oriented perpendicular to the ceiling joists, rafter ties should be installed just above the ceiling joists. The ties usually rest on the joists.

* When rafters are installed on 24-inch centers, rafter ties are typically installed every other rafter.

* It’s not unusual to see rafter ties of either 2-inch by 4-inch or 2-inch by 6-inch. The 2006 IRC requires them to be at least 2-inch by 4-inch.

In summary, collar ties and rafter ties perform different functions but are both essential roof-framing members.

Found the above on the net while reading the thread and thought it looked good enough to post. Regarding the 3/12, 12/12 roof, the first thing that jumps out at me when I see it is that it's not symmetrically loaded which is a red flag to me that means proceed with caution. As pointed out above in the thread if the spans are small it's probably no big deal. If they start getting large then there is more grounds for concern. As I'm still learning (a long way to go) wood engineering there's no way I'd proceed on this one without consulting one of the engineers I work with on a regular basis.

 

[Rio]

The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...

Collar Ties vs. Rafter Ties

by Nick Gromicko, Rob London and Kenton Shepard

Collar ties and rafter ties are both horizontal roof-framing members, each with different purposes and requirements.

Collar Tiescollar rafter tie

Collar ties are designed to tie together the tops of opposing rafters. This helps brace the roof framing against uplift caused by wind. Collar ties must be placed in the upper third of the roof.

Other facts about collar ties:

* They may or may not be required, depending on jurisdiction. InterNACHI inspectors should not call out lack of collar ties as a defect unless they know that collar ties were required in the jurisdiction in which the home is located at the time the home was built.

* Collar ties are probably not needed if metal connectors were used to fasten the rafters to the ridge.

* Where they are required, they should be installed on every other rafter where rafters are on 24-inch centers.

* The 2006 International Residential Code (IRC) requires they have a minimum nominal dimension of 1-inch x 4-inch.

* Collar ties, contrary to popular belief, do not prevent walls from spreading.

Rafter Ties

Rafter ties are designed to tie together the bottoms of opposing rafters. This helps keep walls from spreading due to the weight of the roof. When the walls spread, the ridge will sag. A sagging ridge is one clue that the home may lack adequate rafter ties. Rafter ties form the bottom chord of a simple triangular roof truss. They should be placed as low as possible in the roof framing.

Other facts about rafter ties:

* Rafter ties are always required unless the roof has a structural (self-supporting) ridge, or is built using engineered trusses. A lack of rafter ties is a serious structural issue in a conventionally framed roof.

* In most homes, the ceiling joists also serve as the rafter ties.

* Where rafters are oriented perpendicular to the ceiling joists, rafter ties should be installed just above the ceiling joists. The ties usually rest on the joists.

* When rafters are installed on 24-inch centers, rafter ties are typically installed every other rafter.

* It’s not unusual to see rafter ties of either 2-inch by 4-inch or 2-inch by 6-inch. The 2006 IRC requires them to be at least 2-inch by 4-inch.

In summary, collar ties and rafter ties perform different functions but are both essential roof-framing members.

Found the above on the net while reading the thread and thought it looked good enough to post. Regarding the 3/12, 12/12 roof, the first thing that jumps out at me when I see it is that it's not symmetrically loaded which is a red flag to me that means proceed with caution. As pointed out above in the thread if the spans are small it's probably no big deal. If they start getting large then there is more grounds for concern. As I'm still learning (a long way to go) wood engineering there's no way I'd proceed on this one without consulting one of the engineers I work with on a regular basis.

 

[DRP]

1. Let's use Francis' numbers 25 psf, no reductions, seismic C, 90mph. Feel free to use other numbers.

2. 10 psf will cover DL, no don't pick up any diaphragm action here just load it

3. Let's use #2 SPF, snow, repetitive member

4. 2x12, for insulation, keep it simple 2' oc spacing. How did I arrive at those sizes, typical,... academic exercise, pulled out of air feel free to use something other. Different depth plumb cuts are ok, ridge must simply be deeper than the deepest rafter to support bottom edge of plumb cut. If that offends drop the dormer rafter a size,we'll insulate the level ceiling there, it'll still check on this building.

5. Not allowed, Footnote Table R802.5.1, Why, the tie tension and bending moment get out of hand if they are restraining the rafter thrust from high on the rafter. Is that the situation here, no.

6. top of fl deck, anything, put the ridge at 35'. You have width and pitches, sufficient info for general plate and ridge heights, I used 12/12 and 24' wide, 3/12 to make it easy on you. I sketched quickly, go grand call both overhangs 2'.

7. I was tempted to say sheathed with a tarp. What I'm asking for is that no diaphragm action be used in your analysis of the dormer roof. 50' long, call this the entire roof. IIRC the engineered dormer in the photo I posted was about 24' long, ridgeboard. The framing under the ridge is temporay construction support, it was removed. I stated that the shed roof diaphragm was probably sufficient to take care of lateral loads but upon thinking more I don't believe we even need to bring that into the equation, I'm calling it gravy for the purposes of this discussion, don't use it.

8. There was no center wall in the original photo of that engineered shed dormer. For what its worth that build was the subject of a feature article in a trade mag. The article was on design rather than structure. However over 350 home companies were advertising in that magazine at that time. I know many of their design depts read those articles. Not one wrote in commenting that they were concerned with the design. Having built basically the same detail for a number of them over the years, neither am I. Walls conventionally framed 2x6- 16 oc fully sheathed.

9. Your baby, the moment I break out my calculator here it's going to get off track but I do have a fair idea, let's see what you come up with. I've been showing what has worked and been standard among many thousands of homes. Go back to the beginning, I didn't really disagree with you did I? Show your calcs.

10. Why do you think the connections are anything higher than the heeljoint table would cover? If this were a 3/12-3/12 would the tension be higher or lower? Now look at the heeljoint table, it's using 16 commons, we'll be shooting .131's. Make the conversion, I can do it from a referenced table. You can go round the horn and use the yield limit equations if you want to do it the hard way. Double check the heeljoint table, I'm not factoring diaphragm action, now check the table again, it's light isn't it? "Rule of thumb"... pull the trigger at least half again.

11. Vertical deflection of rafter over a wall is zero. Horizontal deflection of the wall does occur, I was wondering who would make it that far. For the loads above it is within acceptable limits though isn't it. Then we do have the shed roof diaphragm in the "real world".

Actually scooting around a plan with these drawing programs give me headaches, I learned with slide rule, pencil and parallel bar. I'm perfectly capable of drawing a phantom clevis with paper and pencil . No interpolations from different conditions has occured here, standard practice. I've simply asked for you to show proof.

12. Third time's a charm, I've got others depending on my income. Y'all take care.

 

[brudgers]

The "we've" in this instance is the industry as far as I can tell, I've gotten this detail many times and as I pointed out this was engineered. Please explain the reason or math that makes it fail in your opinion. This was prescriptive to my understanding up until the '06 cycle. Very often the rafter depth is determined by insulation requirements, that was my comment on them often being deeper than structurally necessary. In the pic I posted also notice where the tie is joining the main rafter. I'd agree with Mule here.

"The industry" built homes in New Orleans below the flood level for many years and nobody got killed.

And then one day it started killing people.

That's why it's called "a recipe for disaster."

 

[brudgers]

Nothing says the beam needs to be anything more than a 1x but you need to consider it.

"Framing members shall be at least 2 inch nominal width" [2305.1.2.1]

 

[brudgers]

If there is no ridge board then each set of rafters will be considered a (separate) truss, and engineering is required for trusses.

And the truss would not be statically determinate.

 

[Francis Vineyard]

I could probably cut and paste all the good points made on this thread and write an article very similar to one I found written in JLC 9/93 by Robert Randall, a structural engineer from Mohegan Lake. N.Y. He noted observations of homes eventually showing the effect of undersize framing and poor construction. I’ve notice sagging roofs without dormers when I did home inspections. Wish I had taken notes of the rafter size to confirm this observation. During my search online many of the articles resulted with an engineer dispensing the same advice as dhengr; in fact they sound just like Dick on this message board. Quoting from this article in reference to my OP; “Except for very small dormers, I would not recommend using a header between doubled rafters to support the top of drop-ridge dormer rafters. Although this practice is common these header are usually undersized and a sagging roof or cracked rafters can result.” Emphasis added. PM me and I’ll send a copy of this article from JLC Troubleshooting Guide; Archive Members can read it online. The image is very poor owing to resolution limits to post. My learning curve shot up again from this, thanks to everyones input and dhengr for bringing your expertise from Eng-Tips Forum to here; I'm a frequent visiter at that site too.

View attachment 313

View attachment 313

/monthly_2010_12/Dormer.jpg.0073e738993a4addfd8ca0f5ee9de043.jpg

 

[peach]

"Framing members shall be at least 2 inch nominal width" [2305.1.2.1]

IRC R802.3 "Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter"

 

[alora]

IRC R802.3 "Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter"

Ok, so ...

IRC R802.2:

"Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."

 

[Rio]

Ok, so ...IRC R802.2:

"Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."

Just for clarification, a 3:12 pitch is equal to a 14% slope, not a 25% slope. A 25% slope will have a 6:12 pitch ( about 5-5/8" units vertical to 12 units horizontal).

 

[alora]

Just for clarification, a 3:12 pitch is equal to a 14% slope, not a 25% slope. A 25% slope will have a 6:12 pitch (6 units vertical to 12 units horizontal).

Just for clarification, you may not know what you're talking about.

 

[Rio]

I've attached a diagram showing what I believe is of a 3:12 and the resulting slope. Please let me know what is incorrect about it so I don't repeat the error. 14 degrees in 90 degrees is equal to 15.56 in 100 so the slope is, by my calculations and per the diagram, 15.56% View attachment 678Thanks!

View attachment 316

3_12 PITCH.pdf

3_12 PITCH.pdf

 

[Francis Vineyard]

Rio

25 percent slope equals 14.056 degrees! alora quoted from the code.

 

[Darren Emery]

It looks like your mixing degrees and percentages.

To convert rise:run to percentages, divide the first by the second.

3/12 = .25

4/12 = .33

5/12 = .416

6/12 = .50

etc...

It's percentage of rise to run as a ratio, not percentage of angle of degree to angle of degree.

 

[Rio]

Thanks to all for the clarification of the clarification, lol.

 

[peach]

Ok, so ...IRC R802.2:

"Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."

your point alora exactly is what?

Really low slope roofs are sometimes treated as walls.. that's why the reference to Chapter 6.

 

[GHRoberts]

Really low slope roofs are sometimes treated as walls.. that's why the reference to Chapter 6.

Perhaps like floors.

 

[peach]

exterior walls.. not floors.. Chapter 6, not 5