Author Topic: ridge board/beam  (Read 23455 times)

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Offline devildog

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ridge board/beam
« on: June 03, 2009, 06:34:28 PM »
I did a search on this and couldnt really find an answer, maybe someone can help.
do you need the ridge beam only if you want an open ceiling ?  Ive noticed some builders use beam and some use 2x boards.
I only assumed, because I dont know, that you have to use collar ties( I think this is the name) if you use a 2x. I actually like this look, and it would seem to be less cost and easier than the beam.
thanks for your help! oh, and this is for the V.C.
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Offline Don_P

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Re: ridge board/beam
« Reply #1 on: June 04, 2009, 05:32:06 AM »
A few definitions might help to start;
A ridge board is a 1x or 2x and is not structural, it simply provides something to nail the rafters to. It is not absolutely neccessary and can be replaced by a gusset at the peak of each rafter pair as in a truss.
A collar tie is under the ridge board, in the upper third of the height and is to prevent uplift from tearing the rafter pairs apart. They are required at a minimum every 4' but can be replaced by straps over the ridge as wall. The gusset can also serve this function.
A rafter tie is at the bottom of the rafters and prevents the rafter feet from sliding outward as load is applied to the roof. It can also be the floor or ceiling joists. The rafter ties are on each pair and may not be raised more than 1/3 of the way up. As they are raised the rafters must be upsized because the tie is inducing a bending load into the rafter.
I think that covers the basics of a ridge BOARD roof. Typically this is prescriptive, following provisions in the codebook.

A ridge BEAM roof works a little differently. The beam is sized to safely support typically half the width of the roof area it is under, 1/4 of the roof on each side is supported by the walls. The rafters are basically hanging off the beam and so horizontal thrust is eliminated. If the ridge can't drop the walls cannot spread. There is then no need for ties across the lower third, there still needs to be something up high to prevent the roof from blowing apart if there is uplift or if the building inflates. Often this is through bolting of large beams to the beam or straps over the top but it can be through the use of collar ties. You need to support the ridge beam at its ends on columns that deliver the beam load down through the building to adequate footings. Typically this is engineered with each component sized for adequacy.

Well, I didn't get beyond definitions but hopefully it will help  :)

Offline John Raabe

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Re: ridge board/beam
« Reply #2 on: June 04, 2009, 09:23:36 AM »
The VC (Victoria Cottage) plan has been designed with ridge beams rather than collar ties. This is because the fully developed plan could not have collar ties without restricting headroom.

Actually a collar tie to perform its duty best ties the roof rafters together in the bottom third of the roof height (not the upper third). You can normally do this tie on every other rafter pair, or 48" o/c for rafters at 24" o/c.

The VC plans also have a simplified studio house plan for a stand alone 16' wide building. In the "alternative cross section" on page 1 of the big sheets, I have a modified layout that triangulates the rafters at each rafter pair. This tie is higher than a normal collar tie but since it works more like a truss it has been approved in most locations. If your inspector wants an engineer to stamp this configuration you might be better off to go to a local truss company and have them design a truss to your local loads and then they will provide the engineering work for the inspector. Truss company engineers have tricks and can often design a lighter truss than the continuous 2x8 rafter that I show. In some cases the manufactured trusses can cost less (installed costs) than a rafter setup with plywood gussets.

In buildings where the collar tie can work in the lower third of the rafter height and still provide plenty of headroom, such ties work very well and can provide an open ceiling feel with a place to hang lights and fans that are more problematic with a ridge beam.

A 14x24 office/studio

And here is a scaled detail of the rafter tie and how it can be used


This from our sister subscription site: List of the 42 downloads from PlanHelp.com

« Last Edit: June 04, 2009, 09:56:59 AM by John Raabe »
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Offline Don_P

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Re: ridge board/beam
« Reply #3 on: June 04, 2009, 10:43:21 AM »
There seems to be a bit of confusion of the terms collar tie and rafter tie, this is very common and leads to much confusion, I wish there were better terms. I'll quote from the current code to hopefully clear it up. You can find this in chapter 8 of the IRC beginning at section 802.3;

Framing details: Rafters shall be framed to a ridge board or to each other with a gussett plate as a tie. Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter...

Ceiling joist and Rafter Connections: Ceiling joists and rafters shall be nailed to each other and the rafter shall be nailed to the top plate. Ceiling joists shall be continuous or securely joined together where they meet over an interior partition and are nailed to adjacent rafters to provide a continuous tie across the building when such joists are parallell to the rafters...
 Where ceiling joists are not connected to the rafters at the top wall plate, joists connected higher in the attic shall be installed as rafter ties... Rafter ties shall be a minimum of 2x4" lumber. Where ceiling joists or rafter ties are not provided, the ridge formed by these members shall be supported by a wall or girder sized in accordance with accepted engineering practice.

Collar ties or ridge straps to resist wind uplift shall be connected in the upper third of the attic. Collar ties shall be a minimum of 1x4 inch lumber and shall be spaced not more than 4' on center.

(I did some editing to try to make it read easier)

Hope this helps clarify the difference between the two and their functions. Rafter ties, lower third or below, each rafter set, thrust resistance. Collar ties, upper third or higher no more than 4' apart, uplift resistance :).

Offline devildog

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Re: ridge board/beam
« Reply #4 on: June 04, 2009, 02:44:32 PM »
Thanks Don P and john,
 youve both helped me understand better. I would think using 12ft walls with loft floor being about 9ft after floor joist and decking are put down that id have plenty of room for collar ties(1/3 down from ridge board). and in the open area having rafter ties ( 1/3 of the way up from top plate). Like I already mentioned I kinda like the look of the exposed boards.
I 've got another question. what is the best method  or code way of attaching these? nails, bolts,....?
Some people spend an entire lifetime wondering if they made a difference in the world. But, the Marines don't have that problem.
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Offline Don_P

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Re: ridge board/beam
« Reply #5 on: June 04, 2009, 07:07:00 PM »
IIRC you're in Hillsville? I'm next door in Grayson Co. Our inspector is pretty much by the book, you all have been through a few, I don't know what yours is like. I think the one everyone was complaining about is gone. Crossing each county line up here is like entering another world enforcementwise. This design would be outside of the prescriptive code so don't be too suprised if you need an engineer. Do keep your silt fences up and in good repair, your county is hot and heavy on erosion right now, they got busted by the state.

I haven't been able to access the Virginia online codebooks today, I've used it this week hopefully they'll be back up, this is the full list of VA books, you'll want to look in the residential book, chapter 8 is roof construction;
https://www2.iccsafe.org/states/Virginia/

For right now I've been quoting from NJ which is on the same year version (06) and reads very close, the tables I'll send you to look to be identical stock versions, look in chapter 8 (keep trying the Virginia link though);
http://www2.iccsafe.org/states/newjersey/NJ_Residential/Residential_Frameset.htm

Table 802.5.1(2) is the rafter span table we use, 20 lb Live Load(snow)/10 lb Dead Load, confirm this with your inspector. Notice at the bottom of the table is a rafter span adjustment factor that needs to be multiplied by the table's allowable span to to get your allowable span if you raise the rafter tie.
Table 802.5.1(9) at the end of the rafter span tables is the connection minimums for the rafter tie and your floor joist connection to the studs if he allows them to be called the tie (this is where you may run into engineering) Read and understand footnote G. By raising the tie the forces on the connection are increased and it requires more nails (no bolts here, many nails are actually better). Notice also that the table is calling out 16d common nails, this is not a regula hand drive sinker or a gun nail but a honking big common nail. There is an interchange chart I can dig up a link to if you want it. I'll give you a little aside here, I've calculated the forces before and the codebook is actually light on the number of nails, they are counting on help from the roof ply diaphragm, I wouldn't, don't skimp  ;).  Remember lower is stronger so don't raise rafter ties any more than you have to.

The collar tie nailing schedule is in chapter 6 with the main house nailing table, R602.3(1), all the way at the bottom. ignore the rafter tie spec I'm sure you'll see there, the one I gave you above is the correct one in chapter 8. You'll also see the rafter to ridge specs in the chapter 6 table.

Offline John Raabe

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Re: ridge board/beam
« Reply #6 on: June 04, 2009, 08:50:16 PM »
Thanks for the info Don_P.

I've been known to use the term collar tie to apply to any type of rafter tie.

These diagrams might help:


From Wagnor, House Framing


From Thallon, Graphic Guide to Frame Construction
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Offline Don_P

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Re: ridge board/beam
« Reply #7 on: June 05, 2009, 05:05:02 AM »
I'm equally guilty John, I've been trying to use the precise terms since the requirement for both and the specific seperation of terms in the code, this has all cropped up in the past few code cycles and often has inspectors as confused as the public. The collar tie and raised rafter tie adjustments are both fairly recent changes. What you suggested above was code in the 2000 and previous versions. The tables and text have all been changing since that time. Often in the field we learn about code changes the hard way.  Like I said, I wish the terms weren't so similar. Thanks for putting up the pics. That is an excellent set of graphics and captions  [cool]

Offline devildog

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Re: ridge board/beam
« Reply #8 on: June 05, 2009, 02:49:35 PM »
Thanks againfor all the info, Im trying to start thinking about  everything I know I dont know now ,so when the time comes I wont be completely lost.
don, glad to know I have a neighbor on here with local and code knowledge. Are you building right now? I have talked tothe building inspector his name is jim whitten. He  was very nice and said  if I would have appllied for my building permit before may 1 2008, I didnt need aplan. but now I do, I told him I purchased plans on the intrnet and he said and acted like aslong as I had aplan bring it in and in about 15min. Ill have my permit. I hope its that easy.
                                                         darrell
Some people spend an entire lifetime wondering if they made a difference in the world. But, the Marines don't have that problem.
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Offline Don_P

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Re: ridge board/beam
« Reply #9 on: June 05, 2009, 07:36:06 PM »
That sounds great, and it is a good idea to make all the mistakes you can on paper. Back when I built 20 years ago our inspector then sent me over to the restaurant and told me to sketch it up on a napkin. I do get along fine with our current inspector, I was one of his first assignments and he's still there, that's gotta count for something. He's not that much different he'll just ask for engineering if something's outside of the codebook provisions so I figured I'd prepare you for that just in case. I've been working in the county to the north and he's pretty easy but that can also bite a person, an owner builder there forgot to put any vents in his plumbing and it didn't get caught, the plumbing didn't work and the sheetrock was already done. I actually don't mind a thorough inspection. The majority of our inspectors do come from the trades so they've been on both sides of the counter. I'm a real small residential contractor, if you get stuck feel free to holler, I might not be able to help but I can certainly commiserate.
 We adopted the 2006 IRC in May '08, but we had previously used the '03 and '00 versions. The ICC updates it every 3 years and we adopt it statewide 2 years later after review. Before that we used the old CABO 1&2 family code for a long time. I grew up learning the SBCCI code that ran from NC south and we built under the UBC codes out west, so I kinda had some background with all three model codes before the merger and the ICC codes came out.

I have ripped out scale sized material before and framed a scale model with a hot glue gun. That is also a good way to help know what you don't know  ;).

emcvay

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Re: ridge board/beam
« Reply #10 on: September 29, 2009, 03:25:38 PM »
OK now I've got myself asking questions *snicker*

I'm building the 14x24 and am under the impression that the loft joists are providing this function.  Am I wrong?

Offline MountainDon

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Re: ridge board/beam
« Reply #11 on: September 29, 2009, 04:21:36 PM »
I'm not sure exactly what you mean.  

Rafters as drawn in the Little House plans use a ridge board. The load on the roof can be broken down into a downward vertical force on the wall and a horizontal outward force on the wall top. Your loft joists/beams keep the walls from being pushed apart at the upper end. A well designed and built side wall transfers the load to the foundation.

Using a ridge beam the loads are transferred down from the ends of the beam through the end wall to the foundation. Most of us owner builders do not find ourselves needing a ridge beam. The ridge board is more or less just something to help tie the upper ends of the rafters together.

Does that help?


« Last Edit: September 29, 2009, 05:55:30 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

emcvay

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Re: ridge board/beam
« Reply #12 on: September 29, 2009, 05:01:21 PM »
Thanks Don, that is what I thought.