I build milled log homes and typically the width of the homes are 24, 26 and 28 foot. All have 10/12 or 12/12 roof pitch, with 2×12 rafters. Recently the company I buy product from says all ridge beams that do not have a tie at the “plate level”( a second floor joist or beam system) must have a ridge beam in the cathedrial areas designed to support the span/ load as a beam, not considering any collar ties as support. This is what they have been told by numerous inspectors in the NC and SC states. I have search the new 2003 IBC code and find nothing stating this. I do find that it reads the ridge formed at the rafter shall be “desiged with accepted engineering practices”. This in their interrpation means a engineered LVL or Lam beam…. in no way can a single 2×12 or double 2×12 be used at the ridge. I do use a collar tie usually 2×6 at 8 foot above the plate line, working out to approximately 2/3 up from the plate line with the remaining 1/3 from the collar tie to the ridge. Ties are 2 foot or doubled at 4 foot on centers. Rafters are 16 or 24 inch centers.
Any one else have this problem? I am currently having a structural engineer calculate the ridge and his read on the code. …. the latest project is a 12/12 pitch 24 foot wide no loft and the company says the ridge is to be a LVL 27 inches deep with a width of 6 inches and a length of 34 foot the building is a Chapel 24′ x 34′.
Edited 8/19/2004 11:25 pm ET by Ranger
Replies
Collar ties set as high as yours are generally don't qualify as structural ties designed to prevent the walls from spreading. They could, and I emphasize could, also create excessive loads on the roof rafters. Collar ties as you describe would not pass inspection in my area.
It may be that structural design, code compliance, and/or liability issues have caught up with the company designing and selling these homes, and the results are from them taking a closer look at their designs and how they comply with the code.
Perhaps as the company has expanded, it's now looking to incorporate some sort of umbrella-type of national code compliance in their designs.
I think thats why god invented scissors trusses.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
Ranger, I think I can add some insight here.
Collar ties in the upper 1/3 of the roof are intended to resist uplift of the rafters...or pulling away of the rafter from the ridge. There are two types of ridges, ridge beams and ridge boards. Ridge boards can be puny little things...1x8's for example. They carry no gravity loads. The two rafters theoretically press together at the ridge. In this case, gravity loads are translated to horizontal and vertical loads at the wall line. The vertical loads are resisted by the wall and the horizontal loads, which tend to spread the walls apart, are resisted by ceiling joists, tie beams or the like...collar ties will not work to resist these horizontal loads because they are too high up the rafters to be of much use.
Now, for ridge beams, the ridge is sized to carry the tributary area for the portion of the roof that it supports. If sized correctly, the beam carrys half the vertical roof loads and the 2 walls carry the other half. There is no spreading forces with this situation since the only way the walls could be spread is if the ridge deflects downward. A properly sized ridge and proper rafter to ridge connection is important in this condition.
It sounds like in your case, with a cathedral area, a ridge beam is required unless you want to break the space up with tie-beams at the top of the wall elevation
Hope this helps.
you want to break the space up with tie-beams at the top of the wall elevation
The cabin I am currently building for myself has a 10/12 pitch roof and a laminated ridge board (lightweight)to nail the rafters to.
I designed the (4x8 42" o/c) tie-beams to make the thing structurally sound. They will also become the (2x6 t&g) floor support for a loft. I have two 24' beams spaced to tie the open living/kitchen span together. Once the whole thing is finished it should be plenty strong. My only fear is the longer unsupported beams would do a serious twist or something.
I have used a ridge beam many times, but it is a chore to get one up there when craneless........plus the tie beams serve double duty as a floor.
It sounds like you have the right idea. Lightweight ridge boards require something such as the tie beams that you mentioned. The connection between the tie beams and the wall and/or rafters is critical. If you're worried about the long beams twisting, timber blocking can be installed by bolting them through the long beams at the mid span. This should help with any potential twisting
timber blocking can be installed by bolting them through the long beams at the mid span.
Could you explain what you mean by timber blocking? Thes 24 footers are totally exposed except where they tie into the wall.
I'm not sure if I read the original question right but here's what I meant about timber blocking.
First, I assumed that you were worried about twisting of the long tie beams...this might or might not be a concern depending on what kind of beams you use.
If you're using some beams where twisting is a problem, you could add blocking from the center line of one beam to the next adjacent beam and continue on down the line in this manner...centerline blocking. To keep the timber atmosphere of the house, you could use blocking that is the same size as the tie beams. Doing this will help restrain any potential for twisting of the main tie beams.
Lurch is totally tright, and you could read more by doing a search here on the terms Collar tie, and Rafter tie, and structural ridge.
A collar tie is defined as lurch mentioned, in the upper third of the rafters. A rafter tie does the structural work you are looking for to be able to eliminate a structural ridge. It lies in the lowest third of the rafters, preferably at the plate level to resist the outward thrust created by gravity on the roof system and transfered to the walls.
An engineer could calculate some factor for the resistance to those thrust forces inherent in a solid log wall that does not exist in a simple doubled 2x4 top plate on a framed wall, but he still needs to account for every pound of thrust applied.
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ranger... almost finished with a structural ridge job..
the building is 20' wide.. the unsupported span is 27'..
the PE called out an Anthony Power beam.....7" x 18" total length of 39'..
roof pitch is 12/12..
so , yes.... what they are quoting to you is not unreasonable
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike, on that picture "rd11 powerbeamjpg
I was framing by the hour for this guy, and had the exact setup as this picture
Built my gables and man handle the laminated beam into position.
Pulled a string at the ridge beam from gable to gable on both sides of the
rafters. Then proceeded to cut each HAB(height above beam) so that I would
have a straight ridge line. My boss at that time said I should have cut them (HAB)
the same so that when the beam settled it would be straight. I think he was
more worried about the amount of time it took to do this. So my question is
to you and anybody else is, what is the "proper" way to do this. If you have
any deflection in your beam and your rafters were straight before the deflection
you'll have a sag. Every laminated beam that I've seen has at least 1/2" crown
in it,and From the picture it looks like you cut your rafters "straight" from gable
to gable.
butch... we're not framers... so i don't know any production tricks.. but once we got that sucker set. there was nothing more we could do with it....it was what it was
we strung it from gable to gable and measured our total run on several rafters.. they were all right on....
the top seat cut was all over the place.. at first we measured from the front edge to the strung center line.... then we tried measuring from the strung centerline to the top of the beam ( or HAB ).. the final solution was to measure the diagonal from the strung centerline to the front edge of the beam ...
we used a tail jig .. pulled the overall length and then laid out the digonal measurement with a Big 12 speed square... with that beam... that span... i don't think we got any downward deflection..
and we did pairs of rafters ... about every 8' so we got no sideways deflection..
once we got on a roll .. it went pretty fast and we really didn't care where the beam was ... the ridge was straight and true... the beam was pretty straight also.. just not true.. but once the crane unhooked.. we had no way of effectively moving the beam around.. it was supported at each end and about 1/3 from the north end on 6x6 built up posts......
you can see the diagonal measures written on top of the beam for each seat
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 8/21/2004 7:53 am ET by Mike Smith
Mike,
Nice pics and timely to the discussion.
So the horizontal framing from rafter to rafter in the done pic is a ceiling only, not structural at all?
Talk about the load paths to the foundation; I interested in the header and it's support for the large window, and where the jacks landed to carry the load down.
Why do you think cutting the birds mouth at the top was problematic? I'm pretty sure I understand what you did, but I am not sure I understand why there was any discrepancy other that that there was some and sometimes that's just the way it is. No one is really sure what is causing it, and sometimes, the fact that there are several things causing it makes it difficult to identify.
BTW, the ridge looks like joinery to me. Fine job.
EricEvery once in a while, something goes right!
eric.... the beam on the south gable was supported by a built-up 6x6... bearing on a double 12" LVL bearig on doubled jacks bearing on squash blocks in the i-joists... bearing on double studs on the first floor
the off-center built-up post is bearing on a 6x6 post in contact with the 2d floor Advantech.. bearing on a 3x3 footing formed into the slab
the north end bears on 6x6 post to the foundation wall..
the reason for the problem has to do with the north end..
this is not square to the existing structure.. and the north end is clipped.. and the plumb line from the peak to the centerline was off.. all in all i kept everything at that end within 3/4" of where it was suppsed to be.. but that still throws off the rafters ..
hey, we muddled thru..
here's that problematic north gable..Mike Smith Rhode Island : Design / Build / Repair / Restore
Thanks Mike,
I remember viewing those photos.......looked like a crabby situation, or a real head scratcher as I like to say!
>>the off-center built-up post is bearing on a 6x6 post in contact with the 2d floor Advantech.. bearing on a 3x3 footing formed into the slab
So the ridge beam is not clear north to south? Where did the midspan post end up?? In a wall I assume? Could the beam have been sized bigger to carry with out the support. Could you have assembled lvl's to the same effect?
I don't have a lot of experience with engineered lumber that is why I am so inquisitive. Thanks.
EricEvery once in a while, something goes right!
no reason not to break the span..
and the lvl composite would have been a bear to assemble, then pic and set..
anyways.. we didn't design this or we would have been a little more cost-effective..
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike,
Great work as usual. You inspire me even though you aren't a framer :-) I talked to a guy at the I-joist supplier. We use Roseburg I-joists, lvls and glulams. I had him spec a ridge beam for my brothers house. His program told him a 5 1/8" x 16" x 22' would work with about 3/8" deflection over time. I asked him what a 5 1/8" x 18" would deflect and the program told him less than a 1/4 of an inch. When I asked our lumber salesman what the price difference was between a 16" and an 18", he said 1$ per foot. So for 22 dollars, we have almost no deflection. I usually size up one now because it's just really cheap insurance.
We've used Roseburg BigBeams glulams before. We set one that was 36' long and 8" x18" deep. I had a hard time climbing the ladder to set that one. The guys made me set it myself . . . . . naw we had a forklift. It was great walking an 8" beam. It spanned across a 3 car garage.
Here's a pic of the beam we set in my brothers house. Fortunately it was nice and straight.
"but once we got that sucker set. there was nothing more we could do with it....it was what it was"
Mike, that was my point. So I guess what I was trying to ask is you made your rafters
straight from gable to gable, and you weren't worried about deflection?
The reason I'm asking is whether I did the right thing by cutting the HAB so that it
was straight instead leaving all HAB the same thus showing the crown in the ridge.
Does that make sense????
don't know..depends on how much deflection i guess... in this case it wasn't downward deflection it was sideways.. the ridge is nice and straight as a stringlineMike Smith Rhode Island : Design / Build / Repair / Restore
We just did a beam this week,built it on the ground and picked with a crane, just like Mike and I discussed last weekend at Rhodefest.
48' long 14" tall and 5 1/4" wide, LVL,had the same problem as mike keeping the top of rafters the same and the added problem of two different roof pitch to still keep the ridge line the same.
We put a temp support to hold side to side, slight differences in with of the beam made for some problems but not to bad.
We didn't worry about the deflection for up or down in cutting the top cut of the rafters.Vince Carbone
One other thing where the beam is supported of course is over a window at each end and a doorway at the masterbed room.Vince Carbone
Vince,
Nice work. Looks clean. I think there is nothing better looking than a house when the roof has just been framed.
I have a question for you. Did you butt the rafters to each other on top of the ridge beam, or lap them? I like to lap them and then block over the beam because it's faster for us than butting them.
Your framing sure looks clean :-)
Thanks Tim,
the rafters butt at the ridge except where there are doubles for sky lights on one side and not on the other.Vince Carbone
Your, and Mike's, pictures have helped me out immensely on my little project.
Thank you.PAHS Designer/Builder- Bury it!