I'm really hoping someone will jump in here and help me out on this....The house design is a high posted cape, 4' knee wall, 24'w x 32'l. 3 bent, 2 bay. Common rafters from ridge beam step lapped into top plate. Ridge beam will be supported at each gable end and at it's center point. My question: Is it preferable to have the tie beams interrupted and tenon into a continous vertical ridge post, or break up the ridge posts and have them tenon into the top and bottom of continous tie beams?? If that's the case, would radial/tangential shrinkage of tie and ridge beams leave the interrupted ridge posts not in complete bearing contact with the tie beam and ridge beam, creating a non-supportive situation? Frame will be mixed oak.
Am I correct in thinking the supported ridge beam will eliminate all rafter thrust at the plate?
I want to keep the frame design in the traditional manner. How was this framing situation addressed 200 years ago?
I will have the final design reviewed by a structural engineer, I'd just like to know the correct way to approach this before $60 per hr. drawing begins. Thanks, Ron

I prefer the continuous ridge post. I think if you use a nice long spline, you can break up the tie and keep a lot of the tensile strength. How are the rafter peaks connected to the ridge? If they can pull away, then you'll get rafter thrust. If you can secure them really well, it's less of a problem. What I'm thinking about is a hidden metal strap running from the top of one rafter, up over the ridge, and to the top of the other rafter on the other side of the ridge. CB.

Derek and daiku, thanks for your input. No drawings yet Derek, and as I am severely "computer challenged", it's highly doubtful that I'll figure out how to post pictures any time soon.
daiku, rafters will be tenoned to ridge as shown on pg. 38 of Jack Sobon's "Historic American Timber Joinery". Yes, I am a card carrying "Sobonite", a hard core traditionalist, and am very reluctant to break up that tie, as I don't believe it was done that way in the past. But I like your suggestion of the Simpson strap. I don't like using hardware in my frames, but if it's hidden and will make the frame stronger, I can compromise. Thanks to both you guys for your help. Ron

Ron,

to play devil's advocate--

I don't really consider a 4' kneewall and a structural ridge to be a traditional framing pattern.

It is my understanding that a structural ridge eliminates all thrust. For this to be the case, the peak of the rafters needs to be supported by the ridge --usually accomplished by sitting them on top of the ridge. The ridge also cannot sag, or the rafter peaks drop, pushing the rafter feet out and the wall right along with them, resulting in derek's observed slowly collapsing house.

As daiku said, you could instead fasten them to the ridge in a way that they cannot withdraw from the ridge. In effect, hang them from the ridge. This means a tension connection. The strap daiku mentioned may be your only hope here, because I would hate to assemble a common rafter/ structural ridge roof with wedged thru tenons into the ridge beam (4" tenon is not a tension connection).

So, in my opinion unless you support the peak of the rafters with the ridge (not just lean the rafters against it -- tenon or no tenon) you don't have a structural ridge and you have thrust. And thrust is bad with 4' kneewalls. Really bad.

Also, that ridge is going to need to be stout to carry half the roof load and span 16'. I'm not an engineer, but I don't think an 8x12 is even close. I'm guessing 8x16 or better in mixed oak. I haven't seen any ridges that big in old frames. I have in some new stuff and it can look pretty good, but it ain't traditional.

As for shrinkage, stacking interrupted posts on beams in green oak might not be good. If you have a deep tie and a deep ridge (24-28" combined depth for this example's sake)they could easily lose an inch and a half of height. That drops your ridge an inch and a half, and if your wall plates don't drop the same distance, your rafters are now too long and you have thrust. If you do interrupt posts, use very dry stock and keep the depth of beams in the stack to a minimum. I don't like more than 16".

Have to run, but I'll be back

cheers,

Gabel

What is the intended pitch of this roof?

Jim Rogers

Gabel's answer is right on. If you want to eliminate thrust, fully support the tops of the rafters. This means a stout ridge and you should probably plan on braces from post to ridge for racking reasons as well as stiffening. Make the braces as long as possible and accordingly fat in section. I, too, favor continuous post/interrupted tie. As always, it's best to get an engineer's input (one who KNOWS timber frames!) for beam sizing. All the best.

Posting drawings should work just like posting pictures. My drawing software (Vectorworks)lets me export drawings to other formats such as DWG/DXF, JPG & TIFF. Here is an example...

Tom

Yea Derek, that sounds about right, but this frame design is veering towards something from one of Benson's coffee table books,(..."not that there's anything wrong with that..." ), but it's not how I want to build. This frame design was not my idea. The original plan was to go with a gambrel roof, queen posted supported purlin plates defining break in pitch, and nice short common rafters, but the architect kinda changed their minds with his design, which incidently did not include a ridge . I had doubts about this framing scheme all along, and Gabel, the issues you brought up pushed me over the edge. The gambrel I know is rooted in tradition, and I have confidence in the design. I am going to try to convince the clients to stay with the gambrel.
Gabel, kudos for the very helpful response. You really brought up some relevant considerations when using a structural ridge. I get your point of resolving the thrust issue before it ever reaches the plate. In regards to your comment about this framing scheme not being traditional, and I've also seen it expressed in these forums before, I am curious as to why? Why wasn't a structural ridge beam used in the past?My observations of old barns in my area of N.E.Ohio back up your statement. The ones I've seen are common rafter,supported mid span with purlin plates braced and strutted down to a dropped tie beam.
Jim, the proposed roof pitch is 7/12, which further adds to the problem. Ron

derek,

That's a good solution to house and birdsmouth the commons, spiking to the ridge, and strapping over the ridge to the other common. I like it.

Ron,

I wasn't trying to talk you out of anything, just present a few relevant thoughts.

Don't forget that the architect and client both trump the framer when it comes to design issues. How important is having a traditional framing pattern to the client?

You can definitely build a good, strong, and attractive timber frame with a structural ridge. Derek described a pretty good recipe, and for what it's worth, I think the frame derek described is simpler (read cheaper) than the one you are describing. May or may not matter, but budget is pretty important on most jobs I work on.

Keep us updated.

Happy solstice all.
Here comes the sun.

Gabel

Gabel, I agree Derek and daiku made some good points and gave very helpful advice. But I tend to be drawn to and have faith in frame designs with a historical track record that backs them up. From what I've read,(someone please dive in here and correct me if I am wrong), a design with a structural ridge,with either continuous or interrupted support posts, was not built much or at all in the past. I believe that this framing option to resolve plate thrust certainly occurred to the framers of the past, but they chose not to build that way. And I'm sure they had their reasons why. And I assume they aren't good.
I'm not pushing the gambrel on the clients, it was their first choice, not mine. Ron