I am trying to design a frame for a 22X32 foot, 1 1/2 story with a full plate set on top of a 3 foot kneewall, 4 bents total all 8 x8 posts and 8 x8 beams(?). I would like to use 4X6 whitepine rafters with an open mortise at the top and the same size collar ties. My problem is I know the closer to the center (lower down from peak) the ties are the more they help prevent rafter spread. But that starts to reduce my head space below. The rafters would be just a little over 13 feet long from plate to peak. If I set the ties 3 feet down from peak that still leaves decent head space but I am not sure if that will be a good solid roof. The roof will be an 8/12 pitch with the rafters 2 feet on center and will be covered with Foam panels and then metal for a house in Vermont. Any opinions on this would be welcome. I would rather overbuid some then cut it close as I am cutting my own timbers so within reason a little bigger timbers or closer spacing is O.K.. Also would like to have 2, 6 foot wide gable dorms in each roof, but am not sure if they would affect what I have planned. If my idea seems structurally sound I would skip a pair which on the 2 foot spacing will leave my 6 foot for the dormer. Thanks much for any thoughts.

jcahill:
Years ago, there were tests done on collar beams and their locations. It was found that they are not in tension (preventing rafter spread) until they are within 1' above the plate. Any location in elevation above that they are in compression, preventing the rafters from sagging from the weight of the roof, live load and dead load.

You may need to have your design reviewed by an engineer who is familiar with timber framing and have your beams/timbers correctly sized for the loads and spacing.

Good luck with your project.

Jim Rogers

I would think about changing the roof to a 12/12 and drop the knee wall to 2' or a little less,(the distance from the top of the tie to top of plate), would help those engineers.

How about a set of crucks on the 2nd floor supporting a purlin plate to carry those rafter, with the dormer stretched out to most of the middle bay? This would add to the U.S. inventory.

I agree with Tim here, crucks or similar arrangements are ideal if you wish to have a totally open attic space. If you don't have or don't want to deal with curved timbers, the Germans have an equivalent system that is essentially the same thing.

I think this was about 22x32... but with a full length ridge.
2' knee wall.



You'll likely need to up the size of the posts and the bent ties, depending on how you control the outward thrust.

You could also go queen post truss for the middle 2 bents, depending on how your floor plan works out.

I'm in BTV. Send me PM me if you want to talk about design.

Isn't the outward thrust 0 when you have a supported ridge beam like that? From what I've been told, if the rafters are fastened on top of the ridge beam properly, then the ridge beam and the rafter sills split the vertical load without having any outward thrust, thus allowing for a higher knee wall. Please correct me if I'm wrong.
Rusty

yup, if you hold up the rafter @ the peak and @ the foot, and the posts under each run to the ground, you have theoretical 0 outward thrust (from gravity loads, not taking into account wind). in the example i posted the center 2 bents are clear span, so they act as trusses. they carry the ridge load to the tie beam, and if the tie beam is not strong enough to carry the load (which it is for roof load) - then it would go into truss mode and i have to take the outward thrust into the posts.

To all, thanks for the advice. I have done 2 small timber frames and both used this type of roof but it sounds like it will not work for a larger building. So the principal rafter (i think this is the correct term for large 8x8 beams? that go top of each post, meet at ridge) and purlins is what I need. With this sort of span would I also need a king post or queenposts support? I plan on 3 bays 12, 8 and 12 again and the interior walls on second floor would be on same demensions so the posts will not be an obstruction as long as the tie that connects rafters and post can be set high enough for a door. Also it seems as though purlins, running parallel to the ridge would make dormers diffucult to frame since their ridge would run at right angle to the purlins. Could you place pulins down from the ridge until they would interfer with dormer, then run rafters down from the last purlin but at right angle to ridge to a plate? to frame in dormer opening? I kinda have my heart set on gable dormers and 1/1/ stories. I wanted a 3 foot knee wall and was planning on a brace from top of each beam to post extending above beam, I could sacrifice and go to a 2 foot knee wall but i know that will reduce my headspace above. Thanks for all the advice, sure learning lots. Joe

Why not run common rafters with a ridge or principal purlins?

Running principal rafters as you suggest with common purlins does not change the need to contain outward thrust unless your posts holding up the ridge run to the ground. And common purlins are a pain to join into the valleys you will need on your dormers.

This is something that I've done that has dormers in the center, with a knee wall high enough for working upstairs. This is a barn, but you'll get the idea:

I think I see what you are talking about from your excellent graphic. So what I took to be a rigge bopard sandwiched between the ends of rafters, like conventional is really below and is supporting the rafter pairs at the peak. So it is like another bent that runs in the center of the buildin, down the length and it supports the common rafters at the peak. Would I still need collar ties? I like this very much as I agree with your thinking that it will simplfy dormers. The only difference is I planned on 2 dormers per side each 6 feet wide. I had also planned on a chiney centered both length and width which would not work but that can be shifted forwardon one side with dormers to right and left. Thanks, I am starting to seem some possibilities. Joe

No, you don't need the collar ties with a ridge. Depending on frame design you may need to have a properly designed bent with a tie beam that is in tension... but that is another matter.

I'm trying to piece together some of the bits and pieces from this thread with regard to collar ties. Jim cited a study suggesting that a collar tie is only effective in preventing spread at the rafter base if the tie is within a foot of the base. The conclusion being that any collar tie position higher in the triangle only serves to prevent sagging of the rafter under load (with the force on the tie being compression).

Later in the thread, Mike says that collar ties are unnecessary with a supportive ridge.

So, taking both points into account, am I to conclude that the only reason I see so many roof structures with collar ties higher up in the triangle is because the rafters are undersized for the load?

No, I think a lot of people do not understand how a collar beam works. I find a lot of stick builders just do not want to grasp the concept.

The German "Kehlbalkendach", literally Collar Beam Roof, is a roof system that relies on Collar ties to somehow strengthen or stiffen the roof structure. The simple Kehlbalkendach consist just of rafter pair joined with a collar somewhere between the mid point and top third of the rafters. The idea here is not that the ties will resist thrust at the plate, but rather create another triangle that stiffens the roof against live loads (triangles are immutable) essentially allowing them to get away with using smaller rafters.

The spans handled by this simple roof structure are surprising.

Variations of the Kehlbalkendach are perhaps the most prominent roof structure in German speaking countries. For larger spans purlins are used to support the collars.

a collar tie works well against live loading. This is because under wind loads, the tie transfers load to the other rafter in the pair, and so 2 pieces of wood can be used to share a single load in that way.

Kevin - I would work to have properly sized rafters, and not rely on the collar tie for stiffening.

DL - all the triangles make some great bracing.
In stick framing they make a great place to hang the sheetrock. ;-)

I'm sure they are offering some sort of stiffening or bracing. The rule I've always worked with is lower third of overall vertical rise.

I'll try an throw a few examples at my FEA software and post some deflection images. In some cases on large spans I've seen bending requirements go way up at the rafter foot as the really stiff upper triangle does more harm than good (when the tie is placed too high).

Yes, the rafters need to be made of the proper size to support structural loads on their own. In the German system mentioned, the point is to resist wind loads, keeping in mind that the roofs they are used on are very steep, often steeper than 45 degrees. Essentially they are acting as another level of bracing, using the rafter to pairs to brace each other. They are also almost always placed about mid span if they are not supported in any way by purlins, and many times may serve the extra function as joists for a loft floor.