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: