Bent raising question

[Sly]

Hi everyone,
I'm new to the forum and recently started getting interested in timber framing. During the day I'm an architect and woodworking has always been a hobby of mine so i quickly took interest in timber framing. Plus who doesn't like building stuff!
But enough intro...

I am currently working on a 12 x 16 cabin to be used as a hunting camp/winter refuge. It is similar to the Sobon cabin except that i wasn't planning on using a single top plate to rest the rafters on. Rather the rafters are incorporated into the bent and the bents would be tied together with connecting beams and purlins on the rafters. If my description isn't clear let me know.

My question is: should I assemble the bents and rafters on the ground and try to raise the whole thing or raise the "wall part" of the bents connect them together and drop them rafters in afterwards. My gut feeling is that if we raise the whole thing, to much stress will be put on the rafter/post joint and it would be safer to raise it separately but i'm sure one of you has some insight on this.

Thanks in advance!

Sly

[dukndog]

First, Welcome to the FF Sly!!
Second, I've not been involved with a raising yet, but would tend to agree the stress on the rafters would be pretty high attached to the bent while raising.
I'm sure your question will be answered by someone with more knowledge shortly.

DnD

[Brian_Weekley]

Welcome, Sly.

A picture is worth a thousand words--it would be helpful to better understand your rafter/purlin/bent design.

[Sly]

Ok, i hope this works!
I sketched out the design and uploaded it. I gave it done on sketchup but it's not up to date. i'll update it to the final design and upload it



 

[Jim_Rogers]

The only way you could raise this design with the rafters attached is to put some series well attached "strong backs" across the joints. And you'd probably have to leave them there until after you have all the purlins in and everything is secure.

The next problem is putting the purlins in.

If you stand up bent one, and it is braced off. And now you're going to have to stand up bent two you're going to need to have your connecting girts in place in bent one and ready to be slid into bent two as it's being raised. With any braces that you may or may not have between bent two and the connecting girt.

And bent two will need to have just as many strong backs as bent one, while it's being raised.

If the purlin system is a drop in system then you have seriously weakened the rafters, especially the rafters for bent two.

At your location your snow load should be considered, and the rafters should be up-sized over what is shown in the book.

I have some serious concerns over your design changes.

Good luck anyway.

Jim Rogers

PS Welcome to the forum.

[Brian_Weekley]

Sly,

I'm not an expert by any means and currently building my own version of the "Sobon shed" also.  However, I question the use of 6x6's throughout.  He uses 8X8's in his book.  Mine is slightly larger (14x20) and he recommended that I actually use 8x10's for the posts and tie beams.  Depending on the snow loads in your area, you probably should run that by an engineer.  As far as your raising question, the rafters would typically go on after the frame is raised.  I'm sure the experienced timber framers will weigh in with better advice.

[Sly]

Hum.. My original intend was to have drop in purlins with dovetail joints so the rafters can't move away from each other. But i see the point made about the middle rafter being weakened. Is there another of connecting the purlins aside from drop ins?

At least my original question is answered. From your comments, it sounds simpler and safer to raise the walls and roof separately.

I have only the floor done anyways so i can still play around with the design.
Thanks,
Sly

[Jim_Rogers]

Well dove tail joints are not the best. The reason why is that they shrink from left to right like this:



The red dimension shows the way they will shrink. And if they do shrink then they will/could pull out of the rafter. Although the roof decking will hold it somewhat.
But if you're going to drop in dove tails then you need some width to the rafter in order to create a good tail.
If it is a 6x6 then there isn't much wood there to make the tail mortise on each side.
And by cutting away the top surface of the rafter you have reduced it down to a smaller weaker size.
Let's say you will cut into the rafter 2" on each side for a two inch tail and mortise. That only leaves 2" full up the middle. Now you're basically asking a 2x6 to hold up the entire center section of your roof. Not something that will work very well at all.

In my area we needed a 4x6 to hold up a 2' span when using boards for a roof deck. You're asking a 2x6 to hold up a 8' span.

I guess my question(s) is why don't you just do it as drawn in the book?

It is a proven design.

I/we have built it here:



Jim Rogers

[Sly]

I don't have the Sobon book, i've been using a book by Todd Benson. but point taken! i'll double up on rafters and take out the purlins instead. i'll most likely have to splice together two pieces to make the two top plates since i don't have 16' pieces. 

Thanks for all the input!

Sly

[Jim_Rogers]

Buy the book.
You can get it cheap online or ebay.

[sap retreiver]

Hey guys, I too am new and thought of the same design for the 16x24 sap house I'm designing and already milled out for. How do i go about making the same four bent design, or is it not possible?
Yes i now know i should have bought some books or just gone to an engineer. My backround of carpentry and a little architecture apparently has gotten me a little too deep.
I also was going all 6x with 4x purlins but its also red oak.
  Thank you for any help, back to the drawing board

[Jim_Rogers]

Do you have 24' plates and sills?

If not then you're going to have to cut a scarf.

Jim Rogers

[Dave Shepard]

Welcome to the Forum sly. The Benson influence was immediately apparent. Why are you using this over the traditional plate? Not having a plate crates a weird, and in my opinion, uncomfortable, aesthetic at the wall to roof plane intersection.