Topic Options
Rate This Topic
#34062 - 10/22/16 08:17 AM truss-plate-tie joinery
tburt Offline
Member

Registered: 07/31/16
Posts: 8
Hi all,
Background: In the process of building a 24'x36' single story timber frame with dropped ties and continuous top plate. Two thirds of the roof will be 2x8 common rafters, but the last third will be 6x8 principle rafters with purlins to allow for a king post truss on the gable end (please correct me if I got the rafter terminology wrong). Roof pitch is 8/12, Vermont snow loads, and rafters will be step-lapped into the plate to create an overhang.

My question comes at the joinery around the king post truss, which will span the full 24' width of the building.

I have assumed that the dropped tie (8" below bottom of the plate) used for the rest of the building would not be the best design for the free spanning kp truss - I read somewhere that the extra forces exerted by the rafters at the plate could be problematic - but if this assumption is wrong, then that makes things easy!

In "Historic American Timber Joinery", this is the only end-post tying joint that seemed to allow for a step lap:



My worries with that joint are that it: (1) is difficult to assemble (requiring the top plate to be slid on from the side); and (2) if there is significant horizontal thrust on the step lapped plate, then it is only 1 pin holding it to the tie.

So, I then thought about a raised bottom chord, but could not find any good information about proper joinery for a span of this size (I saw some other threads on much smaller structures being ok). I am mainly be worried about the strength of the tension joinery.

My current idea is a "raised chord" that is raised just enough rest on top of the plate, but is tenoned into the rafter. Because of the 6x8 rafter and 8x8 tie, there would be extra wood for the pegs to hold onto.




I would love to hear your thoughts about solutions and/or historic precedent for this type of joint. Thanks!

Top
#34063 - 10/22/16 08:34 AM Re: truss-plate-tie joinery [Re: tburt]
Will B Offline

Member

Registered: 10/02/02
Posts: 179
Loc: Massachusetts
Tburt:
Use the English Tying Joint, or one of the other solutions that bring the principal rafter into the top of the tie, which is at the same height or above the plate. There is not nearly enough relish, IMO, for the joinery to resist the thrust on an 8:12 roof in VT, nor does the tie appear to be doing anything to keep the plate from going out. Your 2x8 rafters can still provide an overhang and you could add a sprocket to the principal rafter to provide one. However, unless this is an unheated and uninsulated building, timbers should not penetrate the insulated envelope in your climate, and the eaves should be formed by the SIPS or other insulation matrix.
_________________________
Will
www.heartwoodschool.com

Top
#34064 - 10/22/16 09:16 AM Re: truss-plate-tie joinery [Re: tburt]
tburt Offline
Member

Registered: 07/31/16
Posts: 8
Will B, thanks so much for your insight. I definitely see the strength benefit of the principle rafter terminating into the top of the tie, and if necessary, that might the route we go with and use sprockets to make the eve. This end of the structure has no walls, so no concerns about air barriers or thermal bridging.

I would like to find an option that would allow for the rafter to continue past the plate into the overhang, mainly for aesthetic purposes because the entire gable end frame will be visible (no walls).

Top
#34065 - 10/22/16 12:07 PM Re: truss-plate-tie joinery [Re: tburt]
tburt Offline
Member

Registered: 07/31/16
Posts: 8
Maybe something like this?





Edited by tburt (10/22/16 12:09 PM)

Top
#34066 - 10/22/16 04:24 PM Re: truss-plate-tie joinery [Re: tburt]
Will B Offline

Member

Registered: 10/02/02
Posts: 179
Loc: Massachusetts
That looks better, although a full-width cog is preferable to the half-dovetail on the tie-to-plate connection. Avoids the shrinkage problem that would loosen the dovetail. See Fig. 4, page 6 of HATJ.
_________________________
Will
www.heartwoodschool.com

Top
#34067 - 10/22/16 06:07 PM Re: truss-plate-tie joinery [Re: tburt]
TIMBEAL Offline
Member

Registered: 12/10/07
Posts: 1875
Loc: Maine
I'm curious about the king on the end bent, is there post under it for the end wall? Why not have the truss at the next to last bent?

Gone to all cogs with English tying joints.

Top
#34068 - 10/22/16 08:46 PM Re: truss-plate-tie joinery [Re: tburt]
tburt Offline
Member

Registered: 07/31/16
Posts: 8
Great, it seems that the English tying joint with cogs is the way to go - it had not crossed my mind to just lap the rafter over the joint to create the eve, I guess because I hadn't seen it before.

Tim, the space between the last two bents makes up a 12'x24' porch. We want the free span for more open access to that area from the three un-walled sides. The second to last bent is an exterior wall, hence no need for a truss there.

Top
#34069 - 10/23/16 08:55 AM Re: truss-plate-tie joinery [Re: tburt]
Will B Offline

Member

Registered: 10/02/02
Posts: 179
Loc: Massachusetts
Tburt,
The joinery success is dependent on the actual thrust on the foot of the rafter and the shear of the relish beyond the tenon, plus the pin. Put a notch in the tie and shoulder on the underside of the rafter to provide more bearing. If in doubt, get an engineer to design the truss and/or review the design.
_________________________
Will
www.heartwoodschool.com

Top
#34070 - 10/24/16 04:50 AM Re: truss-plate-tie joinery [Re: tburt]
TIMBEAL Offline
Member

Registered: 12/10/07
Posts: 1875
Loc: Maine
For rafter tails on a purlin system, I have used a combination of purlin and short rafters set into the bottom purlin, the rafters plugged into the purling and run four feet to the plate then gave an over hang with exposed tails outside the building. Here we are pre cutting them in the shop. Possible idea for the tails and at the same time using a principal rafter/purling roof system.
https://www.youtube.com/watch?v=zRsWoUKg8to


Edited by TIMBEAL (10/24/16 04:50 AM)

Top

Moderator:  Jim Rogers, mdfinc, Paul Freeman 
Newest Members
Redtop, Stu, HFR, Flange, Phillip_Anthony
4775 Registered Users