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Author Topic: rafter joints at ridge question and answers  (Read 4157 times)

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Offline Jim_Rogers

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rafter joints at ridge question and answers
« on: August 07, 2015, 09:51:27 AM »
Quote
What joint do you use at the rafter peak?  My next project is a TF with no ridge, I am trying to find some drawing of rafter joints and sizing.

I was recently asked this question.

There are several choices you can make. It could depend on what thickness your rafter are as to which size tenon you would use. If you're going to have a tenon and mortise joint.

Normally if the rafter is less then 6" thick I use a 1 1/2" tenon and a 1 1/2" offset to make it big enough to have a peg through it.
If the rafter is a principal rafter then most likely it's going to be something around 8" thick and then you'd use a 2" thick tenon and 2" offset.

I have drawn a lot of "typical" joints and posted them on the Timber Framing Headquarters site.
But here are a few that you could choose from.

 

 

Above is a shot of a pair of rafters that are supported by a ridge purlin. This shows a gap between the rafters on purpose. As the ridge purlin drys it will shrink. If the rafters were butted then they would not be supported by the ridge purlin. You have to allow for the rafter to "close up" when the ridge shrinks.

 

 

Above is two rafters that are just butted to each other and would be secured with some type of metal fastener.

 

 

Next is a simple half lap held with a peg. We used this joint several times and found that the simple peg didn't seem to hold it tight enough and we added at least one screw, above the peg so it wouldn't show from below when you look up. Not the best joint but easy to cut.

 

 

Next is the traditional tongue and fork joint. The tenon is called the tongue and the mortise which is open to the end of the rafter is called the fork. Easy to cut and very strong when secured with a peg that is properly placed.

 

 

Last is a similar joint but the mortise is not open to the end of the rafter, so it's called a blind mortise.

I hope this has helped you to understand the different joints that are available to you to joint two rafters at the roof peak without a ridge beam.

Jim Rogers


Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Online jander3

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Re: rafter joints at ridge question and answers
« Reply #1 on: August 07, 2015, 11:18:33 AM »
Thanks.  This answers my question.


Offline canopy

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Re: rafter joints at ridge question and answers
« Reply #2 on: August 12, 2015, 04:43:38 AM »
Why aren't these joints traditionally shouldered? What I mean is if leaving the part shaded in green as a shoulder instead of removing it to get down to the perfect timber there would be a larger bearing surface and it would give a more snug fitting appearance like shoulders elsewhere in a timber frame.



Offline Jim_Rogers

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Re: rafter joints at ridge question and answers
« Reply #3 on: August 12, 2015, 09:29:55 AM »
You can cut it that way, if you wish. It would make it stronger.
I choose to reduce each side but many/most do not.

Jim Rogers
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Offline Roger Nair

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Re: rafter joints at ridge question and answers
« Reply #4 on: August 12, 2015, 06:36:04 PM »
For those who might consider a half-lap joining there are methods that will hold the joint together.  I have personally worked on barn frames some now 170 years old that had half-laps secured with 3 or 4 wrought iron nails that penetrated through and clinched over, no telling how long the nails will continue to hold.   Other techniques could involve an over sized peg, with a stopped end and the other end kerfed and wedged or a stopped peg that runs well past the opposite side with a crossing wedge as a stopper.  Some framers have used undersquinted abutments against the blades with common pegs.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

Offline Jim_Rogers

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Re: rafter joints at ridge question and answers
« Reply #5 on: August 14, 2015, 10:43:57 AM »
Thanks for those tips Roger.

Yesterday, I traveled to Heartwood school to offer our tools for sale to the students there, and they were in the process of cutting rafters.
Both tongues and forks.

I helped a woman student by being a "butt clamp" (I sat on her timber) so that it wouldn't slide around on her saw horse.
She was cutting the reduction on the bottom side of the fork.
Other students in her area of the yard we also doing the same joint. Including her husband.

Here are some pictures of the joints and students:

 

 

 

  



  



  



  



 

As you can see most were cutting the reduction on the bottom of the fork as I have drawn it.

This is a decoration more that any thing else. But I feel it looks nice when it's all put together.

 

 

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline canopy

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Re: rafter joints at ridge question and answers
« Reply #6 on: August 14, 2015, 09:37:40 PM »
That looks good. For comparison, I prefer the simple, blockier look without cheeks even more so I optimized out the cheeks on both rafters. On the fork I used a shoulder as previously discussed. On the tongue I made a backing angle. I chose an angle such that the bearing surface of the fork is 90 which is nice when plunging the fork with a chain mortiser. Oversize timbers on both rafters are compensated for automatically just like the cheek method. It looks like this under the peak:




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Re: rafter joints at ridge question and answers
« Reply #7 on: August 14, 2015, 10:03:54 PM »
Here is the look installed. Also note the collars have no gaps at the top of the pocket because I laid them out from the top instead of the bottom. The top method compensates for oversize timbers the same as the more standard bottom layout method and was discussed in a previous thread. Any gaps in the photo I believe are due to wane which in hindsight I wish I had insisted fully dimensioned timbers, but it's really hard to find teak big enough in these sizes and lengths.



Offline Jim_Rogers

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Re: rafter joints at ridge question and answers
« Reply #8 on: August 15, 2015, 09:05:08 AM »
Nice looking roof you've got there.
Thanks for sharing the photos.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Sedgehammer

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Re: rafter joints at ridge question and answers
« Reply #9 on: November 12, 2019, 07:44:15 AM »
Quote
What joint do you use at the rafter peak?  My next project is a TF with no ridge, I am trying to find some drawing of rafter joints and sizing.

I was recently asked this question.

There are several choices you can make. It could depend on what thickness your rafter are as to which size tenon you would use. If you're going to have a tenon and mortise joint.

Normally if the rafter is less then 6" thick I use a 1 1/2" tenon and a 1 1/2" offset to make it big enough to have a peg through it.
If the rafter is a principal rafter then most likely it's going to be something around 8" thick and then you'd use a 2" thick tenon and 2" offset.

I have drawn a lot of "typical" joints and posted them on the Timber Framing Headquarters site.
But here are a few that you could choose from.

 

 (Image hidden from quote, click to view.)

Above is a shot of a pair of rafters that are supported by a ridge purlin. This shows a gap between the rafters on purpose. As the ridge purlin drys it will shrink. If the rafters were butted then they would not be supported by the ridge purlin. You have to allow for the rafter to "close up" when the ridge shrinks.

 

 (Image hidden from quote, click to view.)

Above is two rafters that are just butted to each other and would be secured with some type of metal fastener.

 

 (Image hidden from quote, click to view.)

Next is a simple half lap held with a peg. We used this joint several times and found that the simple peg didn't seem to hold it tight enough and we added at least one screw, above the peg so it wouldn't show from below when you look up. Not the best joint but easy to cut.

 

 (Image hidden from quote, click to view.)

Next is the traditional tongue and fork joint. The tenon is called the tongue and the mortise which is open to the end of the rafter is called the fork. Easy to cut and very strong when secured with a peg that is properly placed.

 

 (Image hidden from quote, click to view.)

Last is a similar joint but the mortise is not open to the end of the rafter, so it's called a blind mortise.

I hope this has helped you to understand the different joints that are available to you to joint two rafters at the roof peak without a ridge beam.

Jim Rogers
Jim or anyone else if you could please answer. Thanks



Offline Jim_Rogers

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Re: rafter joints at ridge question and answers
« Reply #10 on: November 12, 2019, 09:39:16 AM »
Normally everyone calls that a bird's mouth. But to be correct it is not a bird's mouth but a crows foot cut.

check this out for better description and drawings:
Another Noob with questions in Timber Framing/Log construction

Jim Rogers
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Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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Re: rafter joints at ridge question and answers
« Reply #11 on: November 12, 2019, 09:48:43 AM »
Appreciate that I does! thanks. I didn't think it was a birds mouth, as I thought I knew it, but just couldn't think what it was. With 3x12 rafters how would you fasten that to the midspan purlin beam? Still a timber lock screw? With a 3x12 sitting on the exterior wall with no rafter tail, what measurements would you use for the birds mouth? Thanks again!

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Re: rafter joints at ridge question and answers
« Reply #12 on: November 13, 2019, 01:47:44 PM »
Appreciate that I does! thanks. I didn't think it was a birds mouth, as I thought I knew it, but just couldn't think what it was. With 3x12 rafters how would you fasten that to the midspan purlin beam? Still a timber lock screw? With a 3x12 sitting on the exterior wall with no rafter tail, what measurements would you use for the birds mouth? Thanks again!
@Jim_Rogers

Offline Jim_Rogers

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Re: rafter joints at ridge question and answers
« Reply #13 on: November 13, 2019, 01:53:38 PM »
Appreciate that I does! thanks. I didn't think it was a birds mouth, as I thought I knew it, but just couldn't think what it was. With 3x12 rafters how would you fasten that to the midspan purlin beam? Still a timber lock screw? With a 3x12 sitting on the exterior wall with no rafter tail, what measurements would you use for the birds mouth? Thanks again!
@Jim_Rogers
To fasten it to a purlin you can use a timberlok screw with a minimum of 2" thread penetration into the purlin.
The bird's mouth seat notch into the rafter has to be less than 1/3 up from the bottom. so that the rafter doesn't split from the bird's mouth cut. Normally 1/4 up.
This affects all the elevations of the rafter/plate/purlin, and ridge. Everything is connected and has to be done right to meet all requirements.
Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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Re: rafter joints at ridge question and answers
« Reply #14 on: November 13, 2019, 02:22:45 PM »
Appreciate that I does! thanks. I didn't think it was a birds mouth, as I thought I knew it, but just couldn't think what it was. With 3x12 rafters how would you fasten that to the midspan purlin beam? Still a timber lock screw? With a 3x12 sitting on the exterior wall with no rafter tail, what measurements would you use for the birds mouth? Thanks again!
@Jim_Rogers
To fasten it to a purlin you can use a timberlok screw with a minimum of 2" thread penetration into the purlin.
The bird's mouth seat notch into the rafter has to be less than 1/3 up from the bottom. so that the rafter doesn't split from the bird's mouth cut. Normally 1/4 up.
This affects all the elevations of the rafter/plate/purlin, and ridge. Everything is connected and has to be done right to meet all requirements.
Jim Rogers
Thanks!
That'd be a 13-14" screw then, as these are 3x12s
I designed it for 3" already, so that is right
Yup

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Re: rafter joints at ridge question and answers
« Reply #15 on: November 13, 2019, 03:42:42 PM »
3x12 rafter deduct the crows foot cut. You maybe able to use a 12" screw so that there is still 2" penetration into the purlin.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension


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