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Intersecting joinery question - rafter seat, scarf joint, brace pocket

Started by macke9c1, March 08, 2022, 09:46:47 AM

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macke9c1


I'm in the process of designing and building my first frame (a SYP 14x28 shed) and had a question or request for guidance on what is 'too much'.  

All these joints technically fit and don't intersect with each other but wanting to be extra cautious about taking out too much wood in one area.

The top plate is an 8x8 with an undersquinted scarf joint that I was trying to position close to being over the 4x6 brace, but it also happens that a 3x6 rafter (step lap rafter seat) lands on top of it.

I feel like I've safely overbuilt my shed so I'm thinking this may not be a huge issue, but generally, is there an alternative recommendation or something I definitely need to do differently?







Jim_Rogers

How deep is the lap of the step lap rafter seat?

Plates usually don't have any tension in the timber. So, a different scarf can be used. The scarf you have drawn is for a timber that could be in tension. And it may not be needed in a plate.
Recently, to make it easier to assemble, I've been using a different scarf in the plates.
And they have worked out great for us during the raising event.

It is a brindle half lap scarf. Like this:


 

With this scarf joint you lower the upper end down onto the lower half. And a step lap rafter seat won't take too much wood out of the timber (in my opinion).

Just an idea for you to consider.

Jim Rogers

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

macke9c1

it was 2" but so was the step and glancing back through my books made me realize I can make the step a little smaller and leave more wood in the plate by just reducing the rafter as it gets close to the plate.

I was also worried about squeezing in the undersquinted plate pieces together and on the posts and braces at the same time, this plate scarf joint seems to alleviate all of that since I didn't have any tension there.

I'm trying to leave the tails as thick as possible for aesthetics and strength since I'm running them out a full 2'.  

Does anything here look off? I was trying to find a rule about how much you can reduce a beam and where that reduction should start and end without loosing it's tension/compression strength in the middle.

3"x6"x12' rafter (including the 2' tails) on a 12:12 roof.

I appreciate all the help, sometimes you get an idea and put your blinders on and don't think about other solutions.



 

Jim_Rogers

Normally, the step and the lap are the same thickness. Your drawing is labeled 2" tail and then the dimension is labeled 2 3/4". The lap should be 2" not 2 3/4" and you can make the step 1 1/2". You can make it 2 3/4" if you wish but it isn't an easy dimension to verify.


 

In the above picture, Jack Sobon is checking the dept of his lap into the timber with his framing square (in this case 1 1/2" deep). You place your square on the surface of the lap and it should meet the corner of the plate, where his finger is. If the square is proud of the timber, then you need to trim a little more. Until it is flush.
When you do a 2" lap then you use the other leg of the framing square to check your depth into the plate. In my toolbox I don't have any metal tool that is 2 3/4". So, you'll have to create one that you can use to verify the correct depth.

I'm not sure about the strength of the 2' overhang as I don't know what the snow load (for your area) or roofing materials loads are in pounds.

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

macke9c1

I totally get the standard measurements are easiest to verify with the tools you have.  The example in Beermer's book has 2" lap and 1 1/2" step but the tail is 18", and I'm in central Georgia so our snow load is a dusting :D 

I just figured I'd make the lap bigger so the tail would be thicker since it was longer than his example. Again, maybe one of my many overkill measurements "just to be safe". 

Thank you for the advice Jim!



 

everythingwood

Quote from: Jim_Rogers on March 08, 2022, 11:26:01 AM
How deep is the lap of the step lap rafter seat?

Plates usually don't have any tension in the timber. So, a different scarf can be used. The scarf you have drawn is for a timber that could be in tension. And it may not be needed in a plate.
Recently, to make it easier to assemble, I've been using a different scarf in the plates.
And they have worked out great for us during the raising event.

It is a brindle half lap scarf. Like this:


 

With this scarf joint you lower the upper end down onto the lower half. And a step lap rafter seat won't take too much wood out of the timber (in my opinion).

Just an idea for you to consider.

Jim Rogers
This is exactly the joinery I will be using on my woodshed.  Rolled the first 1/2 of one plate into the shop last night. Then it’s on to the smaller timber’s and braces. 

canopy

"I'm trying to leave the tails as thick as possible for aesthetics"

One thing you can do is return to full dimension below the seat as shown below. Also notice the rafter is higher than the plate which is another technique to allow thicker tails.



 

Since you have a high pitch roof another thing you can do is make an aesthetically pleasing rafter peak. Make one side shouldered with a 90 degree mortise. The other side the tenon is angled per roof pitch to suit. Below is an 11 in 12 roof:


 

Don P


kantuckid

Chiming in ;D:
 I have something close in mind using 4x6 rafters with a current project. I plan to use an 8" log/timber screw, one per rafter seat to pin the rafter to the seat.
 On my homes great room I used pole barn nails ~ 15 years ago for same purpose, also in EWP materials same dimensions. 
The seat will be cut into top of a stacked log wall using tie logs to help hold the walls. 
Kan=Kansas;tuck=Kentucky;kid=what I'm not

canopy

Quote from: Don P on March 08, 2022, 08:38:00 PM
one pin at the peak, let it rotate
It sounds good in theory, but in practice to get even 1/32nd of an inch of travel at the peak peg hole the plate would have to move an inch (which it wouldn't) and only then if the rafter stayed completely rigid (which it wouldn't). So I'd opt for as many pegs as standard joinery guidelines call for--1, 2, 3. But I like your thinking as at minimum it could be used to optimize peg placement for best rotation allowance.

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