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Cutting Step Lap Rafter Seat #32291 05/22/14 03:42 AM
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ATC Offline OP
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I there a trick or technique to cutting this, either with a chisel or power tools?

This is the only picture I have of the joint in question.



What I'm wondering is, on a say 8:12 roof, the inside of the joint would have to be cut at a 33.69 degree angle. Is there a tool that is used for this, or do some people cut a guide block?


Thanks

Re: Cutting Step Lap Rafter Seat [Re: ATC] #32292 05/22/14 01:15 PM
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Dave Shepard Offline
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You have to come up with the dimensions by laying out the joint on the face of the plate. Then you use a combo square to test the joint. I will try to post a better reply when I'm not on my stoopid phone. It is not difficult once you know how.


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Re: Cutting Step Lap Rafter Seat [Re: ATC] #32295 05/25/14 08:10 AM
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Jim Rogers Offline
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When I was taught how to cut a step lap rafter seat, I was told to first layout the seat as if it was a 12/12 pitch. Then when you change the pitch to 8/12 the lines will move back away from the eave edge of the timber.

To layout a seat you use your framing square and a set of stair buttons.

Do you have any?

The picture you posted, I created.

I can create one for you at 8x12 pitch and or make a measured drawing of the distances of the of each of the cuts.

But I can't do it until the end of the week, as I am away from home on a job.

It isn't difficult once you have the button. They attach to your framing square and make it easier to layout this join.

One of the first things you need to tell us, and understand is that the thickness of the tail that laps over the plate is usually the same depth as the step.
The two most common are 2x2 or 1 1/2 x 1 1/2.
Which do you intend to use to make the 8x12 pitch?

Jim Rogers


Whatever you do, have fun doing it!
Re: Cutting Step Lap Rafter Seat [Re: ATC] #32296 05/26/14 05:45 PM
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TIMBEAL Offline
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Steve Chappel has a great chart in his Timber Framer's Workshop book Trigonometric Ratios pg. 211

under 8/12 pitch roof you will find the numbers .5547 under Sine and .8321 under cosine. You can figure the dimensions of the step lapped rafter seat with these numbers. The given will be the thickness of the rafter tail and the thicker section just before the tail with the adzed reduction. Play with the number. Test them and run them backward and forward to ensure they are right. Of note Sine=rise Cosine=run.

I like how under 12/12 you have the same number .7071, 9/12 you have .6 and .8 respectively. and 8/12 cosine is .8321 fun numbers and easy to remember. Just divide or multiple but your givens, rise or run.

Re: Cutting Step Lap Rafter Seat [Re: ATC] #32299 05/27/14 06:39 PM
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Thanks for the help from all of you.

I have stair gauges, and I can do the layout (I think). I'm a Licensed General Building Contractor, so rafter layout is familiar to me. I assume I would use basically standard layout techniques.

I'm intending to move into more traditional building styles, and am planning on starting maybe with a Jack Sobon style shed. I'm not a master with a chisel, however.

What I'm a little unsure about is how to cut the inside "notch" portion, and maintain the proper angle. I practiced on a scrap piece of timber I had, and my angle was a bit....uh....off.

Last edited by ATC; 05/27/14 06:39 PM.
Re: Cutting Step Lap Rafter Seat [Re: ATC] #32300 05/27/14 08:29 PM
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Roger Nair Offline
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My suggestion is to first go simple with cutting and once you get the knack then look for improved approaches. First forget about precise angular descriptions. Look at the rafter, note that: tail is reduced parallel to rafter slope, a cog abutment is at right angle to slope and full dimension rafter beds at slope. So a simple cutting scheme requires a control from slope and cuts square and parallel to control.

So begin with cutting the bed for the rafter tail, you will need a handsaw, chisel, hatchet, slick and combination square. Working to the lines, saw to the shoulders, then chip out waste, and smooth the bed, all the while retaining sharply laid out lines. The next cuts can be controlled with the combo square working against the tail bed.

You will quickly gain the feel of the shape

Re: Cutting Step Lap Rafter Seat [Re: ATC] #32301 05/28/14 01:54 AM
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TIMBEAL Offline
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I have drilled holes in the deep parts of the step, 3/4" on each end.

Re: Cutting Step Lap Rafter Seat [Re: ATC] #32303 05/28/14 02:50 PM
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Gabel Offline
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This is such a cool joint. The inboard vee notch is generally entirely cut with chisel and mallet. As mentioned, cut the lap first to give yourself a reference for your combo square to check the progress on the chopping of the step notch. Stringy, tenacious wood and dry wood is not your friend for this type of work.

Because it resists efficient cutting with power tools it is seen only very rarely in new construction. There are a handful of framers who use it still, but it is pretty much not used in the timber frame industry.

Re: Cutting Step Lap Rafter Seat [Re: ATC] #32304 05/28/14 09:50 PM
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Is this joint's presence in areas with no wind? I imagine a builder would go through this because no spike is necessary.

Re: Cutting Step Lap Rafter Seat [Re: Gabel] #32305 05/29/14 12:46 AM
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Originally Posted By: Gabel
There are a handful of framers who use it still, but it is pretty much not used in the timber frame industry.


Yeah, I was talking to a Timber-Framer buddy a while back, and he was saying that no one makes those anymore. They just cut a standard birds-mouth, and run a TimberLok down through the top plate. eek I'm trying to go a bit more traditional.

I know there are other rafter-to-top plate joints, but I don't know of any that allow you to continue the rafter for the rafter tail.

As far as wind loading, when I go to build a house, I'll have it engineered. I don't know if a wood peg would suffice, or if you'd be required to use a Simpson type hurricane tie. Anyone else have experience with that?

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