step lap rafter seat

[Alexis]

Hello,

I would like to know your procedure for cutting the step lap rafter seat. I will build a sobon shed and cannot seem to understand how tu cut the seat part... thanks

[kfhines]

Saw to the lines on the out board leading edge first and clean out the bottom. This will give you the reference to hold the 90 degree shoulder too. The rest is all mallet and chisel work. It's a little slow going, take your time and keep checking the angles.



kfhines

[routestep]

I've been cutting step lap seats in a plate for the past few days. My shoulder is a little sore from the chiseling. Here is what I do:

From the size (depth) of the rafter and the pitch angle I lay out three lines on top of the plate from the reference edge (arris) and one line on the outside of the plate down from the reference edge.

There is the inner most line on the plate. This is where the bottom of the rafter first enters the plate.
The middle line is where the bottom of the step lap is. I use a drill, hand brace and 1/2 inch wood auger bit and drill a series of holes the width of the rafter to make cutting the seat easier. My rafters are 4.5 inches wide and I have been drilling about seven holes along this center line. To get the right depth I count turns. I've been drilling down 1.75 inches in about 40 turns. The third line is the outermost and is closest to the reference edge.

The innermost line and the outermost line define a "V" shape notch into the wood. The middle line that I drill along define the Bottom of the "V". I then start chiseling out the "V" notch. The bottom of the "V" is 90 degrees, so I check the angle with either a framing square or combination square.

I take a hand saw and cut in from the edge (arris) of the plate. You cut to the outermost line on the top of the plate and cut down to the fourth line on the outside reference face. The saw kerf will be at the angle of the rafters. I generally cut several kerfs so I can chisel out the waste. The rafter tails pass into this notch and allow the roof to overhang the walls. If all goes well the angles will all be 90 degrees.

If your roof is a 12 in 12 pitch the "V" knotch will be symmetrical.

Hopefully I didn't confuse things for you. Some one might be able to post a few pictures showing the steps.

[moonhill]

I hate to admit this but I used a skill saw to define the bottom of the V,  I first bored a 1-1/2" on each end of the 6" wide pocket about 3/4+ the depth with a boring machine.  I was working spruce so after 2 out of 20 pockets I pulled out the skill saw again and set the angle and plunged the other two/36 cuts.  Then chiseled the scrap out.  If it was pine I would have only bored the two holes, spruce is a tough wood, tenacious it is.  I like these pockets.  These didn't have rafter tails so just the V's in the top of the plate.

Tim

[routestep]

I had to make some steplap that were 7.5 inches wide and thought about using the skill saw. But with the 4.5 inches wide ones, I guessed they were just too short to bother with. But reading your post, maybe I'll give it a try. I have five move to do.

[moonhill]

I have used a cordless saw, they have the small 5" blade, maybe it 5-1/2". 

Tim

[Jim_Rogers]

Alexis:
The first thing I have to ask you is do you know how to layout a step-lap rafter seat?
If you don't I can help you with that.

Well, if you do then the first thing you do after laying it out, is cut the lap. That is the part of the seat where the rafter tail will "lap"over the plate. Hence the name.

To do this you usually use a hand saw and saw inside the lines the two edges. And then remove the center section with a chisel.
Here is some pictures of this process that I took with a very cheap camera many years ago, and scanned into my computer. That is Jack Sobon doing the demonstration:



Usually the lap and the step behind it are the same thickness/dept that is if the lap is 2" then the step is usually 2" as well, (or 1 1/2" x 1 1/2").

To check if the lap is the correct depth you can use your framing square like this:



After you have made your lap correctly then you make the step. Starting out with your chisel in this position:



And it's just a matter of chisel work to remove the wood at the correct angle:



You can check your work with a combination square like this:



When you have it all done you can check your rafter tail to the seat like this:



Above the sample rafter tail is only about 3' long, made to use to check seats....

Keep asking questions.....


Jim Rogers