The Forestry Forum is sponsored in part by:

iDRY Vacuum Kilns


Forestry Forum
Sponsored by:


TimberKing Sawmills



Toll Free 1-800-582-0470

LogRite Tools



Norwood Industries Inc.




Your source for Portable Sawmills, Edgers, Resaws, Sharpeners, Setters, Bandsaw Blades and Sawmill Parts

EZ Boardwalk Sawmills. More Saw For Less Money!

STIHLDealers.com sponsored by Northeast STIHL


Woodland Sawmills

Peterson Swingmills

 KASCO SharpTech WoodMaxx Blades

Turbosawmill

Sawmill Exchange

Michigan Firewood, your BRUTE FORCE Authorized Dealer

Baker Products

ECHO-Bearcat

iDRY Wood Lumber Vacuum Drying for everyon

Nyle Kiln Dry Systems

Chainsawr, The Worlds Largest Inventory of Chainsaw Parts

Smith Sawmill Service

Dynamic Green Products Inc.





Author Topic: Length of rafter using stepped-lapped rafter seat.  (Read 6875 times)

0 Members and 1 Guest are viewing this topic.

Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Length of rafter using stepped-lapped rafter seat.
« on: June 19, 2011, 04:07:59 PM »
I'n building a variation of the Sobon shed, but the roof pitch is 9/12, not 12/12.

In looking at page 123 in his "Timber Frame Construction" book he has a great diagram, but I need to adapt this to a 9/12 pitch roof. 

I can calculate the rafter length from the peak to the outside of the plate, but I'm having trouble figuring out how long to make the rafters. Everything after the rafter seat is overhang, its figuring out where the rafter ends, and the overhang begins since I'm no longer ending the rafter at the outside edge of the plate.

I did search the forums for answers, and found this:
http://www.forestryforum.com/board/index.php/topic,20158.msg288169.html#msg288169

However, that great explanation assumes 12/12 pitch..correct?

Also, the slope angles inside the rafter seat now change, in this case 37deg, not 45..correct?

I've got three bents up with the tie beam (plate), still need to cut the rafter seats. If I cut the two opposing rafter seats, can I use the 45degree in the center of the seat cuts as total span, rework my rafter lengths based on that, then total rafter length would be from the top to the first 90 degree cut (that sits in the seat)??


Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #1 on: June 19, 2011, 06:09:33 PM »
Wow. I just cut a stepped-lapped rafter seat and it was not easy. The front half is easy since the sides can be cut with a saw to lines that insure the angle is correct. The back half, being just a pocket was difficult. I ended up using a drill and forstner bit to remove some of the material. Still difficult though.

Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #2 on: June 20, 2011, 10:39:23 AM »
Answering my own post about the difficulty in cutting the stepped-lapped rafter seat. Found this was addressed a few times in this forum with some great suggestions. I'm going to cut another test today. More optimistic this time. The lap is easy, two or many saw cuts and chisel out the wood. The pocket I'll use a drill to remove the deepest corners to start, then chisel out the rest.

jeff

Offline Jim_Rogers

  • Board Moderator
  • *****
  • Posts: 7641
  • Age: 68
  • Location: Georgetown, MA
  • Gender: Male
  • Keep your chisels sharp.
    • Share Post
    • jrsawmill.com
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #3 on: June 20, 2011, 07:24:59 PM »
If you're all set with this please let me know.
If not then maybe I'll have to talk to you about it.
I do have a method of laying out the seats at a different angle then 12/12 but it has been described to me to layout the 12/12 first, in pencil very lightly on the top and side of the plate, near where the actual 9/12 rafter seat will be so that you can use it as a reference.

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

Offline Jim_Rogers

  • Board Moderator
  • *****
  • Posts: 7641
  • Age: 68
  • Location: Georgetown, MA
  • Gender: Male
  • Keep your chisels sharp.
    • Share Post
    • jrsawmill.com
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #4 on: June 20, 2011, 07:29:33 PM »
Do you have a set of "stair buttons"?

Shown in post by Lj about doing his stairs for his daughter's house.

What is your lap dimension?
What is your step dimension?
What is the width of your rafter?
What is the size of your plate?
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #5 on: June 21, 2011, 06:47:43 AM »
 Hi Jim,

Thanks for taking the time!

I spent yesterday practicing a few stepped-lapped rafter seats.  I figured out how to get the angles in the seat by drawing one at the end of the timber where I could draw the lines on the side for my angles, which gave me where to draw the lines on the face. I practiced a couple and using a short rafter end it fits well. I've got my 9/12 as far as the joint goes.

I don't have a set of "stair buttons".  I can use the square to get the various angles of the needed cuts.

What I haven't figured out is rafter length. Using one of the many online rafter length calculators I can get a length based on pitch and span, but that's figuring regular stick-built rafters that just end at the outside edge of the plate. With the stepped-lapped rafter seat, if the rafter is measured at the top of the rafter starting at the peak, where on the rafter does it end?

What I was going to try, and this may be incorrect, was to use a use a rafter calculator (I'm lazy) with my pitch and use still use the outside edge of the plate to calculate the span. Then to address my above issue, then just mark the end of the rafter. When cutting the rafter, where it sits in the seat, I'd draw out my cuts so the mark designating the end of the rafter ends up at the outside edge of the plate.  Cut one then cross my fingers.

Is there any easier way??

Here's my plan: I'm using 4x6 rafters and plan on spacing them 24" on center. Did Sobon use rafter ties on every set?? From the barns around here, I see rafter ties on every other, or every third set. I guess I'll do the same. Sound reasonable? When the rafters are up, I'll strap and put on metal roofing. Not sure if I should use 1x or 2x strapping. I plan on calling the metal roofing manufacturer near here to ask them what they recommend for their product.


Where I'm at:






Offline Jim_Rogers

  • Board Moderator
  • *****
  • Posts: 7641
  • Age: 68
  • Location: Georgetown, MA
  • Gender: Male
  • Keep your chisels sharp.
    • Share Post
    • jrsawmill.com
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #6 on: June 21, 2011, 07:29:35 AM »
Well, there could be an easier way. But I need some answers to the above listed questions.

Also, we usually never put together a frame without all the joints cut. Cutting step lap rafter seats on plates up in the air is not going to be easy at all.

Using an online rafter calculator will get you the rafter lengths based on the outside location of the plates, that is run or span. But I'm not sure if it will help you locate the location of the seat on the rafter.

We'll also need to know the amount you want the rafter to overhang the plate.

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

Offline witterbound

  • Senior Member
  • ****
  • Posts: 443
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #7 on: June 21, 2011, 01:13:39 PM »
Take a few minutes to learn to use sketchup. You can draw your "problem" in just a few minutes, and get precise measurements.

Offline witterbound

  • Senior Member
  • ****
  • Posts: 443
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #8 on: June 21, 2011, 01:28:01 PM »
Or, mock it up on the ground and get your measurements.  The easiest way would probably be to draw it out with chalk and a chalk line on your garage floor. 

Offline Jim_Rogers

  • Board Moderator
  • *****
  • Posts: 7641
  • Age: 68
  • Location: Georgetown, MA
  • Gender: Male
  • Keep your chisels sharp.
    • Share Post
    • jrsawmill.com
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #9 on: June 21, 2011, 05:31:44 PM »
I have drawn this frame many times, in my drawing program.

I have modified one to be what I think he wants but without the exact specifications, I can't give him exact answers.

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

Offline piller

  • member
  • *
  • Posts: 44
  • Location: East TN
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #10 on: June 21, 2011, 06:02:17 PM »
OK, a 9/12 pitch is 36.86989 degrees according to my calculator ( ArcTangent(9/12) ), definitely much easier using a rise of 9 and a run of 12.

I believe that you want to figure out where to draw the line for the step on your 4" x 6" rafter    As Jim mentioned, we need a few values to solve the problem.

What is the width of your structure between the two top outside corners of your plates?  Is it 12' ?  

What is your rafter layout design at the step:  for example, your rafter is 6" wide.  Perhaps you are going for a 2" relief to be cut on the bottom of the rafter, a 2" step in the middle of your rafter, and a 2" thick section that remains at the top of the rafter.

Do you want your rafter to be flush with the top outside corner of the plate or can it stand above the plate, and if so how much?  

OK, let's say that you have a 9/12 roof pitch, you want the rafter to be 2" thick above the step, you want the top of the rafter to be flush with the outside top corner of the plate and your building is 12' wide.   I made a little spreadsheet to solve for the values and I get:

Plate Lap cut vertical:    2.500"
Plate Lap cut horizontal:  3.333"
Length of rafter from peak to plate corner:  90.0"
Length of rafter from peak to step line projection across rafter:  87.3333"
Rafter peak is 54" above top of plate.

When I plug in a 12/12 pitch I get:  
plate lap cut vertical = plate lap cut horizontal = 2.828"
rafter length to plate corner: 101.823"
length of rafter from peak to step line projection across rafter: 99.823"
Rafter peak is 72" above top of plate.

I'll try to attach the spreadsheet, I think the numbers are right but no guarantee, they might be completely wrong, impossible, please check results, etc.
--chip

Note:  I reformatted the spreadsheet to clean it up a little June 22, 2011 at 12:00 pm


Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #11 on: June 24, 2011, 08:38:18 AM »
Hi Guys,

Jim, I can appreciate cutting all timbers before raising anything. My excitement got the best of me, I needed the beams out of the shed and needed to work with my friend's schedule on using his tractor. I plan on setting up staging to cut the rafter seats and raise the rafters.

The width of the structure from outside edge to outside edge of the plates is 172"

What is your lap dimension?     3 3/4" across top of plate**
What is your step dimension?   3 3/8"  across top of plate**
What is the width of your rafter? 4"
What is the size of your plate? 8"

Overhang? 12" or so.

**I arrived at these measurements by drawing out the cut for the lap & step on the side of a timber, using a bevel gauge & 37 degrees. Then transferring the lines across the timber. I cut one in some scrap and a test cut rafter sits well at 37 degrees. The numbers differ from what I get using the nifty spreadsheet from piller (Chip). ?

Piller, what is "step line projection" My structure is 172" and I'm using 4x6 rafters. Rafters will be cut as outlined in Sobon's book. 2" deep step 2" deep rafter tail, flush with plate.

When I plug 172" building width into your spreadsheet, I get 3.33 Plate Lap Horizontal cut, which is much less than the 3.75 I arrived at using my side-layout technique. Is this set up for rafter top flush with plate?

-Jeff

Offline piller

  • member
  • *
  • Posts: 44
  • Location: East TN
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #12 on: June 24, 2011, 04:06:34 PM »
Hi, I checked the numbers, I think the spreadsheet is right.  Your rafter is 6" deep and has a 2" rafter tail that will be flush with the outside top corner of the plate and a 2" step.  Your roof pitch is 9/12. 

You should have two layout lines on the top of the plate, measured from the outside top corner of the plate.  The first line is 3.333" from the corner, the second line will be another 3.333" (or 6.666" from the corner).  Then, on the outside vertical surface of the plate you will have a line 2.50" down from the outside top corner of the plate.   

This is a rise of 2.50" and a run of 3.333" which works out to a pitch of 9/12, and these are the right numbers for a 2" thick rafter tail section. 

What I am calling the "step line projection" is the perpendicular line drawn across the rafter that includes the 2" step line.

Now, what you might want to do is to verify all of this using a framing square, a large sheet of paper or plywood or similiar, and a pencil.   It should only take a couple of minutes.

1) Draw a straight horizontal line across the paper. 

2) Draw your 8"x8" square plate under this line. 

3) Draw the 9/12 roof pitch line from the outside top corner of your plate (to be more precise you could make it 18/24).

4) Use your framing square and draw three lines below the 9/12 roof pitch line that are parallel to the 9/12 roof pitch line with a 2" spacing between lines:  this is the cross section of your 6" rafter.  If you draw your lines right your rafter cross section should be exactly 2" + 2" + 2" = 6" deep and the lines should cross the top and outside surfaces of your plate at the positions given above. 

5) Draw the step line projection across the rafter:  line up one edge of your framing square on the 9/12 roof pitch line and slide the framing square up and down the 9/12 roof pitch line until the 90 degree leg of the square crosses the intersection of the top of the plate line and the parallel line 2" below the 9/12 roof pitch line.  When this is lined up draw the step line projection across the rafter. 

6) Now determine the step line projection position on the rafter, the outside top plate position on the rafter, and the end of your tail position on the rafter (measured from the rafter peak):  First find out the distance from rafter peak to outside top plate corner:  Your building is 172" wide, so half of that is 86", this will be your "run" distance.  You have a 9/12 pitch, so find the rise as follows:  rise = 86" * 9 / 12, this is 64.5".  OK, now you have a right triangle with a 86" run and a 64.5" rise, the hypotenuse (rafter peak to plate outside top corner) length will be the square root of (86 * 86) + (64.5 * 64.5), which is 107.5". 
OK, now you need to know where to draw the step line going across your rafter.  Look at your drawing and measure the distance along the rafter top edge (your 9/12 roof pitch line) from the top outside corner of the plate to the step line projection.  Subtract this value from 107.5" and you should have 104.833". 
How long from the rafter peak to the end of the rafter tail?  You said you wanted the rafter to continue 12" past the outside top corner of the plate, so this distance is 107.5" + 12" = 119.5".

That should give you all the values you need.  The bottom corner of your 2" step will be located inside the plate here:  4.533" horizontally from outside face of plate, 1.6" below top surface of plate.

Let me know if this makes sense or not and how the project is going.  The picture you posted looks quite nice.  By the way, what are the posts standing on, are the posts tied down?
--Chip





Offline Jim_Rogers

  • Board Moderator
  • *****
  • Posts: 7641
  • Age: 68
  • Location: Georgetown, MA
  • Gender: Male
  • Keep your chisels sharp.
    • Share Post
    • jrsawmill.com
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #13 on: June 24, 2011, 05:27:26 PM »
Do you have a set of "stair buttons"?

Shown in post by Lj about doing his stairs for his daughter's house.

What is your lap dimension?
What is your step dimension?
What is the width of your rafter?
What is the size of your plate?


When I asked the above questions I was referring to the dimensions shown in this drawing:



In order to figure out all the stuff that Chip has mentioned you need to understand these dimensions.

The reason it is called a step lapped rafter seat, is because the rafter tail "laps" over the plate and extends beyond the plate to create an overhang to shed rain water off the side of the building. The reason that it called a "stepped" lap rafter seat, is because when you look at it, the back part looks like a stair step. (At least that is what I think I was told was why it's called what it is called.)

Yesterday, I was at a Timber Framing school with the tools we sell, and I questioned one of my favorite timber framing teachers about what happens with a rafter seat when you change the pitch of the roof. Especially from a 12/12 to a 9/12, as we have here.

And he explained it to me as Chip has stated.

The location of the "step projection line" has to move back on the top surface of the plate, but it has to be on the top surface of the plate. This had me confused for a while.
Moving this line back does change the amount of "tail" the rafter needs to maintain the amount wanted for the over hang.

Here is a cross section drawing showing what happens when you rotate the rafter on the plate arris:



I'm not sure if you can see it here or not. The plate is all in blue lines. The rafter is all in red lines. I have added the new "step projection line" location with a black line.

Here you can see if you just rotate the rafter at the arris the "step projection line" point moves below the surface of the plate. This would be difficult to cut and establish correctly and easily. So it make it easier you move this point back as Chip has mentioned.

This put it at the location 3.333" or 3 5/16" back from the arris.
(Everyone knows or remembers to convert a decimal inch to a fractional inch you multiply the decimal amount by 16ths, right?) (point 333 times 16 on your calculator equals 5.328 which means 5 sixteenths plus a little that gets rounded down).

I haven't taken the time to verify the dimensions that Chip has posted, but his math looks good and I'd say he has it right.

Thanks Chip.

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

Offline Preserved

  • member
  • *
  • Posts: 31
  • Location: New Hampshire
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #14 on: June 25, 2011, 04:09:00 PM »
Thanks Guys,

I need to play with this.  Chip, I've got some poster board, I'll draw this out as you suggest.

I've got work to do!

-Jeff

Offline piller

  • member
  • *
  • Posts: 44
  • Location: East TN
  • Gender: Male
    • Share Post
Re: Length of rafter using stepped-lapped rafter seat.
« Reply #15 on: June 25, 2011, 06:25:24 PM »
Jim, nice pictures!  Also thanks for pointing out the decimal to 16ths conversion.

Jeff: poster board should be perfect.  Keep us updated.
--Chip


Share via delicious Share via digg Share via facebook Share via linkedin Share via pinterest Share via reddit Share via stumble Share via tumblr Share via twitter

xx
i hate step lapped rafter seat

Started by Alexis on Timber Framing/Log construction

1 Replies
1549 Views
Last post June 08, 2011, 01:20:13 PM
by beenthere
xx
step lap rafter seat

Started by Alexis on Timber Framing/Log construction

6 Replies
4186 Views
Last post December 14, 2008, 12:58:12 PM
by Jim_Rogers
xx
Rafter Seat Cutting Procedure

Started by brendonv on Timber Framing/Log construction

5 Replies
969 Views
Last post January 09, 2017, 07:31:03 PM
by brendonv
xx
Sizing the step lap rafter seat

Started by canopy on Timber Framing/Log construction

2 Replies
713 Views
Last post December 06, 2013, 08:24:09 PM
by canopy
 


Powered by EzPortal