iDRY Vacuum Kilns

Sponsors:

Laying out rafters

Started by Dan, July 03, 2006, 09:34:27 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Dan

I`m in the process of building the shed from Jack Sobon`s book, the frame is up and I am ready to cut the rafters. I understand the standard math formula to determine rafter length, but I`m not sure it is the same when using a step lapped rafter seat. Any help would be greatly appreciated :)

Jim_Rogers

What do you need help with?
The top or the bottom?
Or overall length?

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

Dan

Trying to figure the length of the rafter from the peak down to where it sets into the step lapped part of the rafter plate (The peak is an open Mortise and Tenon).
I thought I had it figured out, but when I checked my dimension against the dimension in the Sobon book, it differs by a few inches. My rafter plates are 12 feet apart , and the step lap is set in 2 13/16" on each side.  Hope this makes sense, Thanks.

Jim_Rogers

On page 123 of his book is the rafter length and layout drawing.
It is cut just as shown.
Which is for a tongue rafter, but it's the same for a fork rafter except the mortise/fork is on the other side.  So they are both 1 1/2" off the layout face, that is one of the gable ends for the middle rafters and the outside for both end pairs. (also be sure to make all forks for one side and all tongues for the other).

If you square of one end, and call it the top of the rafter whether it is fork or tongue at this point doesn't matter. Then measure down the 9' 3 13/16" or (111 13/16", however you want to measure it) and that will be the end of the rafter or overall length. Measure back from the overall length mark 12" and that's the location of the step in the rafter seat.

You posted above that you've raised the frame?

Usually we don't raise the frame until all the timbers are cut, so that we can test fit them before hand to insure that they all fit exactly.

What we've done before is put together the sills all floor joists. Then sit on top of these the plates, with all the rafter seats cut into them. Align the outsides of the plates to the outsides of the sills, without any posts there, as this will be the exact location only taller in elevation. We used some small pieces of stickers nailed onto the outside of the sills to prevent the plates from sliding around while we were test fitting rafter pairs.
Then we'd assemble the rafter pairs, check them for being 90° to each other at the peak and then set them onto there rafter seats on top of the plates and see if they drop in as they are suppose to.
We would tap in a peg at the tongue and fork connection but not drive it in much, just enough to hold it together while we did a test fit. And then drive them out after with a peg pusher ( a smaller dowel, either wood or metal). That way we could test fit each pair together including those with collar beams, and test their connection to the rafter seat to be sure the rafters aren't too wide for the seat. If the rafters are too wide for the seat we plane off the none layout face of the rafter tail just where it enters the plate to the correct width of 4", and retest the fit.

Have you cut the rafter seats in the plates?

If you need further help ask another question....

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

Dan

Thanks for the reply Jim, yes, the rafter seats are cut into the rafter plates.
I was hesitant about cutting the rafters before the plates were in place just in case my width was a little off, the thought of setting the plates on top of the sills never crossed my mind while I was cutting the frame but it seems obvious to do it that way now...  I will cut my rafters to the dimensions in the book as it looks like my frame is right on as far as the dimensions go.

Could you explain how that dimension is determined?

Thanks,

Dan

mark davidson

Dan,
you can also draw your rafter and plate using graph paper to find dimensions. Use the biggest scaling that will fit on your paper. the graph paper squares are usually 1/4" and a common scale is 1/4" = 1'. However if you can enlarge your drawing to 1/4" = 6" then you will be more accurate. I often use a mix of scales in drawing with the joinery drawn at 1/4" = 1" and the building dimensions drawn at 1/4 = 1'.

once you have your situation drawn(you will be working in the view called "end elevation"), your rafter length is part of a triangle. The sides of the triangle run down plumb from the peak, level from the plate and along the outside edge of the rafter. One of the problems with timberframing is that sometimes it's not so clear where this triangle should be placed in relation to joinery and which side of the timbers etc. generally, I would place the plumb line at the center of the building, the level line at the top face of the plate, and the angled line at the outside face of the rafter. If the rafter crosses above the outside edge of the plate(always the case with a birdsmouth cut), then the triangle's level line may need to be raised above the plate...
Once the triangle is placed onto your drawing, you can figure the dimension from the peak to the outside top corner of the plate using the right angle triangle formula (A squared plus B squared equals C squared when C is the hypotenuse, or long side of the triangle). Then it will be possible to see how the joinery of the rafter relates to this triangle and you should be able to transfer dimensions and angles from your drawing to your rafter.
As buildings get more complicated, drawing things out is invaluable. For smaller buildings like the Sobon shed, I like Jim's idea of taking the actual piece and placing it onto the actual plates when they are sitting on the sills. Hard to miss when using the building itself as your drawing board.


Don P

One other way that my dad showed me years ago. We sometimes took scrap to make trial ends out of, then used 2x4's or scrap ply, whatever would work, nailed to them to make the length. Test fit that mock up and then use it as a pattern.

Jim_Rogers

Step lap rafter tail length.

In order to figure out the step lap rafter tail cut location you need to know several things.
First of course is the roof pitch.
In this case it is easy as it is a 12/12 pitch so the angles are 45°, and 90°. You also need to know what the size of the step is, or how much will be overlapping the plate. This is the "lap" in the name stepped lap rafter tail, or stepped lap rafter seat. Usually the step is the same amount as the lap so it is easier to figure and cut. So the lap is 2" and the step behind the lap is 2". This tiny triangle, with two legs being 2" has a hypotenuse of 2 13/16".
This is shown on the drawings on page 123 of Jack Sobon's book.
You also need to know the width of the building called the span as half the span is called the run. In this case 12' so half, the run is 6'. As this is a right triangle with a 45° angle the other angle has to also be 45° and therefore each leg, the vertical side (a) has to be the same as the horizontal side (b) the run, 6'.
So if we draw a right triangle with two legs of 6' and connect the hypotenuse (c) you have the rafter top edge to the outside of the plate.
As shown here:




Now knowing the dimension of the run is 6' (b) and that the other side (a), which is also 6' and also called the rise, we can figure the hypotenuse using the formula c = the square root of "a" squared plus "b" squared (6x6) + (6x6) or better still (72x72) + (72x72) as 10368. And using a square root function on a pocket calculator we see that the square root of 10368 is 101.8233. So that's 101 and a decimal of .8233 inches. To convert .8233 inches to a fraction so we can use it with our standard tape measure you multiply it by 16 for sixteenths of an inch (.8233 x 16 = 13.1728) and rounding off it's 13/16'' so 101 and 13/16" is the rafter length to the outside of the plate. It can also be shown as 8' 5 13/16".
Now we know the lap is two inches and that means the other leg of this tiny triangle at the edge of the plate also has to be two inches.



So we subtract two inches from the total and then 101 13/16" is now 99 13/16" or 8' 3 13/16".
Adding the 12" the designer wanted for roof overhang gives us 111 13/16" or 9' 3 13/16".

That's how the rafter length is figured and how to locate the position of the step on the rafter tail.

Hope this helps.

Jim Rogers

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

Jim_Rogers

On the rafter length dimension in the above drawing you'll see the length noted as 9' 3 7/8" with a minus sign after the 7/8" that my cad program gave me. This minus sign after the dimension means it is just a little less then 7/8" and more than 13/16" (remember we rounded off).
Most times I'd cut it to 7/8" and then fit it up. And if needed, then trim back a 1/16" to get to sit or seat properly.
It's usually better to have a little extra to trim off at fit up, then to be too short.

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

Raphael

  If I was in your current position I think I'd cut one pair and round up a couple people to help test fit them.  Once you have the first pair right you can use them to check the remaining rafter pairs as you cut them.  As Jim said, leaving a too much wood is always preferable to too little.  I've got a sharp block plane I keep on hand during final assembly, it's a better tool to have up on a frame or ladder than a framing chisel.  My Sorby would punch right through a hard hat while the block plane will simply bounce off.

Raising Safety 101:  Nobody should be under the frame when an edged tool is being used aloft!!!  Also things should be kept quiet so the person(s) working on the frame can communicate clearly with those on the ground.

Got any pictures of the frame?
Keep us posted on your progress.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

Dan

Jim, Mark, Don ,Raphael, Thanks for the help! I`m confident that I can cut these rafters correctly now, but I will make the first one a little long just in case... :)

Jim, it was the 2 inch dimension on the step lap that had me confused, your explanation is excellent, it cleared up my confusion right away.

I`ll try to post some pictures of the frame soon,I will have to read up on how to do that.

Once again THANKS !!!!!!!


Dan

Phorester


Hate to barge in with an unrelated comment, but all this interesting discussion  of measuring, subracting, angles, hypotenuse, triangles, etc., etc, brings to mind a career day I was doing several years ago at a local elementary school. 

While wanting in the lobby of the school to be taken to my assigned room, two boys, I'd say 5th grade level, came out of the office. 

One had an aggravated look on his was face and was saying to his friend;   "I don't know why I have to learn all this arithmetic!   When I grow up I'm gonna be a carpenter!"








Don P

Its like deja-vu ... all over again  :D

Thank You Sponsors!