Square Rule Part Two

[mmhailey]

Well I am at the point where I need to start thinking about laying out my roof system.



I am working from a set of plans, that uses full 8X8 timbers. I have not closely inspected the rafters, and king posts yet, but if they are like the posts, and girts, a couple are running from 7 13/16 - 8 3/8. I have to brag on  my sawyer, because most of the timbers are exactly 8", and they are, as perfect as my framing square. Actually I really can't believe after 1 year stickered that they are as perfect as they are.

The plans have the geometry for the king posts and rafters figured for full 8X8. How do I go about cutting everything down to 7 1/2" . In the above sketchup pic, this gable end is my reference end

[swampfox]

Howdy MM,

If I were you, I would lay the timber down and cut the ends (longer than need be) to get a square clean surface.  Take a level and create a plumb line from top to bottom (outside of kp to non reference) on both ends.  Then connect the lines with chalk.  You have just established a center line reference. 

Work off of centerline "X" to each side.

[Jim_Rogers]

When we had to do a king post for this frame:



We just picked on side of the king post and said that it was the adjacent side, as the face toward us was the reference face.



Hope that helps....

[mmhailey]

Just got back from the shop, having stacked the posts and girts I have complete, and put the 6 rafter timbers out on cribbing.  I also brought the 3 King Post timbers in and put them so I could survey them as well.

I found that I have no undersize roof timbers. The kp's are exactly 8X8 and square as if they were extruded. Three of the rafters are exactly 8 and the other three  are 8 on one side and range from 1/4 to 7/16 over in the other.

 







I guess I am back to why cut these back to 7 1/2"? Looks like I can just cut the oversize ones back to 8" and call it good.

What do you think of that as a plan? 

[Stephen1]

I would think....Why cut more than I have too. eh eh

[Jim_Rogers]

You can do what you want.
But you'll have to beware that you are changing things and you'll have to keep track of the changes.

When a designer, even if it's you, decides on a framing "rule" like '2" off the layout face and then 2" thick', or "reduce each timber to the next 1/2" smaller in size at the joint" you are establishing dimensions for all your joints and timbers lengths. Now if you have one end of a timber housed and the other end not housed then the lengths are different then another timber in the same place in another bent.

What you're doing is making each timber a different length and then you'll have to make sure you have the right length to each one.

Having a standard, and applying that to every timber, makes all the dimensions work out the same.

Again it's your frame you can do what you want. Some times changing one little thing makes a lot more work for yourself keeping track of everything then the time it would take to just chop out the housing and been done with it.

And how is it going to look when some timbers and joints have housings and some don't? It's going to be definitely different then any other timber frame.

Jim Rogers

[mmhailey]

Jim I am laying out and cutting the rafters and king posts. I have everything cut on 7 1/2" up to and including the bottom chord of the Kingpost truss





I have housed everything up to this point.  The pic shows two of the reference planes I am dealing with I believe. I also have this gable end as a reference and everything inside is referenced off this end which is the south end.  The purlins, and ridge will be housed into the rafters and king posts to the 7 1/2" perfect timber.

I guess what I am doing is rather than going for the perfect 7 1/2" timber in the rafter I am going for the perfect 8". I suppose this is  just showing my laziness, for not redoing the design, and also I am not sure I want to give up the 1/2" of rafter.

On another question on 2" off reference???

The original design had the rafter's 2" tenons  centered. I plan on moving them to 2" off ref as I have done throughout the rest of the frame. Do you think there was some structural reason for the tenons being centered on the rafters?

[witterbound]

So, when you draw up plans for square rule, when you're going to cut everything back to the perfect timber size, say 7 1/2 inches, do you draw the plans up showing timbers are 7 1/2 inches (as opposed to 8 inches)?  Then you pull your dimensions from the 7 1/2 inch plans, and cut your housings to whatever depth is needed when you're cutting the frame (and don't show the housings in the plans)?  AND, when you've got a "centered" piece, like a king post, do you center the 7 1/2 inch timber on the girt, and assume in the perfect timber there is 3 3/4 on each side?

[Jim_Rogers]

In the past everything was set up to use the framing square, 2" wide body or the 1 1/2" tongue. So that you could easily use that tool to layout and verify your timbers.

Below is a shot of a tenon on a short timber, it is 1 1/2" off the reference face and the framer is checking the tenon at the shoulder by placing the framing square tongue on the tenon and sliding the try square over the surface of the timber to see if it will clear the framing square. If it hits the framing square then the tenon is not 1 1/2" off the reference face and it needs to be improved to be correct. If there is a gap between the two squares then they have cut away too much wood.



And, below is a picture of the framing square being put into a mortise to check the width of a mortise, to see if it is truly 2" wide:



When you create tenons you need to make sure that they are the correct thickness. This is done with a caliper gage like this:



After the caliper has been set to the framing square which checked the mortise, like this:



If you have an 8x8 and take away 2" from the center for the mortise, then you have 6" left. 6" divided by 2 for the front and back means 3" off the layout face and then 2" mortise/tenon and then whatever is left on the back. To check your work you'd need a piece of steel that is 3" wide exactly and something easy to use. We don't usually have such pieces of steel on hand, in our tool box and easy to use. We could do it with the two inch body of the framing square and the 1" ruler of the combination square. But that could be difficult to use two tools in one.

Also, it was mentioned to me lately that when you center your tenons and mortises you're putting the strength of the joint right where. the timbers, are going to check to the heart/pith in the center of both timbers.

If you follow the 2" by 2" layout you'll be off center and the center drying check/crack will not effect your joint strength.

I hope that help you understand why we layout the way we do.

[Jim_Rogers]

do you draw the plans up showing timbers are 7 1/2 inches (as opposed to 8 inches)? 

No you draw full 8" timbers with housings at every joint.

[Jim_Rogers]

Then you pull your dimensions from the 7 1/2 inch plans, and cut your housings to whatever depth is needed when you're cutting the frame (and don't show the housings in the plans)? 

Layout is done from the reference face/ adjacent face edge known as the arris. And if the timber is wider then 8" then yes, you cut a deeper mortise housing then 1/2".

You have to cut away whatever it takes to make the joint right.

[Jim_Rogers]

  AND, when you've got a "centered" piece, like a king post, do you center the 7 1/2 inch timber on the girt, and assume in the perfect timber there is 3 3/4 on each side?

No, a king post will have joints on one side pulled from the arris to be 7 1/2" off. And it will have joints on the adjacent face which are 1/2" off the adjacent face.

Does that make sense?

It works:

[mmhailey]

Jim,

Do I have this idea correct. I exaggerated the King Post Width, just to make it clear in my mind. In other words the Right/East Side of the King Post is all off the arris, and then we pull the dimensions for the geometry cuts from that edge Right?

[Jim_Rogers]

Technically yes you are correct.

However your timber should not be so oversized that it would ever look like that.