Dealing with rafter crown

[canopy]

The top edge of rafters need to be a perfectly flat plane so the roof is flat. Like any timber, rafters have varying amounts of crown. Chalk lining and planing off the top crown seems straight forward (see red area in illustration). But on the bottom the crown creates a shallow birdsmouth pocket where it sits on the plate. Just 1" of crown can dramatically reduce the bearing surface as can be seen on the illustration where the plate doesn't reach full depth on the seat. How is this best addressed? In my case the rafters are 6x7's with a 5' tail past the plate.

[Satamax]

Well, a roof needs to be approximately flat. Not perfectly flat. Tho, one inch of crown is big. You can't straighten with your purlins? What type of building is this? What covering material? Insulation? Suspended floor?  There's is sometimes ways to straighten the rafters. Sometimes, if it has too much crown, you just discard.


Roofing screws are quite nice at straightening timbers that are crooked.

[Jim_Rogers]

As your rafter's top surface is the reference edge you need to measure down from that edge to create the crow's foot seat for the rafter to sit on your plate.

As shown here:



 

Whatever the seat come out to be is what it is. After you have cut your seat and you are erecting your frame you may need to put a come-a-long or some other device on your rafter and pull it down to set it on the plate completely before you insert your 10" long screws to hold it in place.
Once it is in place, and after the roofing is in place the load of the roof should hopefully hold it down and flat.

You may want to hold off of trimming the top surface until it is installed and see exactly how far it is out of plane, as trimming it now may cause it to crown more, or less.

I don't know for sure I've never worked with the type of wood you're using.

Jim Rogers

[canopy]

It would be ideal to bend the crown out, but that sounds really, really risky. Do you guys believe bending a seasoned hardwood 6x7 is a reasonable proposition? I would think it calls for an awesome amount of force pulling in an oblique direction so hopefully the rafter would bend enough before something on the frame breaks--and then wonder whether the pegs and screws could hold when releasing it. It also seems layout would be very error prone to judge where to put the birdsmouth and collar tie mortise so after bending they ended up in the right spot. In fact I envision the layout line would be an arc drawn by hand approximating where and how much bending would take place. And pocket angles would be a little undersized to account for bending stretching them out somewhat. Approximating how much and where is mind boggling. This whole approach does not feel comfortable just thinking out loud.

Another approach I wonder about. In framing we normally layout to the perfect timber--1/2" less than the actual dimension. Couldn't this be done with rafters, assuming rafters were appropriately sized that losing that 1/2" was still within passing strength margins? That would keep the gap down shown in the illustration in the case of imperfect rafters. And so I am a little curious why this isn't a standard procedure like most others places on the frame.

Roof will have 1" T&G decking over the rafters. I don't know how much tolerance there is until rafter unevenness will make it look wonky and tiles lay too unevenly on top. Thanks for the caution to plane afterwards though it sure doesn't sound easy to do planing way up in the air at an odd angle instead of on sawhorses beforehand.

[Jim_Rogers]

Do you guys believe bending a seasoned hardwood 6x7 is a reasonable proposition?

That all depends on how far apart the "fixed" points are and the devices you have to pull it into place.

It also seems layout would be very error prone to judge where to put the birdsmouth and collar tie mortise so after bending they ended up in the right spot.

When I layout rafters I assume that they are fairly straight. And layout according to the layout procedures. That is the top of the rafter, which is the roof slope is the reference face. And the seat is drawn down from that. The mortise for the collar beam is also draw down from that. Cut all joints and collar beam lengths based on your drawings and dimensions.

After all parts of one assembly are done, you can then do a fit up test. From your drawings you should know your outside of plate to outside of plate dimension.

Like this:



 

This drawing shows your pair of rafters dimension should be. If you put a pair of rafters together, you could set up a pair of short sample "Plate" timbers at the correct distance apart and test fit the pair to see if they will seat correctly. If the pair your testing is "too wide" then you can trim the collar beam tenon/shoulder to allow the pair to come closer together. If the pair are "too narrow" you could shim out the collar beam shoulders to make it the right width.

I normally do this type of fit up with just about every frame I assemble in order to ensure that when the frame is up that the rafters will seat correctly and the roof planes will be flat.

You're going to need to be up on the roof applying the decking and when you find a high spot you can plane it down and make it straight. You really won't know how much it is out until you assemble it.

Another approach I wonder about. In framing we normally layout to the perfect timber--1/2" less than the actual dimension. Couldn't this be done with rafters, assuming rafters were appropriately sized that losing that 1/2" was still within passing strength margins? 

This was done with your rafter design at the peak and at the collar beam. But in order to have a long tail overhang, that you requested, the rafter seat, which is called a crows foot, was needed. In order to correctly layout a crows foot seat you need to know where it is. This was shown in the previous eaves detail that you use a right triangle with the dimensions shown to create the point where you then draw the plumb line and the level line of the seat.
If you don't draw the seat plumb line and level line from this point, then your rafters won't all be the same. This method eliminates the variability of the bottom surface of the rafter, as all measurements are taken from the top down.
This is a standard method of rafter layout, at least it is my standard method of rafter layout when a large overhang is requested.

I can show you pictures of frames we have cut using this method and it works.

Jim Rogers

[witterbound]

I doubt you can bend a 6x7 seasoned hardwood, and then attach it with a screw in a manner that keeps it straight.  I'd plane it, then layout the birds mouth based on the remaining width of the timber the way Jim has described.

[Jim_Rogers]

One way to be sure is to set up a test. Put two timbers the same distance as your fixed points and then pull down on it with something and see if it can be bent.

We have done tests like this with softwoods.

Jim Rogers

[canopy]

I like the idea of setting up a couple properly spaced and leveled blocks as test plates. And it seems like it might be good to make the collar after this test, after assuring the distance is true just in case. Pictures are always nice if you can dig them up.

My layout will basically be the same process, though I will layout from a chalk line rather than the top edge as my timbers aren't real accurately cut. Ideally I would like to chalk line, then plane the reference edge true, then layout but you seem to say the planing step might cause some reaction in the wood.

I am also using birdsmouth housings as shown in the illustration for visual appeal and maybe could add a little protection against shear as long as the rafter tail sits flush on the housing instead of having the gap that I am trying to eliminate in this discussion.

Quick question about this comment:

"You're going to need to be up on the roof applying the decking and when you find a high spot you can plane it down and make it straight. You really won't know how much it is out until you assemble it."

What would cause such unevenness given layout was made to a good reference edge?

[Jim_Rogers]

What would cause such unevenness given layout was made to a good reference edge?

The crown in your rafter that you mentioned.

Jim Rogers

[Jim_Rogers]

For setting up a pair of rafters for test see this story:

https://forestryforum.com/board/index.php/topic,52291.msg754380.html#msg754380

Jim Rogers

PS. with pictures....

[canopy]

Jim, you are dead on. I chalk lined and planed a crowned 6x8 tie beam so the top could be level for floor boards. At the center it needed 1/2" off. After completing planing, the beam immediately bowed an additional 1/4" in the same direction. That much of a crown creates more of a gap than I would want on roof decking or flooring. I wonder if planing the convex side would result in less reaction in the wood. Or any other ideas on how to get a flat surface.

[Jim_Rogers]

I wonder if planing the convex side would result in less reaction in the wood. Or any other ideas on how to get a flat surface.

I don't know for sure I've never worked with the type of wood you're using.

Sorry I can't be of more help other than saying try it on an extra piece you won't be putting into your house. Or that can be cut back to some other length.

Jim Rogers

[witterbound]

Is a 1/4 inch crown really going to be that noticeable, especially if all of your rafters are similarly crowned?  If they aren't all the same, you could arrange them so that the crown diminishes over the length of the roof.

[canopy]

Good idea, thanks.