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Author Topic: Birdsmouth-only Rafter Seats  (Read 2549 times)

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Offline Rougespear

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Birdsmouth-only Rafter Seats
« on: November 29, 2016, 09:31:56 PM »
For most of my rafters I am using a housed birdsmouth.  However, there are 4 rafters in the center of the building where I'd like to use only a birdsmouth to join them to the rafter plate and resist outward thrust.  In this fashion, I would avoid cutting housings into the rafter plate which would later be exposed as the frame is expanded upon and the "birdsmouth"-only rafters removed.  I am aware of the tenancy of the rafter to check/split at the crotch of the birdsmouth.  I am also aware that I would have to map the birdsmouth of each rafter not utilizing a housing because there would be "no perfect timber within" to which to meet accurately.

Any thoughts on this?  I read in Tedd Benson's earlier works where he describes rafters using simply a birdsmouth, but I don't think it's common.  Is the splitting truly an issue?  I am trying to save the look of the rafter plate for when future additions are made to the frame.

Thanks for thoughts.
Custom built Cook's-style hydraulic bandmill.

Offline Heartwood

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Re: Birdsmouth-only Rafter Seats
« Reply #1 on: December 02, 2016, 09:37:55 AM »
Rougespear,
It's hard to visualize this with certainty without a drawing or photo showing sizes, roof pitch, etc., so please provide them when you can. Is the top of the rafter meeting the arris on the plate?
If so, yes, the splitting is a problem if the top of the birdsmouth is doing all the bearing and hasn't been sized to do so; that's what the bottom of the housing on the inside of the plate is for. (We usually cut a horizontal gap on the top part of the rafter birdsmouth to allow for shrinkage so it DOESN'T bear there.)
One solution would be to attach a nice looking chamfered board across the inside of the plate between the remaining rafters to cover the housings and create a reveal.
Another would be to raise your rafters up so there's just a small 1.5" vertical component to the birdsmouth on the inside and no housing (mapping ALL of them), and let the horizontal surface of the birdsmouth take the bearing. You would then have a vertical cut on the end of the rafters at the plate arris, and would need to size that part of the rafter above the plate accordingly to take the load without splitting.

Offline Rougespear

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Re: Birdsmouth-only Rafter Seats
« Reply #2 on: December 02, 2016, 11:04:09 AM »
Hi Heartwood, thanks for your reply.  Here is a photo to support the discussion:



The roof is framed using a common rafter setup, and the birdsmouths are positioned 1/4-1/3 of the depth of the rafter.  I ask about splitting at the birdsmouth because the roof load is only supported at the plate.  As you can see, I have housings in both the top reference face of the rafter plate and the inside opposite the adjacent face, both 1/2" deep.  When I add to the frame in the future, I plan to cut out the rafter plate just inside of both posts seen in the image (removing the scarf and leaving a protrusion resembling the tenon of a Dutch-style anchor beam), and splice 4' additions to the posts vertically on top of the rafter plate.  At minimum, I would like to omit the two housings on the plate above the posts, and at most omit the housings for the two rafters between the posts.  I will likely secure each rafter to the plate with 2x Timberlock screws.

Thanks for your suggestion to cut a gap on the horizontal part of the birdsmouth.  To confirm, this is the part of the birdsmouth meeting the top reference face of the plate yes?  What kind of gap are you suggesting to account for shrinkage on the 8x8 plate?

Thinking out loud here: assuming a nominal 8x8 plate and a 12/12 pitch, how about utilizing a modified step-lap joint?  That is, eliminating the tail of the rafter, and cutting only a "v" in the top of the reference face of the plate.  The rafter birdsmouth could be mapped to the inside of the plate, and a small tenon could be cut to register into the "v"... that should transfer thrust forces from the birdsmouth yes?

Custom built Cook's-style hydraulic bandmill.

Offline Heartwood

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Re: Birdsmouth-only Rafter Seats
« Reply #3 on: December 03, 2016, 10:22:46 AM »
That should work, but I'm confused now; if you're going to cut out the plate anyway why are you worried about what the housing looks like after you remove the rafters? What's going to hold up the opposite rafters? Why do you need a housing on the top of the plate if it's a reference face?
Anyway, the load should be taken by the housing on the inside of the plate and that should probably be 1.5" deep, not .5". You'd need to run the numbers.
The gap on top is usually 1/8" - 1/4", depending on the species.

Offline Rougespear

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Re: Birdsmouth-only Rafter Seats
« Reply #4 on: December 03, 2016, 10:48:42 AM »
To answer questions: the rafter plate will only be removed on one side - on the other it will function as a loft railing; rafters will be removed on both sides of the building as there are expansions to both sides of the original frame, and the roof ridge will be perpendicular to the original frame's; I put housings on the top reference face because I saw them done that way in Jack Sobon's "Historical Joinery" book, as well as them helping to locate the rafters.

However, the more I read Sobon's works, it seems the housing in the top reference face of the plate was theorized being used to provide a good bearing surface on hand-hewn timbers... so I think in the case of rough-sawn square rule frames the housing on the top is not required.  Thanks for pointing that out!
Custom built Cook's-style hydraulic bandmill.


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