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paulz
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# Posted: 13 Aug 2018 11:54
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Time to get a roof framed for the 8x15 leftovers shed (had to buy the studs). Ridge board is 2x8, gable about 3' high. If I put rafters and ceiling joists on 16 or 24 centers can the joists just nail to the rafters and sit on top plates without birdsmouths?
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Borrego
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# Posted: 13 Aug 2018 13:04
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How far is the rafter overhang? Usually nice to have a little seat for the rafter to attach to the top plate, makes it a little more secure...
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paulz
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# Posted: 13 Aug 2018 13:21
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Thanks Bo, about a foot overhang If I do birdsmouth, might the 2x8 ridgeboard, supported at the ends, be enough to eliminate or cut down on ceiling joists, like maybe 2-3, to keep the walls from spreading? No snow.
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Greenland South
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# Posted: 13 Aug 2018 22:06
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Use these instead of cutting a birdsmouth.
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ICC
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# Posted: 14 Aug 2018 01:03
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Quoting: paulz rafters and sit on top plates without birdsmouths?
To me it is not totally clear what that means. Are the rafters going to overhang the wall top plate at all? If so, I picture that to mean the edge of the rafter is to be placed resting on the top outside corner of the top plate.... ??? Then the rfater tie is nailed to the rafter to keep the tail from sliding... ???
The purpose of a birdsmouth is to provide a flat cut that rests on the top plate. That provides a good surface area to transfer the vertical load from rafter to wall without crushing the woodfibers.
The hurricane ties pictured above are to help prevent wind underlift (uplift) from ripping the rafter tails off the top plate. They are not really meant to provide lateral (sliding) resistance. As pictured it is often used for floor or ceiling joists as a positioning device. The hurricane tie will also connect the joist to the wall plate or beam and assist with uplift resistance.
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socceronly
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# Posted: 14 Aug 2018 01:44
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These might save you some time
https://www.strongtie.com/raftertowallconnectors_solidsawnlumberconnectors/vpa_connec t/p/vpa
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paulz
Member
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# Posted: 14 Aug 2018 09:10
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ICC, yes you described what I was thinking, something like these pre-made trusses, only the tie would sit on the plate next to the rafter (nailed to it) instead of under it. The idea being the tie would hold the rafter from squishing into the outer corner of the plate.
http://technistrut.co.za/wp-content/uploads/2018/02/Anatomy-of-a-Roof-Truss.png
To be honest I'm not a great carpenter, had a heck of a time with the birdsmouths on my cabin rafters, although I'd probably do better this time.
Those strongtie connectors are nice, pricey I'll bet.
Thanks for the input guys.
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Borrego
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# Posted: 14 Aug 2018 11:04
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Quoting: ICC The purpose of a birdsmouth is to provide a flat cut that rests on the top plate. That provides a good surface area to transfer the vertical load from rafter to wall without crushing the woodfibers.
ICC is right of course......
And it isn't that difficult to cut a good seat cut, just do the math for the angle and transfer it with your speed square....or if you want to do it a simpler way, take a 2x4 sample and figure out the cut by trial and error (a Tri square will help)....
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ICC
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# Posted: 14 Aug 2018 14:31 - Edited by: ICC
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Maybe this link can help. Play with it a bit with different entry data to see what changes in the displayed image of the rafter. Those measurements can be transferred to the wood and then cut. Borrego's trial and error method works.
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Borrego
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# Posted: 14 Aug 2018 18:10
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What a great tool! Wish I woulda had that a few times.....
That should make it easy, Paul....it's somewhat fun to cut your own rafter seats, and the right way to do it. You can brag to your buddies over a beer....
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Borrego
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# Posted: 14 Aug 2018 18:28
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Here's a good video that shows exactly what to do....
https://www.youtube.com/watch?v=yPL6jOa6AH8
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paulz
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# Posted: 14 Aug 2018 22:27
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Great online calculator, and video. I noticed in the video he cut the birdsmouth with a circular saw by cutting past the intersection of the two cuts on one side so as not to have to hand saw the last bit. Was that just for the video or is that done?
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ICC
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# Posted: 14 Aug 2018 23:33 - Edited by: ICC
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In many cases when cutting notches you definitely do NOT want to overcut even a little bit. But when you look at the normal load paths on a birdsmouth notch, the over cut does not weaken the member in a direction that matters. IF the main load stress was pushing up on the rafter/roof overhang that would be acting to possibly break open the cut. The cut wood fibers weaken the rafter more when uplift force is applied than when down force is applied. Hope that makes sense. That said I have a habit of cutting only to the pencil mark and then finishing it off with a saber saw. Not too much bother especially with cordless tools.
A thought regarding the idea of laying the no-birdsmouth rafter across the outside edge of the top plate and using the rafter tie to hold everything together and transfer the load---- You named the real problem with that without realizing it. All the loads from the rafters to the rafter tie and then to the wall top plates pass through the nails. Nails can take a lot of abuse, bending a little, long before failure as a rule. AWC, the rafter & joist calculator people, also have a connection calculator. It can be used to determine how many and what size of fasteners (nails or bolts) are required to safely handle load. If you use it where the entry field is labeled "side" member, that would be the 1x4 being nailed onto a "main" member (the 4x4 or whatever.)" I've seen people get confused by those terms. Also note that under nail types a box nail is more or less equivalent to a nail from an air nailer. Smaller than a common or "real" nail
In the results the "Adjusted ASD Capacity" value is the pounds that a single nail can support. Overdriving a nail (head of nail below wood surface) reduces the load capacity. Conversely driving a too long nail through the two members and then clinching the point over increases load bearing some. Sometimes you can't get enough nails into the wood space available without seriously weakening the wood. Too many nails crowded together can make splits occur.
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Borrego
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# Posted: 15 Aug 2018 10:21
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Quoting: paulz Was that just for the video or is that done?
Depends also what your rafter size is.....obviously, overcutting a 2x4 would compromise it seriously, overcutting a 2x8 is a different thing. I'm like ICC though and like to finish the cut with a jigsaw, just like cutting stair stringers....
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paulz
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# Posted: 15 Aug 2018 15:28 - Edited by: paulz
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I've cut a couple today using that online calculator and the cut and try method. Looking pretty good, think I got it figured out. Thanks guys!
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toyota_mdt_tech
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# Posted: 18 Aug 2018 09:32 - Edited by: toyota_mdt_tech
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Quoting: Borrego How far is the rafter overhang? Usually nice to have a little seat for the rafter to attach to the top plate, makes it a little more secure...
Not only that, the load is carried over a larger area vs a small corner. Get a "squangle", its an adjustable square that can move to an angle. The nice thing about this tool is, you can eye it to the building, determine pitch or pitch you have now or pitch you want. Lock it down in place and now it can do plumb cuts on both ends of rafters and the part that sticks out is used to mark seat cut/birdsmouth. I have 2 of these and cant imagine building without one.
https://www.amazon.com/Mayes-Level-10231-Squangle/dp/B00004YZP6
This is the woodpecker, same deal, just fancier.
https://www.youtube.com/watch?v=0rfuH_y0jZk&start_radio=1&list=RD0rfuH_y0jZk
I build all my rafters off site. As long as your building is square, you now the pitch, rest is a snap. Measure across from top plate to top plate and then know pitch. Make all rafters identical using this squangle. Take the first pair of rafters (do not cut the tails, run them wild) with plumb cuts at ridge and seat cut. Set a pair of rafters up on each gable end, they will automatically find perfect center. Do same at other gable end. Now lift in ridge board/beam and temp secure it with screws for now. Set it set straight up with bracing, do both end rafter pairs. Now just follow your layout, ie 16 or 24" etc and all will fit perfectly. After all done, figure overhang, use squangle at end rafter tails, make cut line, then his with chalk like and with squangle, make all tails and cut.
I learned this technique from Larry Haun.
https://www.youtube.com/watch?v=4oIq-asaOEk
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socceronly
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# Posted: 19 Aug 2018 13:19 - Edited by: socceronly
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ICC needs a Patreon account for all his posts on here.
I'd send a few bucks a month just for reading all this stuff.
Edit: And several others too....
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