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acerocknroll
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# Posted: 16 May 2017 02:27pm
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Hey guys, been a while but our cabin in MD is coming along slowly but surely...
At this point we have basically built a box with walls, loft joists, and some plywood sheathing on top to give it a temporary "roof" to tarp over. We are now a little stuck trying to figure out the best course for framing the rafters for our gable roof.
Cabin is 12x16 and we are planning on a 12/10 pitch to the roof. We were already planning on removing the sheathing covering the loft joists because we want to replace the 1/2" on there now with 3/4". Our quandary is that we built the loft joists on top of the top plate for the walls, so there is really no "top plate" to speak of at this point. See below:
Could we double up the header joists (sorry if not correct terminology) on the inside to make enough of a seat for the rafters to sit on? Should we then add another top plate on top of that? Could we somehow install the rafters on top of the 3/4" ply or is that a terrible idea? Can we get away with 2x6's for the rafters or do we need to use 2x8's? Any options or thoughts on how to frame this would be appreciated!
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bldginsp
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# Posted: 16 May 2017 07:01pm - Edited by: bldginsp
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Whether you use 2x6 or 2x8 is a function of load and span. In heavy snow country you might need larger.
It's important to have a solid connection between the ceiling joists and rafters. This makes a rigid triangle. You could remove the rim joist, place rafters against the joists and nail them together. You could put a plate on top of the floor plywood and run your rafters to that, but then you would need to make a secure connection from each rafter to each joist, with a strap perhaps.
Or, install a ridge beam adequate to handle the load, then hang rafters off that and let them sit on the plate on the flooring at the walls. Such a beam will need a little engineering for load, span, and footings, but this eliminates the need for a connection between joist and rafter.
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Borrego
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# Posted: 16 May 2017 09:22pm
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Quoting: bldginsp You could remove the rim joist, place rafters against the joists and nail them together.
This is your best bet for strength and overall build quality, it's how a 'real framer' might build it.....
Then block it and put in some A35's or similar and you're good to go....
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toyota_mdt_tech
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# Posted: 16 May 2017 10:25pm
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That rim joist has to go at the ceiling joist end. The rafter tails sets on top of the walls plate.
Look for making rafters or framing a roof in youtube by a guy names Larry Haun. One of the finest builders around. I have his book "The Efficient Carpenter". He is deceased now.
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ICC
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# Posted: 16 May 2017 10:44pm
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Quoting: bldginsp You could remove the rim joist, place rafters against the joists and nail them together.
IMO, that is the best method. That is exactly how rafters, walls and ceilings or floors are meant to be tied together. A triangle with enough fasteners at each rafter tail to joist connection to handle the loads. There are tables that detail the size and number of nails in the IRC. Also tables there to choose rafter depth, though usually rafters are made deeper than necessary for strength when they are going to be filled with insulation. You didn't mention anything about insulation. I mention it because many DIY builders don't seem to give it much thought until after they have built the roof assembly.
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acerocknroll
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# Posted: 16 May 2017 11:38pm
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I appreciate all the feedback so far! I understand that removing the rim joist would be the best method, I don't mean to be contrarian but would the below approach be an acceptable substitute? For a few reasons, removing the rim joist at this stage would prove to be very difficult.
Again, I do greatly appreciate everyone's input so far!
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bldginsp
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# Posted: 17 May 2017 12:10am - Edited by: bldginsp
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Excuse me for sounding harsh, but just because you built it wrong to begin with is no excuse to continue doing so. That said, the problem with the design you show is that the rafters will kick out under load. Toenailing them to the floor is not enough. They need a solid connection to the joists, or, they need a real beam at the ridge, not just a board.
You'd be surprised how easy it is to rip it apart with the right tools- seen it too many times....
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ICC
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# Posted: 17 May 2017 12:34am
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Understand that the weight on a rafter supported roof is transmitted down the rafter to that heel connection. With a roof pitch like yours about half is vectored down to the ground and about half is pushing horizontally out wards. Your design ends up with the toe nailing carrying all the horizontal force. When the rafter heel connections are made to the floor or ceiling joists, with the joists nailed to the rafters those nails are carrying shear loads only. Very much stronger than toe nailed connections.
Sure it will be a little work to remove the rims. You can cut the nails with a sawzall and nail cutting blade instead of trying to pull the nails. Then you can rest assured that you will have a strong properly built roof.
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acerocknroll
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# Posted: 17 May 2017 12:35am
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No offense taken bldginsp, we are looking to learn here
I'll try to work up a new model tomorrow, thanks again.
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acerocknroll
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# Posted: 17 May 2017 12:45am
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Something more like this though?
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toyota_mdt_tech
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# Posted: 17 May 2017 08:54am - Edited by: toyota_mdt_tech
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Quoting: acerocknroll but would the below approach be an acceptable substitute?
I'd say that is the standard.  The collar ties at the top are only needed every 4 feet usually, but adding them to all makes it HD and you way want to trim it out, this would be a flat ceiling area for lights, fans etc.
Before you start, buy a tool called a "Squangle", ie a square and an angle, once you set it for your pitch, ie 4/12 etc, then lock it and leave it, does the ridge connection and the seat cut marks with one tool. Also, the ridge on this set up isnt load bearing either.
When you get a pair of rafters made, try fitting them, make 2 identical, then set the tails on the wall plate, let the 2 ends come together to find center, then slip the ridge in between the ends at the top. Adjust the angle if needed. Once set, build all of them using one for a template. If your building is square, they will all fit perfectly. Fine the centers at each end like I explained earlier, set the ridge in at each ends, secure temporarily for fine adjsutments with a screw and then make it level, mark layout (pull from same end on both sides) and put them every 16" collar tie every 3rd one minimum.
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bldginsp
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# Posted: 17 May 2017 08:57am
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Quoting: acerocknroll Something more like this though?
Bingo
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toyota_mdt_tech
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# Posted: 17 May 2017 02:11pm - Edited by: toyota_mdt_tech
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OK, I see you added the plate on top of the sheeted floor (original design). You need to cut your sheeting back, remove the rim joist, then install the rafters like the latter design
It looks like you are willing to spend lots of extra money on lumber to avoid removing rim joist.
Leave the sheeting up there, its what I did, it makes it easy to build your roof (nice floor to walk on vs walking on rafter ties) plus you have super flat storage in attic. But remove the rim joist you have at the end now in design, cut the sheeting back to clear the rafters (set saw at 1/2" depth, easy deal) and then add rafters right on top of the wall plate and up against the ceiling joist (which will be your rafter ties)
You will end up with your second design and have the full sheeting up in the attic, which makes life so much easier. (and you cant add it later, so best o have it done now, you will thank me for the super nice storage area.
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acerocknroll
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# Posted: 17 May 2017 02:48pm
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Yeah I like the idea of leaving the sheeting up and cutting a little out to get the rafter to the joist. I think we'll go that route. Thanks very much for the sage advice everyone
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toyota_mdt_tech
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# Posted: 17 May 2017 05:44pm
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Quoting: acerocknroll Yeah I like the idea of leaving the sheeting up and cutting a little out to get the rafter to the joist. I think we'll go that route. Thanks very much for the sage advice everyone
You have done well grasshopper. IMHO, best option right there.
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bldginsp
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# Posted: 17 May 2017 10:25pm
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Good luck on your project acerocknroll. We haven't discussed loads, spans and other structural issues. But be aware that the number and type of fasteners joining the rafters to joists is very important, particularly in snow country. ICC referred to the building code provisions on the subject. Always nice to see someone willing to learn. Keep us posted with pics.
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acerocknroll
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# Posted: 18 May 2017 03:38pm
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I'm all ears to discuss those issues, bldginsp. We will be in MD and we do get a couple feet of snow a year. What info is necessary to determine the correct type of fastener?
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ICC
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# Posted: 18 May 2017 06:33pm
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IRC, Chapter 8, table R802.5.1(9) has rafter / joist connections listed.
https://codes.iccsafe.org/public/document/code/553/9838540
Read the footnotes
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