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SantanaWoods
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# Posted: 27 Mar 2015 04:13pm - Edited by: SantanaWoods
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Hello everybody
Please look at the image bellow. (the dashed area is the ceiling). I am having a hard time figuring how the roof rafters should look like for a cabin.
Is A correct or B? to be honest both look wrong to me.
If you plan your walls to be 8 ft tall on the inside, and I assume a floor thickness of 1 ft. If you have a hip roof at 23 degrees (almost 4/12) with 1 ft over hang. How do your rafters look like? How do they meet the edge of the walls?
I guess smarter way to ask this question is :
How do you design rafters for 4/12 roof of 8 ft wall with 21 ft width of cabin?
thanks a million in advance.
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toyota_mdt_tech
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# Posted: 27 Mar 2015 04:31pm
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A is correct. The rafter ties (lower area, on each one, while the collar ties, upper) on every 3rd one at 16" centers.
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SantanaWoods
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# Posted: 27 Mar 2015 04:44pm - Edited by: SantanaWoods
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Thanks Toyota for your quick reply. The confusion comes from how and where rafters join the ceiling joists? If the dashed are my ceiling joist with 2x10 and buildup say 1 ft. exactly how rafters get attached to them? The way I have drawn A I do not see it. Also if my wall is 8 ft on the inside, what would be the height of end of overhang ? same 8 ft as it is drawn in A?
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MtnDon
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# Posted: 27 Mar 2015 05:06pm - Edited by: MtnDon
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A is not exactly correct although it could be done. Rafters are notched to the wall plate. For a detail view of how the rafters tail ends are cut according to the IRC, see FIGURE R802.7.1.1 RAFTER NOTCH in the link. (almost all the way down the page. There is also a drawing showing how the ends of the ceiling joists may be taper cut to fit.
The rafters are nailed to the wall top plates. Hurricane ties are usually used,sometimes they are mandated.... cheap way to ensure better attachment of roof to walls.
The ceiling joist to rafter connection is called the heel joint. Use TABLE R802.2.3(9) RAFTER/CEILING JOIST HEEL JOINT CONNECTIONS for the number of nails to be used. Note these are full size common nails, not the skinny box nails found in most air nailers. If air nailing add maybe 25% more nails.
Then on the outside the rafter tails can be left square cut or they can be plumb cut. The soffit area is framed in to provide horizontal soffits, or left slanted. In practice I believe horizontal, not sloped soffits look better. Plumb cut rafter tails make evestrough / gutters easier.
The rafters there don't appear to be notched, disregard that and just use the picture for the soffit types.
Especially when using horizontal built soffits make sure to plan the window placement height to have the soffit clear the window tops.
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creeky
Member
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# Posted: 27 Mar 2015 07:09pm
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as i'm trimming out three buildings this year. those fascia/soffit pics are great. thx.
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MtnDon
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# Posted: 27 Mar 2015 07:28pm
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I did square cut and wish I had done plumb cut...
1. Gutters required odd hangers I don't like as much for a couple of reasons.
2. That angle where the soffit meets the wall is...
a. hard to nail
b. hard to paint when you want wall a different color than the soffit
I didn't think that one out very well. 
Note that if no soffit is used and roof sub-sheathing is visible from under neath that eve strip should be plywood, not OSB. Easiest way is to use a full sheet of plywood on each eve, then switch to OSB.
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Don_P
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# Posted: 27 Mar 2015 11:31pm - Edited by: Don_P
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This might help visualize the rafter/cj (ceiling joist) connection at the heeljoint, the ridge connection and the upper collar tie or strap over the top options at a ridgeboard. The bottom edge of the rafter top plumb cut at the ridge needs to be supported. For a 12/12 pitch 2x12 rafter notice I stacked a ripped 2x to the top of the ridgeboard to get a bearing surface wide enough. You won't be in that situation of needing a very wide ridgeboard at a low pitch, just general info.
Low pitches form a better toggle if you think about them in those terms. The horizontal force in the heeljoint is triple in a 4/12 pitch roof vs the same roof at 12/12, connect the cj's to the rafters well in a low pitch especially. You're forming a rigid triangle of rafters and ceiling joists.
This calc works for common rafters;
http://www.timbertoolbox.com/Calcs/raftercalc.htm
For a hip it gets more interesting, this is a calc I put together to help me keep it all straight.
http://www.timbertoolbox.com/Calcs/irreghip.html
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SantanaWoods
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# Posted: 28 Mar 2015 01:51pm - Edited by: SantanaWoods
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thanks to your generous answers I did some reading and made the following drawing. I marked the dimensions and lumber size.
Two 2x8 at the bottom of 2x6 wall and two 2x8 on top brings the total height to 8' - 6" to give a 8' ceiling. Does this look correct to you?
some questions:
how do you decide how deep to cut the bird's mouth?
roof rafters are 2x10? or 2x6 is acceptable for a 20' cabin width?
ceiling joists I assume would be at least 2x10. (right?)
You guys are fantastic, thank you.
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rwoods
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# Posted: 28 Mar 2015 05:16pm
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your drawing is not correct your birds mouth cut on the rafter should sit on your top plate and your ceiling joist would sit next to the rafter on the top plate also you need to cut the stud length to allow for your top and bottom plates if you want eight feet to the bottom of the ceiling joist
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MtnDon
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# Posted: 28 Mar 2015 08:19pm - Edited by: MtnDon
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Quoting: rwoods your drawing is not correct
Yes, the rafters and the ceiling joists both sit on the upper wall plates.
Quoting: SantanaWoods how do you decide how deep to cut the bird's mouth?
Refer back to the IRC document I linked to previously. The part of the birdsmouth that sits on the wall plate is no wider than the wall plate. The diagram also has dimensions for depth of the cut... the D/4 dimension as shown is important to be no greater than D/4 but could be less. Everything changes as the pitch changes.
Quoting: rwoods you need to cut the stud length to allow for your top and bottom plates if you want eight feet
Use the precut studs that are 92 5/8" long. That's why they are made.
Quoting: SantanaWoods roof rafters are 2x10? or 2x6 is acceptable for a 20' cabin width?
All that is in the IRC pages I linked to before.....
R802.4 Allowable ceiling joist spans.
R802.5 Allowable rafter spans.
Look at those links/tables, they have everything you need to know about sizing for loads. You need to find out what the local ground snow load is to make proper use of the tables. Read the footnotes. Read the other sections as there is much useful and important information in the IRC. That is the information any inspector refers to. If there is no inspection it may be even more important to read and follow the IRC as there is nobody on site to check your calculations or work.
The IRC contains the prescriptive rules, that is what has been shown to work and be safe without needing an engineer; not a how-to-do book. But if read and understood it can eliminate the guessing.
You might try http://www.amazon.com/House-Framing-John-Wagner-Mr/dp/1880029987, it's better than many other books out there.
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MtnDon
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# Posted: 29 Mar 2015 12:06am - Edited by: MtnDon
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For rafter, ceiling and floor spans there is a handy AWC calculator
US snow load info....
If you are going to DIY it pays to take time to learn as much as you can about design as a lot of the info you get online can vary from iffy to dangerous. Asking questions before building is always a great idea!
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SantanaWoods
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# Posted: 29 Mar 2015 03:58pm - Edited by: SantanaWoods
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Thanks a 1000000 times guys. I am really impressed not only by your knowledge but your generosity in sharing it.
I like to learn as much as possible to build it entirely or at least 90% myself (and one lazy but pleasant assistant). Your help is making it possible.
I corrected the drawing accordingly in case it might be useful to somebody else.
One question:
How do you decide which direction to go for ceiling joists? Logically you would one shortest spans, but are there other considerations?
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Don_P
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# Posted: 29 Mar 2015 07:31pm
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Quoting: SantanaWoods How do you decide which direction to go for ceiling joists?
The purpose of the ceiling joists is to tie the bottoms of each rafter pair together across the building, you are building a rigid triangle. Where your ceiling joists lap over one another this would need to be over a load bearing wall. One way to span that 21' with one unspliced joist is to use an I joists (aka TJI), if that is basically unused attic, an 11-7/8" TJI will span that as a ceiling joist and provide a 1 piece clearspanning tie.
The detail I normally do at your soffit and fascia is to align a 2x6 subfascia along the plumb cut rafter tail and in plane with the upper roof surface. Mark the bottom of the 2x6 on the plumb cut end of the rafter tail. from that mark draw a level line across the lower edge of the tail and cut that triangle off the bottom of the rafter tail. Nail on the 2x6 subfascia, a 1x8 fascia then works with a level soffit.
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SantanaWoods
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# Posted: 2 Apr 2015 09:44pm
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Thank you Don. It looks to me that fascia is necessary but soffit is optional (at least for a small cabin) but it is good to know how to go about it in phase 2 of cabin some years later.
Your explanation of ceiling joist and rafter joining and forming a truss is very interesting. Which makes me wonder what happens with a cathedral ceiling. I love the idea of having more open space but I thought it would be much more complicated to build for a novice.
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Don_P
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# Posted: 2 Apr 2015 11:12pm
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When you cannot tie the rafter feet together to prevent them from "toggling" and trying to spread... then hang them from a strong ridgebeam. This is not to be confused with the single 2x ridgeboard typical of a rafter and joist roof, it is a beam sized to carry half the load of the roof spanning between support columns below that are in turn resting over foundation supports under them that carry the beam load in a direct path from beam to foundation. Basically if you can't restrain the rafters, hang them.
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toyota_mdt_tech
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# Posted: 3 Apr 2015 12:01am
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Santana, here is Larry Haun video. Him and his brother are craftsman. Watch all his videos, but the one on laying out rafters is great.
https://www.youtube.com/watch?v=q_6eoL1wZaw
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SantanaWoods
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# Posted: 3 Apr 2015 11:01am
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I watched each of those videos 3-4 times. These guys make it look so easy (which is not). Must be fantastic to be this good a builder. Thanks for all your replies, learned so much.
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