69scode
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Posts:5
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| 02/26/2008 12:25 PM |
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| I found this site a little while ago and i'm glad to see there is a site for sharing information on energy effiency. I plan to build an ICF house next year. My question is; Can I bevel cut the top of the wall to match the rafter angle? I will use Icynene spray foam insulation in the rafter space. My thinking is that there will be a stronger connection between the wall and the rafters if these angles are the same instead of cutting a birdsmouth in the rafter. If this will work, can I use an I-joist for the rafters instead of 2x materials? |
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Alton
Registered Users
Posts:307
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| 02/26/2008 1:58 PM |
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Concrete does not like to be beveled to a thin edge. Thin edges are very weak - think of shearing. Much simpler to leave the ICF wall flat and use a top plate with the appropriate metal clip (such as Simpson Strong-Tie H-1, etc.).
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
334 329-0957 AT&T Cellular |
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69scode
Registered Users
Posts:5
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| 02/26/2008 4:06 PM |
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| I didn't think of the thin edge part. I guess what you are saying is, at the top or inside part of the wall the concrete will taper to a point and it will be weaker the closer it gets to that point. That makes sense. If I use a flat pour with a top plate can I still use plywood I-joists in place of 2x material for rafters? My thought is make life easier on the framer, me and my friends, and to reduce thermal bridging through the rafters. |
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Alton
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Posts:307
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| 02/26/2008 7:35 PM |
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You can use rafters, trusses, I-Joists, etc. provided that you use a top plate and have a way to attach the roof to the wall. You can also have an angled top plate. What I have done is use a double top plate with the upper plate cut at an angle to match the roof. With the taller steel clips that are available today, you do not really need an angle cut on the upper plate. I would set the top plate or double top plate flush with the outside wall instead of the inside edge so that the rafter would clear the wall. After the roof is on then spray foam up against the wood top plates. In my area, we can use treated wood on top of the concrete or use untreated wood with galvanized metal under it.
If you use I-joists, then be sure to find the right clip or arrange to anchor them some way. Do not just nail the rafter to the top plate. Uplift winds can pull out the nails. Nails and screws are best used at right angle to the uplift. That is another reason clips are much stronger than just nailing. With clips, there is no need to cut a birdsmouth in the rafters.
If you need more help, just let us know.
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
334 329-0957 AT&T Cellular |
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Jim C
Registered Users
Posts:18
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| 02/26/2008 7:50 PM |
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It will not be a stronger connection. Typically anything over a 3:12 needs a birds mouth. This transfers the weight into the wall.
For instance if you use a 6:12 and bevel the top of the wall the rafter will want to slide off, now add the rest of the roof and theres quite a bit of wieght there, now you are relying on the clip to prevent this, the clip you use will have to have a pretty substantial horizontal sheer.
It will be much simpler to cut a birdsmouth and use an embedded strap (ex. simpson heta 12). No need for a sill plate, you can also get a galv barier plate to go on the bottom of the birdsmouth |
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Alton
Registered Users
Posts:307
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| 02/26/2008 8:00 PM |
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I have seen I-joists used as rafters. However, they do not lend themselves to having a birdsmouth notch cut into them. By the way, on some of my projects with wide spans, I have found wood attic roof trusses costing less than long finger-jointed rafters. If you do stick build, then do not forget the collar ties.
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
334 329-0957 AT&T Cellular |
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irnivek
Registered Users
Posts:268
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| 02/26/2008 8:49 PM |
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We recently did one with a 10/12 bevel for top plate, used ICF CONNECT brackets for rafter tie down to 11 7/8 I-Joists. The I joists gave a nice flat roof for metal. We had no problem with a concrete "thin edge." Be sure to block the I Joists ends as specified in the manufacturers literature.
In the past we remodelled wood frame houses in CO Springs which were built with I Joists bearing on bevelled stick frame walls, even they held up great with no bowing of walls evident, although this stress should definitely be addressed during design.... |
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69scode
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Posts:5
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| 02/26/2008 10:01 PM |
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| Thanks for the information. If I use I-joists I would use blocking at the ends and where the ICF connecter meets the rafter. I would also connect the rafters with collar ties. My house plan will be a 1 1/2 story where the "attic" space will be finished as a game room and third bedroom. My goal would be as open an area as possible, hence the desire to use I-joists in place of 2x rafters. As far as stressing the walls, I guess that will have to be addressed by the block manufacturer I use. And that will be another post. Thanks again! |
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Alton
Registered Users
Posts:307
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| 02/26/2008 11:32 PM |
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You will need to give some thought as to how you will connect the collar ties to the I-Joists rafters. With 2x6 rafters, you just nail the collar ties to the side of the rafter. With the I-Joists, how will you mate the two together? Get some ideas and get back to us.
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
334 329-0957 AT&T Cellular |
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Jelly
Registered Users
Posts:268
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| 02/27/2008 3:25 AM |
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Posted By Alton on 02/26/2008 7:35 PM
...After the roof is on then spray foam up against the wood top plates. In my area, we can use treated wood on top of the concrete or use untreated wood with galvanized metal under it.
Does it matter what material is used for blocking between the rafters at the top plate? Especially if one is making a non-vented attic and spraying insulation to seal the space?
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woulfcc
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Posts:185
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| 02/27/2008 11:10 AM |
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All the I joist I have ever used can have the web filled with 3/4" or 2x and be cut (birds mouth) or nailed together.
The manufacture has all the details and loads that it will handle.
We have also used micro lams as a top plate and built a hip roof with I Joists with out collar ties, This was a cool cathedral ceiling. |
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Changing how the world BUILDS!
One build at a time.
Woulf c.c. |
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irnivek
Registered Users
Posts:268
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| 02/27/2008 9:44 PM |
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Any time perpenducular wood attachments are made to BCI (Boise) / TJI Silent Floor etc. the center OSB "web" must be filled flush with the top and bottom chord plates. Depending on your series/size of joist, usually 3/4 plywood does the trick and allows attachment of hangers etc.
Check out Boise BCI installation specs. they also have great hints on how to keep a silent floor "silent" and why squeaks exist. |
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Buntly
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Posts:95
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| 02/28/2008 7:17 AM |
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Not sure exactly what kind of situation you have going on, but I have used floor trusses with a pitch built in for this situation. Very simple, no blocking required at heel and we had a pocket at hi end to rest on beam. Plus you have a 3.5" nailing surface for sheating etc. See attached for example. Depending on depth, span, and loading, you could possibly use a flat truss, but then you are limited a bit more to depth.
Bunt |
Attachment: floor roof example.pdf
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69scode
Registered Users
Posts:5
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| 02/28/2008 1:49 PM |
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Posted By Buntly on 02/28/2008 7:17 AM
Not sure exactly what kind of situation you have going on, but I have used floor trusses with a pitch built in for this situation. Very simple, no blocking required at heel and we had a pocket at hi end to rest on beam. Plus you have a 3.5" nailing surface for sheating etc. See attached for example. Depending on depth, span, and loading, you could possibly use a flat truss, but then you are limited a bit more to depth.
Bunt Now that is what I've been talkin' about. I was planning on an 8/12 pitch so this truss idea looks great. Was this truss specific to one manufacturer, or do you think anyone can/will do this? And will I need to add collar ties for strength? I will add a tie for a sheetrock nailer, I am wondering if size matters here. Also, I don't see on this drawing a way to add a soffit without building out. I am looking for some overhang to mount gutters and possibly lighting. Do you think the truss could be extnded past the wall?
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Buntly
Registered Users
Posts:95
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| 02/28/2008 2:11 PM |
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There are many things you could do to get a better nailer for your overhang. If you can go deep enough, I would use a truss rather than a pitched floor truss (shown above). You would then have something to nail your fascia to, otherwise the truss mfgr could plate some material on the truss for an overhang. This type of truss does not need a collar tie. Are you in a high snow load area? This truss was designed for a 25 psf ground snow load. Not sure what you are doing for insulation, but you will need to consider that as well. Truss could be designed to cantilever over wall. Any truss company should be easily able to design this truss. You need to get them span, pitch, depth, etc. See attached 8/12. This is a std truss (2x4, not 4x2) so you have something to nail you fascia to.
Bunt
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Attachment: parallel chord 8.12.pdf
Attachment: parallel chord 8.12. modified oh.pdf
Attachment: parallel chord 8.12. bearing detail.pdf
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69scode
Registered Users
Posts:5
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| 02/28/2008 3:06 PM |
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| Bunt, those drawings are excellent. I'm not sure I want to go that deep on a rafter/truss. I am in central Virginia, so snow load is not a big concern. For insulation I will be using Icynene spray foam. There is a large home show in Richmond this weekend, so I'll talk to some reps there for more input. Thanks for the help. |
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Buntly
Registered Users
Posts:95
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| 02/28/2008 3:51 PM |
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I've used as shallow as 12" deep,......those shown are 24" deep.
Bunt |
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