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Author Topic: Clear Span Roof Truss Calculator  (Read 7777 times)

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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #140 on: November 11, 2019, 12:39:04 AM »
Thanks a lot.

Moved ahead on lot's of fronts with Ben this evening. Decided to drop the 3 metal/wood combination trusses for a variety of reasons. Going with large posts that hold the 2 larger timbered purlin beams 10' apart, then running 3x or 4x12 rafters over them. I think 3x12 DF pass @ 4' OC. If not, what's the widest they can go?

Question though. How does one fasten the rafters to the purlin beams typically? Also the rafters will be 3x12s (maybe 4x if that's the width needed to go to 4' OC ) at least. That's what is in the rest of the house and I think it'll not look right if it doesn't match or what do you think?

Also thinking on this roof system. What about running 2x4s as purlins edge up over the ceiling decking, putting in R13 fibergalss batts in, then screw OSB through the 4" polyiso insulation to the 2x4 purlins. That'd be a R 39.6 roof. Bad thing is the 4" polyiso is a $1.81 per sq ft. 3" I can find for $1.48 used, but in Denver or Dallas. R there is about 30. Doing it this way would be a lot more stable in our windy local me thinks.

Just a thopught, if I went with 2x6s instaed, how'd you fasten them besides drill straight done through it and screw it?

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #141 on: November 11, 2019, 07:12:29 AM »
I'm glad you had a good conversation, it sounds like things are settling. If you want to match the rafter sizes with the others in the house let him know, that might play into his dead loads. The connection details are part of what they will be providing. I can't help with the insulation build up, again work with them on those details.  
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #142 on: November 11, 2019, 09:06:53 AM »
I emailed him on both topics before i posted, but I was wanting some objectivity if possible. That's how the best idea's usually are formed. The main reason we dropped the other idea was it just was going to look too cluttered, but my layout/design passed there design values all the way around. If my memory serves, didn't/don't you use something similar roofing on some cabins you built? I just cannot remember for sure where I saw that.

Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #143 on: November 11, 2019, 04:35:34 PM »
Here is what i found today. I'm sure you know what it is Don, but in case anyone else doesn't. This gets screwed to the roof decking and then you screw into the roof purlin (?).  Would you fasten it in a different way?




Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #144 on: November 12, 2019, 02:50:45 PM »
@Don P Didn't you have some cabins you used polyiso on the roofs? How'd you build those out? Thanks

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Re: Clear Span Roof Truss Calculator
« Reply #145 on: November 12, 2019, 04:49:25 PM »
I'm just back from measuring "granny's house" on this family compound for a remodel. Anyway, this is what we did, 5" of polyiso over the 2x6 T&G roof deck, 2x4 strapping, ply and shingles.


 

 
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #146 on: November 12, 2019, 05:47:28 PM »
Looks like you just glued the iso to the decking, then screwed the 2x4s through the iso to the decking, then screwed the ply to the 2x4s. Amiright, amiright.....;D

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #147 on: November 12, 2019, 09:54:28 PM »
No glue, all screws and nails. Every other 2x4 is over a 4x10 FOHC dougfir rafter so those screws go thru 2x4, foam, decking and into the rafter. every other 2x4 is just screwed into the 2x decking. then the ply is nailed to the 2x4's.

That was the detail on that job, sitting here I can think of a couple of other ways we've done it, just depends on the plan detail provided.
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #148 on: November 12, 2019, 10:50:42 PM »
Ahh k. Ben didn't specify any certain way to do it just yet, he just told me come up with a couple of different ways and he'd see if it works or not. He liked my 2x6 purlin 2' OC with R23 batts and then 2.5" iso glued to the purlins, then 5/8 OSB screwed thru the iso into the 2x6 purlins. That's why I was looking for ways to fasten the 2x6s to the decking. He suggested just cross screwing, but I'm not a fan, so I will use those clips every 2', then drill and screw into the decking and where there are rafters into them at 4 points in the purlin, plus i'll lay a bead of construction adhesive under the 2x6. That's what I came up with so far. Over kill? 

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #149 on: November 13, 2019, 08:55:00 AM »
You'll need to attach the edges of the ply on something like 4-6" centers, 12" in the field so think about getting that done effectively, APA's help desk might have some ideas or their site might have some ready details, apawood.org I think. Glueing on the job is considered structurally to be zero, way too wild an environment. For instance its 12 degrees out there right now and we just decided to hold off crawling up on the roof for a couple more hours, so all depending on the work day you can be trying to apply against weather, temp, wet, etc with unknown gaps and clamping efficiency. Make sure your mechanical fastening is up to snuff, we were screwing those 2x's on I think 2' ctrs but your uplift probably controls, and consider glue to be gravy.
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Offline Sedgehammer

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Re: Clear Span Roof Truss Calculator
« Reply #150 on: November 13, 2019, 10:25:32 AM »
question though. you mentioned 4-6" on the edges and 12" in the field. On a typical house trusses, they are screwed down only where there are the trusses of course, so why all the extra?

Offline Don P

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Re: Clear Span Roof Truss Calculator
« Reply #151 on: November 13, 2019, 10:35:03 AM »
On the trusses, purlins or rafters, 4-6" along the sides of the sheet to keep the edges down and transfer racking loads. In the rest of the sheet 8-12" along every rafter to keep the sheet flat when the racking tries to pop the sheet out of plane. For the other 99% of the time it keeps the sheets flat on the roof from warping. The roofs with 8 or 10 nails in them are the ones you see heading cross country in high wind, a roof gets down pressure but also lift and racking as wind passes over it.
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