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DaveBell
Moderator
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# Posted: 24 Apr 2019 08:15am
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Thinking about using 2x10x20 rafters shed roof style, 16" OC, on a 16x32 cabin. Birds mouth and H1 straps. 16x32 slab. 2x6 wall framing. 16" OC and 16' span seems to be the span limit for #2 spruce fir. Metal roofing, closed cell spray foam, and 6" tongue and grove directly attached inside ceiling. 30 PSI snow load. Hampshire county, West Virginia
Should I have any concerns spanning 16'? Thinking about lateral forces on such a design.
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NorthRick
Member
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# Posted: 24 Apr 2019 04:09pm
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2x10's should be fine for the load and construction you describe. Insulation and venting (or not) of the roof is what I'd be thinking about.
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Brettny
Member
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# Posted: 25 Apr 2019 12:47pm
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Can you add colar ties?
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monkeydeck
Member
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# Posted: 26 Apr 2019 12:02am
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2x12s maybe better choice
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ICC
Member
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# Posted: 26 Apr 2019 12:55am - Edited by: ICC
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The AWC calculator shows the following for S-P-F, #2, 16" OC, snowload 30#, using a deflection of L/240 (the usual for rafters with a ceiling attached to them)----
2x10 can span max of 18'5" if northern timber and 17'4" if southern timber.
2x12 can span max of 21'5" northern or 20'2" if southern.
So 2x10 should be quite usable. 2x12 give that much more safety factor. If the interior T&G ceiling was 2x6 and not 1x6 I would recommend 2x12 rafters.
Quoting: Brettny Can you add colar ties?
Collar ties are not used on a shed roof. In this case "shed" is a style of roof not referencing that the building is a shed. Plus collar ties do nothing to increase load bearing when used in the construction of a gable roof. Collar ties are to "tie" the upper ends of the rafters together at the peak to prevent the roof opening up under high wind conditions. That is all they do. Some folks get them mixed up with rafter ties. Collar ties are located up near the peak; in the upper third of the rafter triangle. Rafter ties are in the lower third of the rafter triangle.
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Princelake
Member
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# Posted: 26 Apr 2019 07:03am
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You could also use i-joists or parallel trusses to get more insulation but also comes at a higher cost.
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DaveBell
Moderator
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# Posted: 26 Apr 2019 07:22am
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Quoting: Brettny Can you add colar ties?
Interesting comment. Collar ties are used at the peak for high wind conditions to increase rigidity. Rafter ties are used to prevent wall spreading caused by the downward force of a Gable roof. (ICC pic above)
Your comment gave me an idea of adding 2x10 ties from the shed roof rafter to the walls. I don't think I need them but might be an idea for a shed roof in high wind areas.
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DaveBell
Moderator
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# Posted: 26 Apr 2019 07:24am
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Quoting: ICC 2x10 can span max of 18'5" if northern timber and 17'4" if southern timber.
How does one figure out if a board is northern or southern?
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toyota_mdt_tech
Member
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# Posted: 26 Apr 2019 08:30am
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Quoting: DaveBell How does one figure out if a board is northern or southern?
accent... 
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ICC
Member
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# Posted: 26 Apr 2019 09:58am - Edited by: ICC
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Quoting: toyota_mdt_tech accent...
The grade stamp tells all that matters. Species, grade, if graded dry or green, the mill, seasoned or moisture content, grading agency
There is SPF, sometimes stamped as S-P-F. Those are northern
SPFs, SPF-S, or S-P-Fs is southern. Sometimes the "s" for south is in parentheses (s)
Douglas Fir - Larch (DF-L) also has subdivisions for north and south, using the same system of adding an "s" for southern
Do a search for lumber grade stamps. There are pages out there that explain the grading marks, some with pictures of examples.
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DaveBell
Moderator
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# Posted: 26 Apr 2019 11:01am
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Quoting: toyota_mdt_tech accent
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