Screened in patio rafter spacning at 48"

[arps83]

I'm trying to get my plans approved for a screened in patio.  I'm planning on using 2x6 tongue and groove boards for the sheathing, and I've got multiple documents that say I can span up to 72" with that.

My issue is that I can't find a good document that lists rafter sizing for distances greater than 24", which seems odd given that the sheathing can go up to 72".

The roof will be built with a 5/12 pitch and is 9 feet from the wall to the peak (18 feet across).  So the rafter will go 9'8".  I'm hoping to use two 2x8s at each 48" rafter location.
I'm not putting a ceiling on underneath, the whole underside will be exposed.

Does this make sense?  And how can I convince the inspector that it's ok?

Thanks in advance!
Rick

[rick91351]

What is holding the 2X6 T&G up?

Can you draw it?  IT is sort of not making any sense to me. 

If it is going to be inspected it will have to be drawen first anyway in most cases to pull permits.     

[arps83]

Sorry for any ambiguity.  Taking a step back, there are 4x4 posts coming up from the understructure of the deck.  On top of those will sit a top plate.  Directly above the posts, on top of the top plate, the rafters will be fastened.  The 2x6 tongue and groove will be the sheathing that makes up the roof.

The design is based off of what's documented here: http://www.familyhandyman.com/garden-structures/screen-porch-construction/view-all.  I just can't find any supporting documentation that allows for the spacing laid out here.

Thanks

[rick91351]

If it were me I would take that plan and article down to the inspectors office and asked him how can I do this here?  Is this satisfactory?

Oh by the way you don't have any funky snow or wind loads to worry about do you?  Our up here are 120 lbs snow load and 90 mph wind.  I know I would have to hire an engineer to do something like that.  By the way I remember seeing that article recently and I liked the looks of it.

[arps83]

The planning and development guy is stopping by tomorrow to go over it with me.  Hopefully that will be enough for him.

120lbs?  Yikes!  I'm in Ohio and he said I need to plan for 20psf live load.

[rick91351]

Hey I forgot welcome to the forum from Idaho!!!

Last year our snow was so slight that it was almost none existent and it is the first year we snow birded.   We thought we were going to get hit with our usual......  Wife has informed me we are not going to do that again.  Az was not for her.  I liked it, she wanted to come home and hug and pine or a fir tree.  Sequoia cactus is not very hug-able!     Actually I think the grand kids might have had something......

Good luck and keep us posted....   

[SouthernTier]

This discussion is more or less related to what you are looking for:

http://countryplans.com/smf/index.php?topic=13025.msg169583#msg169583

For spacing greater than 24", you probably are looking at widths of porch rafters more than just 2X's

The above-referenced discussion basically is saying (read it all, not just this summary) is that you can almost double the spacing width from a span table if you double the thickness of the rafter.   By almost, Don_P is basically saying double minus 15%

For example, if the span tables said you would have at least a 15% overdesign with a 2x8 @24" spacing, than you could do 48" spacing with a 4x8. (no I don't know how I am defining "15% overdesign").

At least that what I think Don_P was trying to say.  If nothing else, try out his beam calculator, although as he says, that would mean diddly to an inspector.

[Don_P]

15% over or under would be to multiply the base design strength value (Fb) by 1.15 or by .85... you aren't going win that way, engineering.

Long link warning, I set this up against you every way possible and it checks, these are the folks that make the codebook tables;

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Spruce-Pine-Fir&size=2x8&grade=No.+2&member=Rafters+%28Roof+Live-Load%29&deflectionlimit=L%2F360&spacing=24&wet=Yes&incised=Yes&liveload=20&snowload=-1&deadload=10&submit=Calculate+Maximum+Horizontal+Span

Some background. When members are spaced 24" or closer, the base design value is adjusted upwards 15% to account for load sharing, the repetitive member factor. Snow load design adjusts that adjusted design value upward another 15% to account for the short duration of snow, the load duration factor.  Even though you can take that one, under the member tab I took live load only rather than snow, so we didn't take that allowable 15%. Then to dock you the repetitive member adjustment, because you are spacing at 4', I clicked wet use which docked you 15%. Then just to be mean, I clicked incised lumber, which docked you 15% more. To continue being cruel I kept the deflection limit at 1/360th of span, you could select l/180 for this situation, this adjusted for the modulus of elasticity. I selected typical lumber store #2 SPF. Strength and stiffness wise the rafters pass easily.