Author Topic: 2 x 10 spans  (Read 3736 times)

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Offline Billisnice

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2 x 10 spans
« on: October 07, 2009, 05:11:06 AM »
Can you span 2 x 10 at 16 foot length on 2 foot center or do you need 16" on center?

thanks

I am looking at a 16 x 20 cabin with a simple shed roof.

Offline MikeT

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Re: 2 x 10 spans
« Reply #1 on: October 07, 2009, 05:29:47 AM »
Are you talking live load?  In my place, my floor joists are 2x10s at 16" o/c.  The roof rafters are 2x12's at 24" o/c for a 12/12 roof.

Offline MountainDon

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Re: 2 x 10 spans
« Reply #2 on: October 07, 2009, 06:19:25 AM »
You can use the calculators at the American Wood Council to run numbers for both the floor and the roof. To obtain a proper answer to the question of what's needed to span 16 feet you need to supply more information. With a shed roof the snow load becomes a very important factor. Available lumber grades also matter; select structural may not be available in all areas or may be quite expensive compared to the common grade #2.

When calculating the span remember that the span distance is measured between the supporting members. That is, if the span from outside of one wall to the outside of the other wall is 16 feet, and the walls are built using 2x6 (1.5 x 5.5 actual) then the actual span would be 16'0" - 5.5" - 5.5" = 15'1"

There are two calculators working in slightly different ways.

http://www.awc.org/calculators/span/reversecalc/reversecalc.asp
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

The deflection value and live load varies with use; floor joists, ceiling, roof with or w/o snow load.  

Examples of code-prescribed deflection limits and live load values are:
    * Living room floors L/360 & 40 psf
    * Bedrooms and habitable attic floors L/360 & 30 psf
    * Attic floors with limited storage L/240 & 10 psf.
    * Rafters  L/240

If the span calculator indicates that a certain size, grade, type of wood would span 15'10" under a certain set of conditions I would recommend going up a size in the lumber or spacing them closer. The calculator gives maximums spans, and in my opinion pushing the limits is not a best building practice. Just my conservative opinion.

Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Billisnice

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Re: 2 x 10 spans
« Reply #3 on: October 07, 2009, 06:26:45 AM »
so 2x 10 at a 16 foot span on 16" center be ok?

Offline MountainDon

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Re: 2 x 10 spans
« Reply #4 on: October 07, 2009, 06:40:31 AM »
I didn't say that.

I said it depends.
It depends on snow load.
It depends on lumber grade.
It depends on lumber species.
It depends on the desired deflection limit.

Right now I am assuming the deflection limit to be L/240 (roof), but I don't know any of the other parameters. Lumber species and grade availability varies across the country. What is used commonly here in NM is different from what someone on GA would have available.

I suggest running your numbers through the AWC calculator. You can then see how changes in one value can effect the outcome. Depending on cost of one type/grade of lumber compared to another it is sometimes more cost effective to decrease the spacing and other times a step up in lumber size may be better advised.



also note that dead load is customary to be considered 10 psf
« Last Edit: October 07, 2009, 06:54:28 AM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline NM_Shooter

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Re: 2 x 10 spans
« Reply #5 on: October 07, 2009, 06:47:23 AM »
Bill, when it comes to structural stuff on this forum, very infrequently will you see anyone say "do it like this"  :D

Are you building on beams, or on stemwalls?  If on beams, your span length is even less as the joists cantilever over.

If it were on beams, I would use 2X10 doug-fir on 16" and not think twice about it.  Do not space further apart than 16".

Look at the calculators that Don provided and play with them a bit to gain confidence.
"Officium Vacuus Auctorita"

Offline MountainDon

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Re: 2 x 10 spans
« Reply #6 on: October 07, 2009, 06:58:46 AM »
Somehow I'm thinking Bill is talking roof, but the others are talking floor. I should have come right out and asked about that for clarity.
« Last Edit: October 07, 2009, 09:27:31 AM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline NM_Shooter

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Re: 2 x 10 spans
« Reply #7 on: October 07, 2009, 09:23:22 AM »
Now that I re-read, I think you are right.  Looks like a roof question to me.... I was thinking floor. 
"Officium Vacuus Auctorita"

Offline Billisnice

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Re: 2 x 10 spans
« Reply #8 on: October 07, 2009, 09:32:24 AM »
That confuses me...lol   If i go 14 x 20 i should be ok with 2 x 10 16" center.  That would be a 14 foot span.

Offline MountainDon

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Re: 2 x 10 spans
« Reply #9 on: October 07, 2009, 09:35:00 AM »
Bill, a 16 foot span may be fine with 2x10. If you tell us whether or not snow load is an issue we could make better recommendations. This is for the roof?
Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline Billisnice

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Re: 2 x 10 spans
« Reply #10 on: October 07, 2009, 09:45:09 AM »
I live in the mts of North GA.  maybe a 6" snow once or twice a year...thanks

Offline MountainDon

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Re: 2 x 10 spans
« Reply #11 on: October 07, 2009, 11:16:49 AM »
That would appear to be in a 5 to 10 psf snow load area, by the maps in the IRC2006.

So if you use the calculator at http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
and plug in the values for ...
southern pine (my guess as to species)
grade #2
2x10
rafters (snow load)
L/240
24 inch spacing
leave the exterior exposure as   no
snow load = 10
dead load = 10,
the calculator comes up with a max span =  22' 3"

Running 2x8's the answer = 18' 8"    Try it see if you get the same numbers.

You can use the same calculator to size the floor joists.


Other questions come to mind when looking at a shed roof. Many roofing materials have a minimum slope recommended. Shed roofs sometimes have rather low pitches where ordinary shingles or even metal roofing is NOT suitable. Sometimes roll asphalt roofing is required with tarred and overlapped joints.

Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?