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wood demension/strength??

A

Anonymous-0

Well-known Member
Hey folks,
can anyone point me to a chart (??) that will compare strength (span length?) in different demensions of wood?
Example: will a 2 1/2 by 3 1/2 be as strong as a 2x6 (1 3/4x5 1/2)? etc...

And maybe what would a 3-4 inch pole compare to.... Started cleaning house (yard) and am looking at adding some roof space.....

Thanks, Dave
 
(quoted from post at 08:32:26 01/16/12) Hey folks,
can anyone point me to a chart (??) that will compare strength (span length?) in different demensions of wood?
Example: will a 2 1/2 by 3 1/2 be as strong as a 2x6 (1 3/4x5 1/2)? etc...

And maybe what would a 3-4 inch pole compare to.... Started cleaning house (yard) and am looking at adding some roof space.....

Thanks, Dave
Click to expand...
lots of span tables on the net. just google them. any good carpenty book will also have them.
 
JML is correct that span tables are the way you figure the maximum span for a particular application. But you need to know the particular species of wood.

If you just want to compare the stiffness of two pieces of lumber with different cross-sections, you can do that by calculating and comparing the moments of inertia of their respective cross sections. (Assuming that the wood species is the same.) That's quite easy to do for most lumber. This Wikipedia article explains the theory; about halfway down you'll find the formulas for different cross sections.
Second moment of area
 
The 2 1/2 x 3 1/2 would be stronger vertically but the 2x6 would be stronger horizontally. Then a 2x6 yellow pine board is stronger than a 2x6 whitewood board.
 
(quoted from post at 06:03:31 01/16/12) The 2 1/2 x 3 1/2 would be stronger vertically but the 2x6 would be stronger horizontally. Then a 2x6 yellow pine board is stronger than a 2x6 whitewood board.
Click to expand...

Talking about rafters for a shed... Next to no snow load and tin roofing.. And 10 or 15 ft span. Think I got it with google tho......

Another twist would be (if it exists) a comparison of steel (angle, I, etc) to wood..... Have a steel handler as convenient as a sawmill....

Looking at a tin roof of 15ft max

Thanks
 
A 10' span I would just use 2x6 treated pine. A longer span would be better using a 2x8. If you go to a metal building supplier you can get a 6" steel "C" which if kept painted will last forever. You don't have to put them on 16" centers either. Depending on expected snow load you can put them on 3'to 5'centers with R-Panel tin on the roof.
 
Pressure-treated pine is typically (but not necessarily) southern yellow pine, which is much stronger than white pine. Still, it's cheaper to just go with bigger lumber unless space is an issue. Of course, Dave is in Germany, so I doubt he can find much yellow pine. Here in Michigan, much of the treated wood is red pine, not nearly as strong as SYP.
 
I recommend treated wood for any exterior use. It will not rot even left unpainted. However the surface can get cracks and look unsightly if a wood preservative or paint is not used. Then condensation develops between the tin and the rafters and starts the rot process on untreated wood. Also if there is any screw in the tin that leak it will introduce water down into the board rotting a big spot on untreated wood. The only draw back is if the treated lumber is fresh from the manufacturer it is soaked with the water and chemicals and needs to dry thoroughly before painting.
 
Dave you need to know what type of wood you can commonly get. Southern yellow pine is just about as strong as Oak but white pine/fur is half as strong.

Also the roof pitch makes a big difference too. The steeper the less load the rafters will have to carry vertically. Some of the load will be transfered laterally.
 
If you are anywhere nearr a building official, buildinginspector, code office, etc., just ask to look in the back of the Code books at the lumber span tables. That'll tell you allowable spans for any type of lumber, for any roof pitch, span, etc.
 
When framing windows and doors in new houses, we always figured 1" per foot of span. These were doubled up 2xs with 1/2" plywood between. In my example, a 6' door would need a double 2x6. An 8' opening would need a double 2x8, etc.
 
Ask your local building official for a span chart.Every region in the
world will vary.If you are going to wing it just use common
sense.Worked for our ancestors and those structures are still
standing if maintained over the years.

Vito
 
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