Beam and Rafter Sizing help

[chasenabby]

All,
I could use a little help!
The cabin below is under construction and is 12 x 12.
I need to know what size logs to use for A,B & C.
Assuming the rafters will be placed on 2'0" centers
We are using red pine logs





 

[Jim_Rogers]

What is the snow load for your area?
What is the roofing materials per pound per square foot?

[chasenabby]

The snow load would be 70#
We will plan on using 5/4 pine boards, felt and asphalt shingles.
Not quite sure how that figures out?

[Jim_Rogers]

Any rafter overhang from the side wall?

With common rafters the boards will run from gable to gable so you won't be able to use them to create the overhang.

[chasenabby]

I guess I am confused.
I planned on using logs for reference "C"  with them extending 16" past the side walls for an overhang.
The plan also was to run the top side wall logs past the ends to create an overhang on the gable ends.
then installing 5/4 boards horizintally gable to gable.
I planned on nailing the rafter logs at the top to the beam and then again on the side walls.

[Jim_Rogers]

I did understand that you wanted to use logs for reference log "C" but your dimension shows it ending at the wall. The actual log length would be 7' 3" + 16" over hang. You have to figure the entire length in the load calculations.

7' 3" + 16" = (87"+16"= 103") or 8'7".

8' 7" length x 2' width = 18 sqft x 85 lbs (70 lbs snow load + 15 dead load which is roofing materials) = 1530 lbs per rafter.

Using DonP's log beam calculator, and entering in the load and span of 72" we now need to select log type. Unfortunately Don's calculator doesn't show red pine. So I used Grade #2 eastern white pine and it says that the log should be 5" to 6" in diameter.
For the rafters.

If I have done it correctly. 
All designs need to be properly reviewed by a structural engineer to insure that the design is good.

Jim Rogers

More later on logs A an B.

Edit: Very big math error on my part. Corrected values are above.

[chasenabby]

Jim,
Thank you so much for your help.
No need to figure out A and B.
I do not have access to logs of that diameter.
I will use regular lumber for the roof system.
Thanks again

[Jim_Rogers]

Sorry about that math error.
I was using a calculator that was suppose to convert feet and inches into square feet and it came up with the 206 number. But of course that was wrong.

To figure A you'll need to do more math.
The load is spread between the rafter foot and the ridge. So you use 6' as half the load on the ridge and 3' on each roof side goes to the plate/wall.

So 6' x 12' = 72 sqft x 85 = 6120 lbs.

Now with that number and the same log beam calculator, under the red tool box on the left side of the forum page, we see that the log has to be between 10" and 11 1/2".
I say 10" as the column on the left says that for full size logs. The column on the right says 11 1/2" for sawn logs. If you're going to cut the top edge of the ridge log then I would say you should go for a larger log and make sure it's going to be ok.

Jim Rogers

[Jim_Rogers]

Now for log B which is short and shouldn't have any buckling you can use DonP's column calculator.

I used some values I found in my NDS book that says red pine parallel with the grain for Grade #2 is 475 (Fc value). MOE of 900,000 and half the roof load which is 3060 Lbs could be held up with a 4x4. But if you have 6" logs I'd use that to be sure.

Hope that helps.
Jim Rogers

[chasenabby]

Jim,
That helps me a ton!
I never would have figured it out on my own.
Thank you very much!
I will post some pictures once we get the logs all stacked and the roof logs in place.
Rob

[Thompson Timber]

Hello forestry forum folks,

I am building a barn for a client and I would like to find some information on references for sizing rafters. 
The barn is 24'wx36'L. It has a 12/12 pitch roof. The rafters are locally harvested north east Vermont spruce and fir. They are supported by a timber plate at the foot and a ridge beam at the peak. This makes them almost 17' in length. The snow loads here are #65lbs/ft.
I am planning to use 6x8s or 6x9s for rafters spaced somewhere between 32" and 48". Planning to use rough sawn 1"8"s to sheath the roof and then screw down corrugated steel roofing on top of synthetic felt paper. What do you think? Is a 6x8 adequate or do I need to go bigger?
Could you recommend a book of a website or somewhere to look up timber sizes for differnt spans and roof angles?

thank you in advance, TT

[Don P]

Check out the toolbox, bottom of the left column of this page, DonP's calcs> beam and column calc's> simple beam, drop down heavy timber calc.
Give it a whirl, post a pic of your plan if you can and holler if you need help