Building a front farmers porch on a cabin in Maine. Planning on using Hemlock logs for the roof rafters, spaced 3′ apart. Log length approx. 8′, porch is 6′. 2′ overhang in front and placing logs on the cabin roof directly over cabin walls (2″ x 6″ construction, 10′ walls) Rough cut 1″ pine boarding and plywood on top of logs, before shingling.
Question is, what minimum log diameter for 5 pitch porch roof. I am presuming 6″ would be OK but.. with all that snow coming off the cabin roof (architectural shingles – 10 pitch)
Replies
With some trepidation I'm going to tackle this question. "Internet Engineering" is not a great idea ... and what's your snow load anyay?
Here is a link to images of a Porch Roof reasonably close to what you are doing. The pitch was 3/12. I can't recall the rafter lengths and don't have my notes with me; overhang was 24 inches. Rafter spacing was 33 inches O.C. The common rafters were 4" × 7" Eastern White Pine.
Second Moment of Area = I = 4 × 7³ ÷ 12 = 114.33333 in^4
Back calculating to find the equivalent diameter:
D = (64 × I ÷ PI)^.25 = 6.94706"
Your pitch is steeper and (without having checked!) I think the logs in the porch images I posted were longer, but your spacing O.C. is greater. It seems like you are in the ball park ... increase the rafter diameter and reduce the spacing to be on the safe side.
However, futher calculations are necessary to ensure your proposed rafters to meet the requirements bending moment, deflection and shear. I'm going to make a note of the values you have posted and check the numbers with the procedure given in the "Log Span Tables".
Rectangular Section to Circular Section Conversion
I'm NOT an engineer and if someone else knowledgeable in this subject posts and says I am wrong, then I'm wrong. Go by their numbers, not mine.
Edited 11/28/2006 11:51 am ET by JoeBartok
Edited 11/28/2006 11:53 am ET by JoeBartok
Edited 11/28/2006 11:53 am ET by JoeBartok
Edited 11/28/2006 11:55 am ET by JoeBartok
Thanks Joe,
Will increase diameter and reduce spacing, just to be on the safe side.
Chuck Clark
Is there a value for the snow load?Joe Bartok
In Maine, use 60 PSF
Thanks netanyahu. I'll try to get back tomorrow with a better estimate for the log diameters.Joe Bartok
Colorado... 80 to 120 depending on county..
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It depends where the poster's cabin is. In Portland and Brunswick, the ground snow load is 50#/ft. At my cabin in the White Mountains of Maine, it's 90#/ft.
I didn't see the latest news after the 50# snow load and also forgot what was going on the roof for decking. So, guesstimating on this basis, throwing in the self-weight and rounding up a hair I came up with a combined load of 210 plf. I used L/240 for the permitted deflection.
According to the Log Span Tables (had to do a bit of interpolating) 6" rafters spaced 3 feet apart should be good for 7.8 feet ... that's treating the rafters as simple beams, uniformly loaded. The values given in the tables are for SPF which has allowable stresses less than Hemlock (is this Western, Northern or, as I'm assuming, Eastern ???). It seems as though my estimate of yesterday was a bit conservative. Or perhaps not.
There's still unknowns here: the rafter end conditions? is there checking? spiral grain? Internet engineering is a baa.....aaadd practice. :) And it would have been nice to check the stresses at the overhang but my hard drive is busted so I can't get at my calculators and I am too lazy to figure this out on a calculator using the formulas in my "Machinery's Handbook". So unless anyone thinks differently: increase the 6" diameter and decrease the spacing as proposed.
If I get a chance later I have a worksheet for round sections (simple beam, unfomly loaded) saved on a CD that I can run on another computer; I'll run the numbers and see what pops out.
PS: If there are notches on the rafters; rule-of-thumb is remove no more than one quarter of the material.
Joe Bartok
Edited 11/29/2006 8:23 am ET by JoeBartok
Edited 11/29/2006 8:25 am ET by JoeBartok
Thanks all.
I believe it is safe to increase the diameter and decrease the spacing. Logs are recently dropped, drying during this winter and installing next spring. Hopefully they will check before installing. Only removing a small amount of log for roof board nailing (flat siding) and possably increasing pitch of farmers roof. Going from a 10' wall to a 7' front porch wall, over a 6' span.
any other info would be appreciated.
Thanks
Chuck
Gave this a quick run through my worksheet. The following permitted values for stresses are from an older copy (2000) of the "Tables" for Eastern Hemlock:
E = 1200000 psi
Fb = 1150 psi
Fv = 80 psi
I assummed a slab 1" deep was removed from the top of a 6" diameter log, simple beam, unifom load, L/240 permitted deflection. Btw, is 6 feet the rafter length or the span. The span or run may be used for calculating snow load but by rights Run/cos 22.61986° should be the length used to determine the dead load.
Density = 30 lb/ft³
Length = 6 ft
Cross Section Area = 25.17686 in²
Second Moment of Area = I = 43.261 in^4
The theoretical load was increased to 335 plf and the beam passed in bending moment, shear and deflection, after that the log "broke". The 335 plf is combined snow and dead load in addition to the self-weight already included in the calculation.
So it looks like you are O.K., but to err on the side of caution stay with increasing the diameter and reducing the spacing.
Joe Bartok
Edited 11/29/2006 10:38 am ET by JoeBartok
Edited 11/29/2006 10:38 am ET by JoeBartok
"Btw, is 6 feet the rafter length or the span. The span or run may be used for calculating snow load but by rights Run/cos 22.61986° should be the length used to determine the dead load."
Chuck: I just had a minute to re-read your post and my dumb question, you already told me. Given a 6 foot span the rafter is 6.5 feet. That's using the 5/12 pitch in the original post. Repeating the spreadsheet calcs gives a max load of 285 plf.
"Going from a 10' wall to a 7' front porch wall, over a 6' span."
I'm in a rush again and probably missing something:
10' – 7' = 3', the run is 6', this is a 6/12 slope, rafter length = 6.70820 ft. the max load is reduced to 268 plf.
Joe Bartok
Edited 11/29/2006 1:05 pm ET by JoeBartok
Edited 11/29/2006 1:09 pm ET by JoeBartok
Edited 11/29/2006 1:12 pm ET by JoeBartok
Edited 11/29/2006 1:14 pm ET by JoeBartok
Edited 11/29/2006 2:52 pm ET by JoeBartok
Reducing the spacing is important to avoid sagging between rafters.the strength of the logs is less important than the connections you use. That could be your weal link
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