I want to build a log home and have a question about the rafters. I want to use an exposed rafter system and would like to use 4"x8" timbers on 48" centers with a 1/2 pitch (12 & 12 slope). The span would be 32 feet between walls, or meeting in the middle at the 16' mark. According to my calculations this will mean that the span will be about 22' 8", not allowing for overhang. Is this span too long? I know that wood species will make a difference, also. What do you all suggest? Would this be okay with most species of wood? Do I need to go with a thicker beam, possibly a 4x10? Is this center to center spacing too far? Is the span simply too far?
HI,
You can make that span work. You will have a little more difficulty getting timbers over 20ft long simply because alot of mills won't go that long and trees aren't always that straight or big enough, but 25 footers are not that hard to track down.
What part of the country are you in? Doug fir is strong, Hemlock is pretty good, SYP can do fine.
WHat is your snow load???
Are you just going with exposed site built trusses? Or is there a loft?
Are you having a bottom chord of 32 ft, or supporting the ridge with a ridge beam?
I had a room built with open ceiling, and 30' across. Used graded 2x12's on 2' centers, supported with a veneer laminated 6" by 18" ridge (24' long). The pitch was 7 in 12.
Seems your 4 x 8 's would need to be a high grade of structural wood (not just any species).
If you use a ridge and a couple of purlins with posts you only have to span 8'; shouldn't have a problem with 4" X 8" rafters.
I would go with a 4 or a 5x10 rafter. It might take a couple more trees, but it will be much stronger & the cost negligible. My house will be 30x36 w/12-12 pitch & I plan on cutting my rafters to a 5x10.
RD
Phil,
We would need a lot more info to advise. wind load, snow load, type and weight of roof material, insulation and interior finish material, etc. all play into sizing rafters. Once you have all of this info you can calculate the value you need to obtain in the rafter and apply it to the calculators you can find at the top of the page for a given specie of a given length, width and breadth.
I would be concerned with how you control the outward thrust in your rafters at the foot. Are you planning to use a tie beam and form a truss or are there walls to buttress the rafters? Will there be a center post? You might want to look at a principal and common rafter system or a principal truss with common rafters between.
Some simple drawings of what you envision would help.
There are many ways to do this, you can even get jumbo SIPS up to 24' long and 8' wide that would span this without rafters.
Bottom line... the span is doable with standard log/timber designs but the method you choose is could be questionable, you don't know until you draw it up and run the numbers.