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Roof spans for large timber

 
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I have searched the forums and perhaps my terminology is incorrect so I’m hoping someone can weigh in. My husband and I are building a 24x30 strawbale building (non-load bearing). We are looking to use 6x6 to frame it out (rough cut, possibly 8x8 as we can get this at a reasonable price - pine/hemlock). My question is roof spans. I hope I’m using this correctly. I see very large timber being used in timber framing for roofs but cannot for the life of me understand the distance between rafters. The only weight issue we have is snow loads... like 100psf. No seismic or wind issues here where I am in upstate NY. Does anyone have a good resource or chart that I can understand? Or know the answer? I’ve been reading “timber framing for the rest of us” but I must be missing something. I’m a long time lurker here and you bunch are the most knowledgeable people I know!

Thanks for... well anything lol
 
pollinator
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Angela Forget wrote:I have searched the forums and perhaps my terminology is incorrect so I’m hoping someone can weigh in. My husband and I are building a 24x30 strawbale building (non-load bearing). We are looking to use 6x6 to frame it out (rough cut, possibly 8x8 as we can get this at a reasonable price - pine/hemlock). My question is roof spans. I hope I’m using this correctly. I see very large timber being used in timber framing for roofs but cannot for the life of me understand the distance between rafters. The only weight issue we have is snow loads... like 100psf. No seismic or wind issues here where I am in upstate NY. Does anyone have a good resource or chart that I can understand? Or know the answer? I’ve been reading “timber framing for the rest of us” but I must be missing something. I’m a long time lurker here and you bunch are the most knowledgeable people I know!

Thanks for... well anything lol



Appendix B is the place to focus for this stuff.

Span is the length a rafter crosses. Using the terminology in Timber Framing FTROU, simple-span is when there is no center support, while a double-span rafter has a center support.

The space between rafters is the rafter spacing, or frequency.

In the example calculations, frequency is treated as a fixed value of 2.5ft, so the calculated values for max bending and sheer loads are multiplied by 0.4 at the end of each calculation, because you have only 0.4 times as many rafters as a 1ft spacing.

If you went through the calculations and found younhave insufficient strength, you could then adjust the spacing using a different multiple, to see if that will give you enough strength. Or, you can adjust the rafter size, or both..


To go through these calculations, you need a design load, and specs for your type/grade of wood.
 
Angela Forget
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homeschooling tiny house fiber arts
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Thank you! Why didn’t I think to look there *sigh*... My timbers are coming from the Amish (whose buildings have definitely stood up to the worst winters) but I’m not sure I’ll know a grade... I’ll have to see what my math comes up with. Thank you for taking the time to correct my terminology and directing me where to look. This could have been why I was so confused to start.
 
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pollinator
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Angela Forget wrote:Thank you! Why didn’t I think to look there *sigh*... My timbers are coming from the Amish (whose buildings have definitely stood up to the worst winters) but I’m not sure I’ll know a grade... I’ll have to see what my math comes up with. Thank you for taking the time to correct my terminology and directing me where to look. This could have been why I was so confused to start.





No problem! In my rush earlier I dropped a couple words, now amended; you are no doubt aware that span is the distance a rafter *spans*, not the linear length of the rafter!


I use #2 specs for timbers that I mill myself, and do my best to 'grade' them to be above that standard.. and then I overbuild, to be safeish.. Of course the actual 'safe' way is spend a b...oatload of money getting a licensed grader to grade the lumber, and a licensed engineer to design the structure.. etc...


The seller should at least be able to ID the species for you, which is some sort of starting point; I would expect they will also be able to select and supply suitable wood for rafters and posts; ie, let them know that the 4x10s would be for rafters at ## span.


Hope the math treats you well. It took me a few reads to get my head around it, and I'm sure I'll be reviewing extensively next time I go to use it..

Look forward to seeing more about your project!
 
Angela Forget
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So I read that appendix.... now I remember why I hated algebra 😭 I even looked up some plug and go tables.  I have a newfound respect for engineers lol.  I don’t mind overbuilding, but I don’t want the roof to be top heavy if that makes sense. I’ll keep plugging away and will post when I have something to show lol
 
pollinator
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As well as knowing your span you will also need to determine your slope, the greater the slope the faster is sheds snow weight and the less snow load buildup.  Also the greater the angle of the timber to the snow load the stronger the wood is, ie imagine a boards longitudinal strength (standing upright) as compared to it's span strength.

100 pounds per square foot is quite a snow load, we are rated the same here.  My house has vaulted ceiling through out much of the house and much of the wood supporting my ceiling is still visible.  This house is quite old and was originally just half the size it currently is and so it has a number of structural styles within it.  The original house side has old hand sawed tamarack beams and rough milled roof supports.  The new side has dimensional roof supports some of which are still open to view.

We have quite a slope on our roof, drops 7 feet in height for each 10 feet in span.  I would suggest a similar slope for your roof and metal roof as it tends to shed snow well.  Even with that steep slope we get clumps of snow falling off our roof that weigh in at as much as a ton or two in a loud sliding whumppp....  The snow hits the ground so hard that it actually shakes the house with the impact sometimes.  Another thing to keep in mind on roof slope in a place with that much snow is how far to overhang your eves.  The further you overhang the eves the less snow you get piled directly against your walls in the winter.  We have a 24 inch overhangs and we get snow built up to five feet deep against the house walls in winter here.

As for spanning wood, I cannot give a simple easy formulae, but I can tell you the sizes of wood we have and the spans they are supporting, might at least put you in a ball park range maybe.

Upstairs the vaulted ceilings are 4 x 6 beams spaced at 60 inch intervals.  The beams are fitted bolted together at the peaks, on the bottom ends they have 16 foot long 3 x 10's bolted to either side of each 4 x 8.  This gives a span of 8 feet from the center to the outside and a fall of 6 feet over that 8 foot span.

Downstairs in the front of the house where it has been added onto we have vaulted ceilings that go up to 18 feet in height and that is supported by two different systems dependent upon where you are.

One system that incorporates into the original structure uses two 2 x 10 boards to span ten feet with a drop of 7 feet over ten feet.  These two 2 x 10's are bolted on either side of a 3 x 10 on the underside spanning ten feet across the room, these supports are spaced at 6 ft apart..

The other system of support is a single 2 x 10 spanning the 10 feet at the same angle 7 foot drop over 10 foot span with no lower supporting system these supports are spaced at 3 feet apart.

The enclosed portion of this end of the houses newer style roof is supported by 2 x 6's spaced at 16 inches, spanning 20 feet at a drop of 7 feet in every ten feet of span. This system also has a major beam support midway that is made of 5 sandwiched 2 x 8's to create a free span ceiling from the upstairs peak all the way to the downstairs wall.

We get an average of 105 inches of snowfall a winter here and we have never seen the roof stressed in any way shape or form.  I would have in the past said that we don't have to worry about earthquakes but we have actually two relatively powerful quakes in the last three years now.  We some floor beams in the last one as they were apparently just sitting on the foundation rather than being attached, but in spite of that our roof is quite strong and stable.  

When I used to build pole barns 25 years ago we commonly used 24 foot 2 x 10's notched to nail to a 6 x 6 post on the end, then it would span 10 feet and rest on top of another 6 x 6 post and then it jutted out over that post to be nailed to then end of the opposite 2 x 10 for the peak of the roof.  This created 10 foot pens on the outside of the building and a 16 foot wide aisle down the center.  We put in these 2 x 10 roof supports every 8 feet and then nailed 2 x 4's spanning them to attach the tin to.  The snow load down there was not quite what it is up here but it still up around 75 pounds per square foot I believe.  These roofs were also not as steep as the roof on our house.

I don't know exactly how helpful any of this might be to you but maybe it will give you some idea.
 
Angela Forget
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That’s exceptionally helpful, thank you Roy! Yes we can get about 300+ inches of snow in a season here. Steep pitch and metal roofs are a must lol. We were definitely going with 24 inch overhangs as we are using strawbale for infill and need to be able to protect the walls.  The 2x10 spaces at 3 feet sounds close to what my husband was thinking, though I may look at the larger timber as well.  Thank you for giving me a good visual, believe it or not I can see that in my head better than all those damn formulas (though I know I will have to suck it up and dig in to my memory of algebra). Thank you!
 
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