I’m trying to find information on how to size a rafter. My application is for a shed roof. I want to use solid timber, green fir, to get a timber frame look. The roof is going to cover a portion of my deck, which I’m planning to screen in. I want to use 36 inch centers for the rafters and the span is 16 feet with a 2/12 pitch. One side will fasten to the house and the other will sit on a support beam. Has anyone ever come across a spanning table that doesn’t require Eienstein to decipher? I’m not having any luck at my local lumber yard. What I get from them is they call to get sizing on a glue lam and make their guesses from that. Any input is appreciated
Thanks, Joe G
Replies
IMO, there are sufficient resources on the web for finding the maximum spans for rafters of various sizes -- when using 'traditional' building methods.
But 'traditional' does not mean 36" centers, and a 2/12 pitch is also a bit off from normal.
It seems to me that you need a qualified engineer to do some thinking for you.
And, in general, engineers don't work for free, and not over the web.
Good luck.
Joe
You can try this.
But spacing only goes to 24" and it is for 2x lumber.
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
Rich
Thanks Rich, I'll look through this for some help
Regards
Joe
Like where are you located? What kind of snow load, if any? What type of roofing and roof deck are you thinking of using?
You might want to consider 32" o.c. in lieu of 36". Could make other building materials work out better. (Not a deal breaker though)
I'm located in the NE section of PA and we do get snow, for sure. The skylights I'm planning to use would fit nicely between 36 inch centers, that's why I picked that. This is a roof over a deck built the same way the sunrooms are installed. Most of the sunrooms are built with wide spacing between the rafters, for obvious reasons. The deck has sufficient structural design to accomodate the load of the roof with snow
Joe, no one on this site can or is likely to give you the info. you are requesting, even with the necessary data for making such calculations, which you have not provided. It would not be fair to them or to you.
Required data. Design live load. dead load and snow load for you area. the type roofing material and roof decking to be used, the allowable or desired deflection? Being in Pa., my guess hurricanes and earthquakes are not a problem.
I did some rough calculations base upon some assumed data, which may or may not be correct for your project. Using lumber timbers with a min. fiber bending of 1200f and a min. modulus of elasticity of 1,200,000e, I'm came up with a minimun of 4x10 joist/rafters @ 3'-0' o.c. There are other joist/rafter configurations, but this shape is the most efficient. THIS IS NOT AN RECOMMENDATION, but is only meant to give you an idea of what you might expect.
The sizing of the joist/rafters is only one aspect to a project as you have described. Others such as the attachment to the existing structure, columns, beams/headers, lateral forces etc.
My best advise is to have someone locally do the engineering for you. It is a simple and straight forward problem but not one that should be guessed at. And if you are in an area that will require a building permit, you may be required to have an engineer sign off on it anyway.
Hope this helps a little. Good luck.
Thanks for getting back. I was looking for some experiences from some of the experts, not necessarily a design or recommendation for sizing the rafters. My "guess" was similar to yours at 4x10 with 36" spacing. Getting an engineer involved to size a simple 12x16 roof is not as easy as you might think. I understand where you're coming from though and I appreciate the feedback. I'm sure there must be contractors out there that have done this before and as with any roof, I would think a good carpenter could look at a job and know what the requirements would be right off the bat. Of course, a sanity check from a qualified engineer is always a good idea. The again, I'm not a carpenter.
My 2 cents
Joe
I come up with 6x8 or 4x10 @36" OC, but keeping in mind that I had to make some assumptions.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Your deck is only 12' wide? You're only going to need 5 of these puppies, so go big or go home.
For a roof snow-load of 2.6-3.0 (you have less than that) and a rafter spacing of 24" OC you need 2x12 D-fir SS for a 16' span. Old-timer's Rule of Thumb: 'To double the joist spacing, double the amount of wood in the joists.'
You're not doubling the joist spacing; you're only going 1.5X so you could use 3x12 on 36" centers by that rule. But since you only need five, and you really want that beefy timberframe 'look', go for 4x12. A 3x12 rafter won't have anywhere near the visual impact of a 4x12.
Note: The above information is provided for reference and planning purposes only. Not for construction use....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I'm with ya on this one. Going big. Input appreciated and it makes sense
Yah, I forgot to mention with 36" rafter spacing you're gonna have to sheathe that roof with 2x6 roof-decking, not sheet goods. But since you're looking for that T-F look, you probably had already planned on that.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I was going to do some sort of T&G. I thought 5/8" thickness would be enough. Maybe not?
If you can find span charts for the lumber species you want to use, you can cheat a bit to get a fairly accurate size for the rafters. Like if your local loading requirement is 40 PSF and the span chart only goes up to 2' O.C. - Check chart for the rafter size for 60 PSF loading at 2' O.C. That should get awfully close to the correct size.OTOH - If you have a situation where you'll have drift loading, the span charts won't do you any good. In that case you'll need an engineer to sort it out for you.
Dignity does not consist in possessing honors, but in deserving them. [Aristotle]
definitely not, unless you like the sagging sheathing between heavy timbers look
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
isn't 3/4 ar 24"OC about the limit???
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
"Some days it's just not worth chewing through the restraints"
I was going to do some sort of T&G. I thought 5/8" thickness would be enough. Maybe not?
Not even close. 5/8" would be so bouncy at mid-span you'd never even get the shingles nailed down tight without a gun.
You need 2x6 (or 2x4; if you like the narrower look) roof-decking. It's a special cut, usually of red pine, and will not sag between rafters spaced as wide as 48" OC. The top face of the T&G joint is square edged; the bottom face is vee-jointed so it makes a very nice looking ceiling over exposed joists or rafters. It's perfect for what you're trying to do.
Oh, yeah--I strongly suggest you stain your rafters and the bottom face of the roof decking before you install it all. Saves the not inconsiderable annoyance of having stain drip off the brush and fall in yer face....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
if you only use 5/8 t&g your roofer better not weigh over 50#.
Sounds like child labour to me....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
probably would be. but a lot better than my fat a** on 5/8" t&g!
One option to avoid the expense of having to go to 2"x6" t and g material would be to run purlins above the wood ceiling and then sheath with plywood.
I don't see how that avoids any expense. With the labor, it is probably more costly.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Most likely true. I only suggested it because in an earlier thread about nails showing through roofing on t and g eaves my suggestion to use 2x material was greeted by derision because of cost. Another advantage of the 2x is being able to extent it out to create eaves without any other structure. Then again I've seen builders do that with osb too!
You've forgotten the look he's after: exposed rafters means inevitably exposed underside of the roof sheathing. 2x6 roof decking is cut so you get a nice v-joint ceiling on the bottom face.
Unless you go to the trouble and expense to fake it by laying something else between the rafters from underneath, whatever sheathing you put on the roof is what you're gonna see from below.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
You are quite right. I withdraw all suggestions of purlins etc. Beefy roof joists, 2x t and g. Sounds like it's going to be a nice place to spend the summer.
My suggestion would be a truss roof.
I know you want the timber look; alternatively you could build up the trusses to look like timber.
The skylight should be framed in properly. Going to 36" rafterspacing will increase the cost, since now you need adequate support for the roofing materials (cross strapping with 2x)
My experience: applied the same as with skylights. In the beginning the customer admired his skylights by looking up; after a while nobody took notice of it anymore. The light was there - that was all that mattered.
from a practical standpoint you could use tables for nominal framing lumber, though it might not be good enough to obtain a permit
hypothetically, if you need a 2x10 @ 16"oc then a 4x10 should suffice @ 32" oc. increase the spacing by 4 inches to 36" oc then you will need 4.25 x 10
you could also play that game with the height of the rafter vs the width, or both
keep in mind the slower the pitch the heavier the snow load at the same span
I think the consenus is 4x10 or 4x12. Right in line with my thoughts. Just needed a little verification.