Building Wood Shed - Need Help Calculating Beam Size That Holds Roof Up

[BoylermanCT]

I started building a wood shed lean to that will be attached on one side to my garage. I dug the holes for the posts today, and plan to get order the lumber on Monday. I need help in calculating the size of the beam that will support the rafters.



I am having 3 6x6 posts spaced 10' apart. On top of the posts is a 23' beam of unknown width and height. The beam will go past the end posts by 18" on each side. The posts are 11' from the garage wall. Based on an 11' span and 24" centers, the charts say to use 2x8 rafters. Rafters will go past the beam by 1'. I will use 1/2" plywood or OSB for decking and then tar paper and metal roofing.

I can't find a chart showing what size the beam on top of the post should be. I know I can do triple 2x12's and have it covered. But that will be a heavy beam to lift into place. Can I do triple 2x10's or 2x8's? I was planning on attaching the beam to the post using a Simpson bracket made for attaching 3 2x's to a 6x6.

I live in CT, and code calls for roof loads of 30 psf. Thanks!

 

[polkwrangler]

2x 10 should be plenty good,i built a firewood storage building with 2 x 10 ,has roof and walls It can hold four truckloads of split oak with all that weight never had a problem.

 

[RNeumann]

Triple 2x12s looks like the ticket. You lift one 2x12 up at a time- build in place.....one person job!!!

Typically the Simpson tie going from the post to the beam is made for dimensional lumber. Since you are sandwiching 3 2x12s you will need to add some plywood to fur out to the post and Simpson width.

 

[Barnbuilder]

How about a double 9-1/4" LVL? Should be more than enough and it would be full length. You can notch the 6x6 so LVL can sit on it. Throw a couple 5/8" galvanized. Carriage bolts through it.

 

[mikester]

I started building a wood shed lean to that will be attached on one side to my garage. I dug the holes for the posts today, and plan to get order the lumber on Monday. I need help in calculating the size of the beam that will support the rafters.

View attachment 625349

I am having 3 6x6 posts spaced 10' apart. On top of the posts is a 23' beam of unknown width and height. The beam will go past the end posts by 18" on each side. The posts are 11' from the garage wall. Based on an 11' span and 24" centers, the charts say to use 2x8 rafters. Rafters will go past the beam by 1'. I will use 1/2" plywood or OSB for decking and then tar paper and metal roofing.

I can't find a chart showing what size the beam on top of the post should be. I know I can do triple 2x12's and have it covered. But that will be a heavy beam to lift into place. Can I do triple 2x10's or 2x8's? I was planning on attaching the beam to the post using a Simpson bracket made for attaching 3 2x's to a 6x6.

I live in CT, and code calls for roof loads of 30 psf. Thanks!

This is a good question for your building inspector NOT the internet full of eager people unfamiliar with your area, climate and codes. The required beam size is based on a calculation of roof pitch, applied loads and allowed deflections...thats why you cant find a chart.

 

[sailfast]

This is a good question for your building inspector NOT the internet full of eager people unfamiliar with your area, climate and codes. The required beam size is based on a calculation of roof pitch, applied loads and allowed deflections...thats why you cant find a chart.

This was an excellent example of one of those eager people offering incomplete half correct recommendations.
The calculations for this can be found on the internet. And, you might want to get the help of an engineer or architect. However, under the circumstances is something you can figure out yourself.

 

[radioman]

With building experience in central nys with similar snow loads, since you have 6x6 posts, nothing wrong with overbuilding. I would just use 2x12s and put them up one by one. I have done it this way. However since you have 10ft spacing of posts and 18 inch overhang you really don't want a slice in the middle completely. I would find a lumber yard that carries 20 footers 2x12. Use 12ft on the outside and one 20 ft inside. Use scrap of 12ft to fill in the 18 inch spacing overhang. Be sure to add metal splice plates in the middle and brackets to hold beam to posts. Follow the beam load carrying screw pattern and spacing. Should be in your local permit office. Can use the decking guidelines. These are the things a local building inspectors look for regardless if you have a permit or not.

 

[EddieWalker]

Your ten foot span isn't very much. For my porch here in East Texas, where snow is not a factor, I went with three 24 foot long 2x8's for my beam on posts 12 feet apart for a total width of 24 feet. What I liked most about that is the 2x8's where one continuous board. I had to go to a store that sells more to contractors and home builders, because Lowes and Home Depot only sell lumber 16 feet long.

First thing that I would figure out is what did people use for the garage doors when building houses in your area? Granted, most garage doors are either 9 feet wide or 16 feet, but once you see what was used there, then it should be easy to determine what will work for your project.

While the size of the beams are important, how you attach your ledger board to the existing building is even more important. Code is changing on this almost every year because of catastrophic failure that has resulting in how the ledger board was attached. Usually it's because nails where used, but the lack of proper flashing that leads to rot is also very common. The ledger board is your lynch pin and keystone to what will keep it in place. Fasteners absolutely have to be structural. Nails and screws are just fasteners, and NOT structural. They can NEVER be used to support a load of any kind. This is also why joist hangers have specific nails and screws that can be used, and only they can be used.

If you decide to go with 12 foot long boards for your beam, be sure to use plywood between the boards to join them together over your center post. 4 feet of plywood on each beam with liquid nails or something similar so that there is no way that the boards can pull apart. Also be sure to nail or screw the boards together top and bottom, ever 12 inches. Be sure to either use metal fasteners to attach the beam to the posts, or notch the post so the beam rests in a L shape cut out of the post. I prefer to do both with 6x6 posts. I set the post so it's a little taller then I need it to be, then I determine my beam height and draw a line across each post to where I want the beam to rest. Then I cut it out and install the first post to the tall L part that I've cut out. Then the plywood, and then the other board. It makes it easy to get a very strong beam without lifting all of it at once.

 

[goeduck]

With building experience in central nys with similar snow loads, since you have 6x6 posts, nothing wrong with overbuilding. I would just use 2x12s and put them up one by one. I have done it this way. However since you have 10ft spacing of posts and 18 inch overhang you really don't want a slice in the middle completely. I would find a lumber yard that carries 20 footers 2x12. Use 12ft on the outside and one 20 ft inside. Use scrap of 12ft to fill in the 18 inch spacing overhang. Be sure to add metal splice plates in the middle and brackets to hold beam to posts. Follow the beam load carrying screw pattern and spacing. Should be in your local permit office. Can use the decking guidelines. These are the things a local building inspectors look for regardless if you have a permit or not.

^^^ good advice. I like the 12/20 length of 2x12s concept. Around here without plumbing and electrical this might not require a building permit unless due to being a lean to. But no reason not to look up local code and keep your nose clean.

 

[goeduck]

First thing that I would figure out is what did people use for the garage doors when building houses in your area? Granted, most garage doors are either 9 feet wide or 16 feet, but once you see what was used there, then it should be easy to determine what will work for your project.

I thought this was a wood shed.

 

[EddieWalker]

I thought this was a wood shed.

Span is a span. All we care about is the distance that the wood has to carry a load between the posts or walls.

The size of the material used to span a garage door opening where he lives will tell him what to use for his wood shed. But if the label of wood shed is an issue, we can pretend that in the future, it might become a garage.

 

[goeduck]

Span is a span. All we care about is the distance that the wood has to carry a load between the posts or walls.

The size of the material used to span a garage door opening where he lives will tell him what to use for his wood shed. But if the label of wood shed is an issue, we can pretend that in the future, it might become a garage.

Ok, got it :thumbsup:

 

[/pine]

Construction tip...When assembling multi-member beams in place on top of posts etc...it's handy to have a quick grip type bar clamp or two and some deep throat c-clamps to draw the members (including plywood flitch plates) tight together with the adhesive for fastening with nails or screws...especially helpful if working alone...

General rule of thumb for typical loads is no more than 25% of the span for joints in beam members away from posts...If material lengths allow scarf joints work well in place of butt joints...

 

[KennyG]

Beams are not that complicated, but it's a little harder to find sizing tables because the configurations are more varied than rafters and joists. I could run through the equations, but Marks Handbook for Mechanical Engineers has an easy to use table (Table 8 in Section 12 on building structural design, Chapter 12 in my 1967 Edition). You only need to design the 10 foot span between posts, since it's uniform. The weight on that section will be 120 square feet of roof (10 x 12) at design load of 30 psf = 3600 lbs. Actually it will be a good bit less because something less than 50% of the load will be picked up by the ledger plate on the building, but let's be conservative and say 3000 lbs.

The table provides maximum safe load on a 1 foot span, so we have to find a value that is 10 times the actual load or 30,000 lbs. From the table, we find that a single 2 x 12 will take 23,900 lbs, so 2 will be way more than we need. A single 2 x 10 will take 16,290, so a doubled 2 x 10 will be more than enough. A 2 x 8 is only 10,200, so it would have to be tripled.

How would I do it? If you are willing to handle a 200 plus pound beam, I would build a beam with two 2x10s with 1/2 inch plywood sandwiched. I would splice the 2 x 10s, just because it's takes some effort for find lumber longer than 16 feet. I would notch the top of your 6 x 6 posts and forget about hangers, just through bolt with a couple of bolts at each post. If you can't handle something that heavy, you would form the beam with one 2x10 layer and the plywood and then add the second 2x10 in place. I like to build beams on flat concrete whenever I can.

Note: This assumes wood fiber stress of 1000 psi, which is not bad, It varies by wood species and quality but this is a conservative design anyway. My edition of the handbook thinks a 2 x 10 is 1-5/8 by 9-1/2 inches but again, close enough for this design.

 

[KennyG]

I just had another thought. There really isn't any need for the beam to be continuous in this application. You could build a pair of 11 - 1/2 foot long beams and butt them up on the center post. Attach each one with two or three through bolts and all will be good. The beams are independent on each side of the center support because they are simply supported at each end.

 

[downsizingnow48]

Just thought I would mention these malleable iron timber washers. I've used them with through bolts to hold 3 2x12's etc together tightly. Instead of nails or screws. It is really easy to get a nice tight beam. Also good for beam to post connections. They come in different sizes.

 

[goeduck]

I just had another thought. There really isn't any need for the beam to be continuous in this application. You could build a pair of 11 - 1/2 foot long beams and butt them up on the center post. Attach each one with two or three through bolts and all will be good. The beams are independent on each side of the center support because they are simply supported at each end.

I am not a builder like many on here but a continuous beam seems to me would stiffen up the structure.

 

[KennyG]

I am not a builder like many on here but a continuous beam seems to me would stiffen up the structure.

I know it seems that way, but a beam, like the top plate on a stud wall, provides no lateral stability. On a post frame structure, the post in the ground makes it rigid and on a stud wall, the sheathing or bracing makes it rigid. The beam only transfers the vertical load from the rafters to the posts. The beam is never going to be tall enough for the fasteners to provide enough triangulation to stiffen the wall. For that matter, if the shorter beams are bolted to the posts the same way as a long beam, they will provide essentially the same rigidity.

 

[BoylermanCT]

Thank you everyone, lots of good info and ideas. KennyG, your calculations are what I was looking for. I just ordered an E Book of the newest version of Mark's Handbook so I have the tables for next project. I think I will go with the 2x10's and notch the 6x6 posts so I have a solid connection and am not just relying on a metal bracket. Plus the 3 metal brackets were going to cost $50 or so and would have to be ordered.

I like the idea of a single beam rather than 2 beams, so I will call around and see if I can find 24' 2x10's at a reasonable cost. I plan to build the beam in the garage and then use the tractor to lift it into place.

I will post pics as the build progresses. I've been on a construction kick lately. Just had a new roof put on the house and garage, my wife and I put a new metal roof on our 12x16 chicken coop, the woodshed is next, and then I demolish my 40x50 barn to make room for a new 30x40 pole barn kit being delivered in mid-November. Busy time on the farm!

Here's the chicken coop with the new metal roof.

 

[goeduck]

I know it seems that way, but a beam, like the top plate on a stud wall, provides no lateral stability. On a post frame structure, the post in the ground makes it rigid and on a stud wall, the sheathing or bracing makes it rigid. The beam only transfers the vertical load from the rafters to the posts. The beam is never going to be tall enough for the fasteners to provide enough triangulation to stiffen the wall. For that matter, if the shorter beams are bolted to the posts the same way as a long beam, they will provide essentially the same rigidity.

I would not put sheathing on a woodshed, you need airflow to dry the wood which is why one beam makes sense to me, at least it is something. Y bracing to the beam, I would do.