Oak cants for garage lean to

[True North]

We have some red oak cants that have been drying for two years. They are mostly 3 1/2" by 3 1/2 to 4 1/2".  I Would like to build a lean to on our garage and was wondering if they could work as trusses.  I have 6' of slab that I would like to cover, and the cants are 8-10' in length.  How do you guys think they would work? 

By the way, I have been blown away by some of the timber frame projects that many of you have posted, and would really like to learn how to do it. Thanks for the inspiration!

[Jim_Rogers]

There is lots of information that you'll need to provide for a guess as to whether or not if they will work.

Span of the truss, spacing from one to the next, snow load for your area, slope of the roof, roofing material to be used.

Just to name a few.

Jim Rogers

[True North]

Hello, Jim. Thank you for your response. The span would be 6 to 8'. I guess that I would be open to suggestions as far as spacing goes.  We can get quite a bit of snow (UP of Michigan). As for slope, I would like to go with the least that would be advisable to maximize headroom. And finally I plan to use metal roofing.

[Jim_Rogers]

We need exact numbers. You'll need to find your snow load. Or tell me what county you're in and I'll look it up on my chart of snow loads for MI.

Also, is it 6' or 8' you need to be precise on what you're going to do. Pick one and we'll try it.

A truss means it normally has a bottom cord. That is a beam that goes from one side support to the other side support. The distance from the finished floor to the bottom of the truss bottom cord is another thing you need to figure out. This distance has nothing to do with the slope of the roof. You need to decide on the slope.

Next you need to decide on what type of truss you want.

There are five basic types:

The first one is the common truss.  While basic and simple, the strength and affordability of this design is unmatched.  As its name implies is the most common truss used in timber framing today and in the past. You can tell it is a common truss by the lower tie beam joining with the two rafters.



The next design is a queen post truss.  It is similar to the common truss in that it utilizes the bottom team beam but it also adds the two posts dividing the gabled space these are known as the queen post.  This works well for highlighting the windows in a great room and adds strength and character as well.



The third truss we are going discuss is the king post truss. The post dividing the gabled space is known as a king post, it is a bold and intriguing design that is commonly used in conjunction with other truss styles. It also often incorporates two struts on either side that not only add strength but character.



The fourth and in some opinions the most dramatic is the hammer beam truss.  This design is the most complex of the five and deserves an engineer's touch to make sure it holds up over time but the results is well worth any effort you put into it.  Common pieces are the hammer beam and the hammer post, and a king post is often incorporated into this upper gabled area.



The final truss design is the scissor truss which has to beams that run from the lower part of the Rafter to other opposing Rafter crisscrossing each other creating a unique and loved look.  While sometimes difficult to figure out the best way to join the timbers in the middle it can solve difficult span requirements while adding character to the frame.



I hope this has helped you to decide.

Jim Rogers

[True North]

Thanks, Jim.

I looked up the snow load under the Michigan residential code, and they list a 70 pound ground snow load, and a 90 pound wind load.

I really like your truss diagrams.  For this project, I would like to just build a lean-to shed roof off of either side of an existing garage.  For the one side, I have only 6' of slab to cover (off the side of the garage).  So the span would be 6' measured horizontally from under the eve of the garage. If, however, I could get 10' long cants to work I would consider going out beyond the slab to make the structure a bit wider. I would like the beams to fasten just under the eve of the garage, and slope down from there.

Hopefully what I wrote above makes sense. I appreciate your help.

[Jim_Rogers]

Ok, now I'm getting more of a picture of what you want.

Something like this:



So next in order to figure the slope we need two dimensions. A and C.
Can you figure these two out and let me know?

Jim Rogers

[True North]

That is exactly what I would like to do!

C is 8'.  I would like to keep A as high as possible while still maintaining enough slope.  Would you have any suggestions on that?

Also, your diagrams are great.  Which program do you use to make them?

Thanks Jim

[Jim_Rogers]

You asked: " Would you have any suggestions on that?
and: Well, it really depends on the metal roofing company specs. Do they have a minimum pitch? I would thing you'd want at least a three or more.

What is shown in my drawing is a 4/12 pitch.

I use a timber framing software known as Dietrichs. It is a professional timber frame drawing program.

Next you said your timbers are 3 1/2 x 4 1/2" is that right?

Jim Rogers

[Jim_Rogers]

I ran the numbers through my beam sizing calculator to see if they would go.

At a 4/12 pitch with metal roofing and 70 lbs snow load a 3 1/2" x 4 1/2" rafter would have to be spaced at 16" on center over a 6' span. Any longer like with an over hang will mean even closer together.

I would think that you're going to also have snow sliding off your garage roof onto this shed roof, so you're probably have to go even closer together. That all depends on the roofing material on the garage and the slope of the garage.

You're going to have to secure the rafter to the ledger plank on the side of the garage as well as on the support beam 6' away from the wall.

Hope that helps.

Jim Rogers

[True North]

Thank you very much for all of your help, Jim. I will think things through and let you know what I come up with.