Unique roof arrangement question

[dmac83]

Hello All!
I have been searching for some time to answer a question I have about a particular roof arrangment. There is an outbuilding on my property that is built with the same style as a barn. I initially thought that the roof was what would be called a "Gambrel" roof, but after a closer look it doesnt appear to be.
Im not intimately familiar with roof design but I do know a few things.
That being said here is what I would purpose.

The lumber used is FIR - dimensions are 3 3/4" X 1 1/4"

There is what I believe to be called the "collar tie" at 41" of height. I would really like to remove this piece to allow for a higher headroom allowance. I understand the purpose there, to give the shape resistance to tension force. What I would like to use in its place is triangular shape gussett on each side of the truss near the top, made of OSB, maybe 5/8 thick with 8d nails give me more head room. I also want to add that these are spaced 32" OC.

I have added 2x6 joists in between each rafter and sheeted the floor with 5/8 tounge and groove.
I live on the west coast, so winter LL is pretty much just rain, and yes when it snows here it is quite heavy, but at the current pitch and plans to use metal roofing I cannot imagine it would see much action in that regard.

Also I would like to add that all joints/nodes are currently scabbed over with 3/8 plywood, and not even that well either..


Thoughts?

 

[RedNeckRacin]

Terrible idea, don't do it. You are going to remove all of the strength of that rafter. You really need to do some reading on rafter design before you remove members. Also, If your floor joists are on 32" centers, you are really close to exceeding the span table in the 2009 IRC. The 2009 irc is what I am held too for building code here YMMV.

 

[Clint S]

Not much of n engineer, but looks like the integrity of your roof will be gone and a good wind would blow it over.

 

[bcp]

I would leave it as is. If I needed to be up there a lot, I would probably put a thin paneling ceiling up so I would just bump my head, not bang my head.

Bruce

 

[Billrog]

The span is minimal just a big garden shed I'd go for it . Certainly snow there is no issue on that roof.

 

[Jim Everett]

Just a gut feeling without doing all the math. You will probably be OK if you do this by making the mods a different way. Do not use OSB for the gusset use plywood -1/2 or better. The gusset height should be at least 18 from top peak to bottom mid point. Screw and GLUE it to the existing rafters. Use enough screws to insure pressure contact between the gusset and rafters. Use a strong structural glue. Screws should be long enough to bond them. The gusset becomes the tension member replacing the tie. Also make a plywood splice/tie plate made to fit along each upper rafter and join to the "wall" stud and the lower rafter section. The "splice" portion should be at least 18 inches on each side of the splice point. Make scab pieces to bring all bond surfaces to the same plane. You have to provide enough rigidity between the gusset and the tie plate at the point where you are removing the collar tie so that all loads become vertical and do not have any horizontal spreading forces . The rafters below the orig tie beam will help here.
good luck and work safely!
Remember that information subjective unless it is backed by MATH!!
Let us know if you do this and how it goes.
Jim E

 

[davedj1]

Look these plans over, maybe it will inspire you
https://www.ag.ndsu.edu/aben-plans/5222.pdf

 

[KennyG]

I'm not sure if this is worth doing, but, given the small size of the building, I think it might work. I think you would need to also add gussets to fully reinforce the lower triangles at each side. That way they would become rigid and minimize the load on your new top gusset. As others have noted, this probably is better done with plywood than OSB.

 

[CincyFlyer]

I also had a 16 x 20 "mini-barn" built the way you describe. If yours is anything like mine (which I gave away), there is zero chance that I would attempt what you want to do. These things aren't built to dwelling code, that's for certain. If you insist upon it, I'd use 1/4" steel plate for the reinforcements, and nothing less.

 

[s219]

Don't see the point -- minimal gain, but Kenny is right, you should gusset both the top and the two side joints. I absolutely would not saw out the cross tie without gusseting those side joints.