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.

 

[Budweiser John]

Your solution is in the reference on post #7.
The total building width is only a little over 11'-6" so not huge by barn standards. Another solution would be to add a center beam running the length of the building ridge and supported on each gable end. LVL beams have pretty impressive span capability.
B. John

 

[quicksandfarmer]

I'm going to be contrary and say that the horizontal piece you're thinking of removing currently contributes nothing to the structure. (And I don't think those trusses were engineered, just built.) A rafter transmits the weight of the roof to the wall. Since the weight of the roof is centered over the center of the roof and not in line with the walls, the difference creates a turning motion in the rafter that causes the rafter to push outward against the wall. Tension in the collar tie balances the outward push of the rafters.

The upper horizontal piece is not needed as a collar tie. The lower rafter, the lower horizontal piece and the vertical piece form a triangle, any horizontal force imparted by the upper rafter is transmitted to that triangle and ultimately to the lower collar tie. So long as the joint where those three pieces meet is strong enough the lower collar tie does the whole job.


What are the magnitudes of the forces? Let's go big and assume a roof load of 60 lbs per square foot. (This is not the everyday load, it's the "Storm of the Century" when there's two and a half feet of wet snow on your roof. Everyday load is probably about 10 lbs/sf). With a span of 22.75" and a spacing of 32 inches each lower rafter supports 728 square inches or 5.05 square feet. We'll call it 300 lbs. Each upper rafter has a span of 43" and supports 1376 square inches or 9.5 square feet, or 630 lbs. At the angle of the lower rafter, a vertical force of 300 lbs at the midpoint requires a horizontal force of 72 lbs and the end to counteract the turning motion. The upper rafter requires a horizontal force of 270 lbs to counteract the vertical force of 630 lbs. At the place where the lower rafter, upper rafter and vertical piece meet the combined loads are 630 lbs vertical and 342 lbs horizontal.

If the piece you are considering removing were acting as a collar tie, at most it would be contributing 270 lbs of tension -- the entire horizontal force of the upper rafter. Removing it shifts that horizontal force to the lower rafter. In either case the vertical force of the upper rafter is borne entirely by the vertical pieces, removing the piece changes is nothing for it. So the question is whether the joint of the two rafters can handle an additional 270 lbs of horizontal force. If the joint is wood on wood in the direction of the force, 270 lbs is nothing, all you have to worry about is the joint slipping. If the rafters are nailed to sheathing they aren't going to slip. If you're relying on nails to prevent horizontal movement I find a good rule of thumb is a 16 penny nail can take 200 lbs of perpendicular force.

If your drawing is accurate the the bottom ends of both rafters are horizontal I would worry about their ability to transmit horizontal loads. Gussets there would help.

The joint at the ridgeline does not have to be modified. The downward force of the weight of the roof tends to push pairs of rafters together, the rafters just have to be fastened well enough to keep from slipping. The placement of collar ties does not change the forces on that joint in any way (as long as the collar ties are sufficient to keep the building from falling down).

 

[dmac83]

Quicksandfarmer: I appreciate the time you put into showing some numbers. Also to everyone, I appreciate the input as well.

I know some may say the juice isnt worth the squeeze but im 6'4 so anytime I have a chance to allow for a little more headroom im game. There is also an ease of installation at the moment because I have removed the almost see through cedar shakes and am currently without roof.

Ive noticed everyone has mentioned plywood as a gusset material choice. Ive read some interesting information lately about OSB and the higher shear strength vs plywood. Actually if you google images Gambrel truss, I cant recall seeing gussets made of plywood.

Based off the information given, I am going to build a triangular gusset for the top portion, and reinforce the all other joints properly with a quality adhesive, OSB gussets, and 16d nails, and then I will remove the upper horizontal member.

 

[Snobdds]

You will be fine adding a gusset at the top. You already have a rafter tie preventing snow loading from bowing out the walls. Collar ties prevent wind load uplift and moving them to the top of the rafter does the same thing.

http://www.finehomebuilding.com/membership/pdf/73042/021240018.pdf