Pre-fab roof trusses, single gusset
Living near the land of pre-fab sheds (lancaster, pa), I decided to take a look at them while planning my 14×20 backyard shop/garage. I noticed that for most (8 to 14 ft wide), the roof consists of 2×4 rafters 16″oc joined at the peak with a plywood gusset. No bottom chords or ceiling joist, some have a single collar tie for whole roof about at mid point of roof height. This makes a nice open space, but does anyone have comments on wisdom/strength of this?
I did some research and found occasional mention of cathedral ceilings/roof constructed this way but few details, other than the critical failure for rafters joined at the peak either together or to a board is the tendency for the roof to flatten and push out which opens up the top joint, and a properly sized gusset eliminates this. Of course, ceiling joists would have the same function.
BTW first post here in many years . . .always got good info before, thanks!
Replies
The gusset isn't keeping the walls from blowing out, that single collar tie and the end walls are.
I would be concerned with a 20' long wall though...
It is common
It is area dependant, and if the size is below the threshold of 200-sf doesn't have to be inspected.
I designed one for my brother that was about that size. It was sized to hold his camping gear. The roof pitch, wall sizing, etc., were all driven by the need to hang his wall tent inside of it so it could dry out after he brings it in after late season hunting ends.
The top plates had to be beefed up to act as beams to resist the thrust loads from the roof with no ties going across.
You can get away with a lot on a small structure like that. But I wouldn't call anything in those sheds a "truss".
What about the shed where Grandpa stores his old undergarments?
14x20 backyard shop/garage.
That calcs out to 280 sq ft, 14 ft span on a 20 ft length is really pushing the envelope, I'd bet dollars to doughnuts you will have a nice apparent SAG in the middle of your ridge after the first heavy snowfall if you try what you described.
Add 2x4s across eave to eave, at least a couple of them. Your can use them for overhead storage.
14x20 backyard shop/garage.
That calcs out to 280 sq ft, 14 ft span on a 20 ft length is really pushing the envelope, I'd bet dollars to doughnuts you will have a nice apparent SAG in the middle of your ridge after the first heavy snowfall if you try what you described.
Add 2x4s across eave to eave, at least a couple of them. Your can use them for overhead storage.
thanks. . . size is the key!
Thanks for all the input. I did some further snooping and think size is the key. Noticed w/ some builders anything over 12' span they go with more traditional ridge board and more collar/rafter ties. No, these are not traditional trusses. I also looked at some existing larger sheds around and there is the characteristic sag in the middle. I do wonder if it is more the deflection of the 2x4 rafters or the scissor- flattening action at the ridge, as a properly sized gusset would eliminate the latter. Anyways, I am going to order up some real trusses to save me some time and labor on the roof, depending on delivery charge, that is!
Good decision...
I just built a 12x16 shed, and when I looked at the cost of building the roof myself (rafters, ridgeboard, collarties) vs buying engineered trusses I only spent about $45 more on the trusses than I figured it would cost me to buy the lumber and fasteners to build a decent roof structure. And I put the trusses up in about an hour by myself, compared to a whole day trying to get all my cuts perfect and assemle the structure of a traditionally framed roof...
For delivery, remember that a 14 foot truss can probably be hauled very easily with a flatbed trailer. I would have paid as much for delivery as I did for the trusses (I live 120 miles from the nearest truss plant) so I picked them up on one of my regular supply runs and didn't have to pay anything for delivery. Just had to drag my 16 foot flatbed car hauler trailer along. I didn't go with the "storage truss' (open area in the middle for attic-type storage), but now I kind of wish that I had...
A properly sized gusset in the middle will NOT keep rafters from spreading, only proper design of a member taking up the horizontal forces will do that. That COULD be a scissor truss, but that is not what you are looking at here.
Wrong, A properly sized gusset will stop the spread
A properly sized gusset will prevent the rafters spreading. The problem is that the properly sized gusset might be deep enough to use half the depth available.
No "gusset" short of a full
No "gusset" short of a full diaphragm will prevent 2x2 rafters from spreading on a 16-foot span with snow load. A combo of properly-sized, properly-connected gusset and properly sized rafters is required.
Ummm, it's not a gusset then, it's a thin joist!