Shed Project: Trusses

[jteneyck]

Here's a link to the last episode:  Floor Framing

The open floor was a perfect work surface to build the trusses on, so that's what I did next.  I used one edge of the 14' width as a reference surface and marked the height of the truss directly on the floor.  After I cut the parts to length they looked like this.



I cut the angles on the top chords with the circular saw using the adjustable speed square as a guide.  The slope is 30°, just shy of 7 in 12.  For the angled cuts on the bottom chord I made a plywood template as a guide for the saw.  I used that same guide to cut the angled 1/2" plywood gussets.  I cut the studs on my chop saw.   Here's a shot with all the plywood gussets installed.



Screwing on the gussets took a fair amount of time.  I used 1-5/8" screws.  



I made 8 of these trusses, 6 with gussets on both sides, and two with gussets only on one side.  The reason why will become clear later on.  Because the roof will overhang the gable ends about 12", the end trusses are constructed differently.  They are 3-1/2" lower and built more like a wall, to support the cantilevered 2x4's to which the end rafters are attached.  I followed the design shown in Modern Carpentry for these trusses, after a discussion with my local woodworking colleagues over a cup of coffee.   Here's what one of them looks like.



You can see how the truss ends at the edge of the floor, whereas lower edge of the top cord of the other trusses sit on the edge of the floor.  The framing in the center of this truss is for a 24 x 30" window.  There will be one in each gable end to help bring in natural light.  Construction was no different than for the other trusses, except I cut the 30° angle on the end of the bottom end of the top chords with a handsaw; just couldn't think of an easy way to do it with a power tool and it only took a minute so it wasn't worth wasting much time thinking about.  

If the weather wasn't so blazing hot I'd be building walls now, but it doesn't look like it's going to cool off for the next 5 or 6 days so there may not be much progress for awhile.  

John

[BloomingtonMike]

No triangle supports?

[jteneyck]

No triangle supports?

I take it you mean diagonals from the lower chord to the peak, for example.  I had downloaded a set of shed plans before I designed the one I'm building.  The trusses in those plans are like the ones I built.  There are all kinds of truss designs, some as simple as what I used.  Diagonals would make the truss stiffer against side loads, like wind against the roof deck or snow lying on it, no question.  But the plywood gussets are there to serve that function, too, as will the sheathed gable end walls.  

If you think I've made a critical error please say so.  I could easily add diagonals on those 8 center trusses.  Thanks.


John

[TDKPE]

I would break that trapezoid flanking the center vertical into triangles with a single diagonal on each side.  Takes the bending moment out of the members and connections. That's for the interior trusses; the gable ends are supported by walls, presumably, and don't need to be trusses.

[BloomingtonMike]

Definitely do more truss bracing research. You are risking out of plane load forces. Id plywood gusset even engineer designed ones if I was building ones myself. Metal plates as well. I never see mfg ones purchased made like yours so id bet that shed plan maker is not backing up the design. Always remember, just because you can does not mean you should. Ive seen a lot of interesting plans. I would have bought engineered trusses though so I am not the expert you need here.

[jteneyck]

I would break that trapezoid flanking the center vertical into triangles with a single diagonal on each side.  Takes the bending moment out of the members and connections.  That's for the interior trusses; the gable ends are supported by walls, presumably, and don't need to be trusses.

That would make it a Howe Truss.   No question it would be really robust, but a simple King post truss is adequate out to 16', assuming the right lumber is used.  


I appreciate both your and Mike's advise.  I'll give it some thought.  

John

[jteneyck]

Definitely do more truss bracing research. You are risking out of plane load forces. Id plywood gusset even engineer designed ones if I was building ones myself. Metal plates as well. I never see mfg ones purchased made like yours so id bet that shed plan maker is not backing up the design. Always remember, just because you can does not mean you should. Ive seen a lot of interesting plans. I would have bought engineered trusses though so I am not the expert you need here

The type of truss I built can be used on some pretty wide spans:  Link  
Lots of shed I looked at don't even use a truss, they just use rafters:  LInk


Some of the online plans use a simple King post truss:  Link.

No argument adding diagonals would make them more robust, I'm just not yet convinced they are needed for a shed in a climate where it is really rare to get more than a foot of snow.  
Thanks very much.  


John

[Phil Thien]

The type of truss I built can be used on some pretty wide spans:  Link  
Lots of shed I looked at don't even use a truss, they just use rafters:  LInk


Some of the online plans use a simple King post truss:  Link.

No argument adding diagonals would make them more robust, I'm just not yet convinced they are needed for a shed in a climate where it is really rare to get more than a foot of snow.  
Thanks very much.  


John

It is an interesting discussion, as adding those two additional diagonals would cost next to nothing in both materials and time.

OTOH, if they're overkill, what is the point?  Kind of reminds me of the joint torture tests where the most important consideration (is the joint suitable for the intended use) is ignored.

My garage's rafter's span a greater distance.

[Bill Wilson]

I can't speak to the physics of truss design, but have a question.  Do you intend to store anything in or hang anything from these trusses?  I know that the ceiling of a shed offers convenient space to put stuff up and out of the way.  Just wondering if this consideration would lend anything to the design of the trusses?

[tomsteve]

its a 14' span. that isnt worth over engineering. add a collar tie halfway up the trusses if youre concerned. done.
i myself wouldnt even bother with a collar tie until i saw proof that these trusses didnt work.
its a 14' span. not 40'