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I'm planning a small roof - 12' x 16', gable style. Almost all the rafters will identical.

Should I build rafter units ahead of time? 2 rafters + a ceiling joist + plywood gussets. I'd make a simple jig on the deck (before the walls are built) to assemble a rafter unit. Like pre-built trusses but DIY.

Or should I put up a ridge beam and install the rafters one at a time?

Below are more specifics about this particular project, but I'm more interested in the general decision making around this question.

Rafters and ceiling joists are 2x6, 24" O.C. Studs are 24" O.C. as well, so each rafter will be right on a stud.

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2 Answers 2

Since you're putting a gable roof on it, I can only assume this is being built for storage purposes, not for aesthetics.

Building it with a glulam ridge beam would be more difficult, but it would be much stronger. The main issue with a ridge beam design would be lifting the beam into position. If you can manage that pretty easily, the rest would be a breeze. I'd put some ceiling joists up at either end, lift one end of the beam over one of the joists, pushing it far enough to clear the ceiling joist on the other side, like so...

B
   B
      B                 
  JttJttJBttttttttttttttJttJttJ     
  S         B                 S        
  S            B              S
  S               B           S
  bbbbbbbbbbbbbbbbbbbBbbbbbbbbb

B = ridge beam
J = joists at the ends of the barn
S = studs at the ends of the barn
t & b = top and bottom plate

... and then lift the bottom end of the beam up with some ropes. The joists will need some strength to support the ridge beam, and you'll need to be able to stand on them, so I'd use three 2x6's @ 9" OC on either end (where the outermost "2x6" is actually the top-plate of the walls on either end of the diagram) with some 5/8ths plywood on top of them. I'd toss in some temporary studs at the center point to make sure the joists hold up to the weight of two people and the ridge beam. Once the beam is in place, put in the ceiling joists and plywood on top of them to make walking around easier (and to create an attic for extra storage,) then put the rafters up.

Grab some metal brackets for the joists, rafter heels, and rafter/ridge connections (it'll cost you an extra $125, but it will make the structure a lot stronger) from a HI store and your little barn thing will be able to withstand some nasty winds.

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You say a ridge beam + collar tie gives more space. Is that because you'd put the collar tie higher up than ceiling joists? –  Jay Bazuzi Jun 27 '12 at 5:05
    
@JayBazuzi Yep... on second thought, though, it's probably easier to just do the ceiling joists (rafter ties.) If you pulled a permit, you'd probably have to get an engineer involved for the city to approve a design without rafter ties. Rafter ties provide horizontal stability to the opposing walls and also prevent the roof from sagging / pushing the walls apart. Might as well use them, toss some plywood on top, and make some attic-like storage out of it. –  Michael Jun 27 '12 at 6:51

It's going to be a lot easier to build in place. Cut everything to precise measurements on the ground and lift it up one piece at a time.

When you get into the preassembled roof structures, you want to rent a crane to do all the heavy lifting. I get the impression you're trying to do a lot of this by hand and hauling up the preassembled structure by hand is a challenge.

The roof is one of the jobs we outsource at our local Habitat chapter since an engineered structure is built offsite, a crane is needed, and we don't like putting volunteers in high places doing critical work. But for a smaller project, with a lot of planning, this can be DIY.

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I was just about to say the same thing. If you are working alone (or with only one helper), lifting the preassembled units is going to be a challenge. Build them on the ground (without nailing them together), then put them back together in place one piece at a time. If you have a bigger crew (and/or a crane), prebuilding might be more manageable. –  Tester101 Jun 27 '12 at 11:47

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