Claudio,
I have not seen in the hardware store anything longer that 16
feet. I will check tomorrow in a lumber store for 21 feet joist.
We bought lots of 24 foot 2x12's for our garage and house. Not everyone
carries those lengths, but they shouldn't be too difficult to locate.
If you choose wood I-joists, those can usually be ordered in lengths of 40'
or longer.
However, I do not understand how the floor joist will serve as
collar ties
Basically the rafters and floor joist form a triangle. The rafters are
attached at the top, and nailed to the floor joists at the bottom. This
keeps the bottom of the rafters from spreading outward, as the floor joists
are holding them in place. Essentially the floor joist "is" the collar
tie. It's a very strong structure.
I planning to have 6 feet walls on the side for the second floor.
Maybe I'm missing something about your requirements... Are you building 6'
walls on "top" of the cinder blocks, and then building the roof on top of
that?
Let's see if I understand this...
You currently have cinder block walls.
The 2nd floor will be built on top of that (You can use the 2x12's @ 16"
OC) for that.
You will be building 6' high walls on top of the 2nd floor deck.
You then want a roof on top of that, but want 8' of headroom in the middle?
If that's the case, you have a couple of options:
1. Scissor Trusses. Quick to install, lightweight, and strong. And, the
truss company will take care of all the engineering for you. But, if you
have to work alone, it could be difficult installing roof trusses.
2. Conventional rafters/collar ties. You can usually move the collar tie up
off the top plates about 1/3 of the total height. In other words, if your
roof rises 6' higher than the walls at the peak, you could raise the collar
ties about 2'. That would give you the 8' headroom you seek.
I originally assumed the roof was being built right on top of the cinder
block walls, with the 2nd floor being framed within the attic space (That's
how we built our garage). If that's the case, you could use the plan I
detailed earlier. You could also check into "attic trusses" which provide
similar space, but I found that stick framing provided a bit more room and
cost a bit less.
Take care,
Anthony
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