[btucker]

I am going to build a double door for a barn that slides on channel. The will each be 6' wide and 12' tall. Any thing I need to know?

[Nwptsailor]

If the doors are being made with ordinary construction grade 2 X lumber , then the styles & rails ( which are the vertical & horizontal pieces respectively) may be susceptible to warping/twisting over the long height and width.
This is easy to combat if you take , say a two by eight for the rails and styles and rip them into several equal pieces . Then swapping alternate pieces end to end , so that the grain goes in a different direction with each piece ( the way a butcher block table looks on its end grain). Waterproof glue every touching side together , clamp overnight. Ensuring everything is laying flat and straight. It helps to clamp the laminated frame pieces to a straight table in addition to clamping the overall assembly.
Once the styles and rails are joined , you can fill in the door with plywood or tongue and groove 1 X material .
Belt sand smooth.
This will keep the doors nice and straight over their lifetime.

[btucker]

Thanks for the advice, but you lost me at end to end. Do you mean that one swaps the ends. I was going to use one buys for the styles to lighten the doors and make them not as wide. Is this wise or will this increase the chance of warping?

[Nwptsailor]

If you take any piece of wood and rip it several times to get equal pieces, you rearrange every piece of the new assembly so that the grain ( when viewed end on ) goes in a opposite or different direction as the adjoining piece.
Flip alternate pieces end to end and rotate each piece as necessary .
Look at a butcher block table end grain , that will give you the idea.

I'd say one byes will not be robust enough . A door that big in 1 X will not last long

[TheSavage]

Personally I am lazy --I pitch the tracks up at the ends , install counter weights , and have a door that's almost on the ground closed and clears the ground as being opened. Really easy to open to.