I have a vaulted ceiling and I will be putting 5" tongue and grove pine on it the ceiling is 36' long. As it is such a long run I want itto be as straight as possible.. My questionis how to start to get a straight line..
"Straight" is all a matter of perspective. What you will probably find is that if you go perpendicular to the end wall, you won't be parallel to the side wall. What you will have to do is figure out a happy medium between the two, which will be to measure the squareness of the area, as well as the length of your width. All this is to say, measure each end of your run from side wall to peak of the ceiling. Are these equal lengths? Measure the end wall to side wall at the ceiling, is this square? If you answer no to either of these questions, then you will have to take an average of these measurements, then choose a starting point that will accommodate the material you are using to the space you have.
Still confused? Understandably, but this is what you pay professionals for, they understand these principals and will accommodate them accordingly.
To simplify, measure from the side wall to the peak of the ceiling, divide it by the actual width of your material. You DO NOT want a sliver at either the start or end of your run, so you will need to cut the width of the first piece to accommodate any weirdness of the wall and to split the difference between the widths of your first board and your last board.
Use a 6ft level (or other straightedge) and check all the rafters to see which ones are high or low. Then plane or shim everything perfectly even at the very beginning and end (at the peak and the walls) You want to basically get within 1/16" and not have any dips or droops. You can get away with some unevenness in the middle, but where the planes meet it really needs to be perfect, because waviness is more visible when there is some other surface to compare to, like at the ridge and walls.
I would probably start at the peak and work downwards, then the last fussy cuts are down where you don't need scaffolding to reach.
My approach is to create the needed flatness by sistering 1X4 slightly below the rafters and using string lines to keep it all straight. Another benefit of this is that if the section is out of parallel, you can make one end of the sistering a little lower which makes it longer thus correcting the problem. Fasten into the rafters, not the sistering- they are not adequate to hold the weight, their sole purpose is to act as shimming. Keep the difference as small as possible- I try to never exceed 1/8" but I'll tolerate 1/4", no more. Past that the fasteners may flex excessively and loosen. This sure beats any other way of shimming or planing the rafters to the needed flatness!