Installing a Drop Ceiling

How do you keep from getting runners under the lights?

I have had requests to post a description of the method I used to 
ensure that the runners of a drop ceiling do not block lights or run
under obstructions that prevent the little hanger wires from being 
mounted to the beams.  Here goes:

One problem with a drop ceiling for model railroads is that, if you
mount lights above the ceiling, it is almost impossible to keep the
runners from ending up running under a light and casting a shadow.
Forget having the lights only be in certain panels of the ceiling.
Even if you make the whole ceiling transparent/translucent/eggcrate
panels, the damn runners end up where you need a light.

Also the runners are hung with soft steel wires from hooks screwed
into the beams of the floor above.  There are places you can't put
these hooks, like a heating duct, and so you can't mount the runner
so that it parallels and runs right under the heating duct either.

Measure the distance of every light and obstruction from one corner
of the room.  You only care about obstructions that run parallel with
the runners, so for heat ducts and lights running N-S, you only need
the dimension from the E or W wall, and for ones running E-W you
only need the dimension from the N or S wall.

Mark off a large square on graph paper.  This should be 6" x 6" or so,
and will represent 4' x 4', the distance between runners on a drop
ceiling.  Mark within this square every obstruction you have measured,
using the dimension you measured _minus_any_even_four_feet_.
If you have a heating duct 9-1/2' from the wall which is 6" wide, then
draw in a 6" wide obstruction the length of the square at a scale 1-1/2'
from the edge of the square.  If you have a row of lights 11 feet from
the reference wall, draw this in at a scale 3' from the edge of the square.

Once all the hazards and obstructions have been draw in, you need to
find the clearest spot to run the runner across the square, and two clear
spots 2' apart for the cross ties on the other axis.  You can orient the
runners and cross ties either way, ie runners N-S and cross ties E-W, 
or vice versa, depending on which works best.

Once you have your best solution picked, measure the scale distance
from the two reference walls.  If it is 1-1/2' for the runners and 6" for
the cross ties, then that is how to run the ceiling, with runners at 1-1/2',
5-1/2', 9-1/2', etc, and cross ties at 1/2', 2-1/2', 4-1/2', etc.  Ignore all 
the advice from the ceiling manufacturer about how to determine the
best way to 'center the grid' on the room.  This will surely put some
runners directly under your lights and cast shadows on your layout.

BTW, this method works very well.  At least, it did for me!