I am looking for ties for new backdrops because we finally have pipe and drape. Our sets are 18 by 45 feet so they will need support across the top. Is cording usable and is there a specific one that it is best to use? We have used cloth and ribbon in the past and it works but I was wondering about other ideas.

Shatcher is right!
I use any black cotton cordage for back drops & drapes, because it is easier to strike or set up. I  avoid synthetic cords or rope, because they are subject to rubbing & fraying easily.
Use a Lark Head knot on the eyelet & a an ordinary Bow knot to tie off to the batten.
( For a Larks Head - fold the cord in half, then put the loop created, put the two pig tail ends thru the loop & tighten on the eyelet. You should have two pig tails now to tie around the batten- ensure the cord is at least 2? long - longer is better, test it with the dia of the pipe]
Also it is an idea to have the pipe marked off in 1 metre increments, the centre in a different colour mark, so cloths can be lined up easily.
For hangings like Legs & Boarders I have started to use plastic electrical reusable ties, which seam to work well for situations where the hanging is normally stagnant.
With the ties on your drop, it is a good idea again to mark the centre of the cloth with a different colour tie.
{To correspond with the central mark on your Pipe [barrel/batten]}.
Also to stretch your cloth when it is IN, get some ?hold on? stretchers or make some up your self.
http://www.hallstage.com/holdon.htm
They are easily made using 2 piece of ply wood, cordage, bolt & wing nut!

I’m organising a series of stagecraft workshops, at the new home of our theatre. {Well at least it is a place to hang our hats at present}
One of the workshop modules is closely related to this topic of ‘ties’.
As I have yet to start a topic I think, I won’t break my own tradition & if Kirk-Stauffer doesn’t mind?
 I will piggy back on to this thread & ask the question of our  greenroom techies & others!
With rope work back stage, I know the knots, splices, whipping, & lashings I normally use. but just out of interest
& Releasing this in most circumstances it is a lost art back stage these days.    
What are the favourite basic ones used by you or techies need to know, tie & understand back stage.
Of course the need for rope work will vary between theatres, however there is still  the requirement for a basic knowledge of them, even in the most hi-tech venues.

The old ‘Monkey fist’ is a rarity these days, I don’t think I have actually made one in the last 20 odd years?
I used to use it a lot when throwing lines & chucking halliards up the grid for pulleys. Which of course is less dangerous than using a hammer or spanner to end of the rope, for a receiver to catch it.
As a wee bloke I learnt knots back stage, long before I joined the Brussel Sprouts [Boy Scouts] as a Wolf Cub back in the 50’s.
Which made it easier when I went up to the Sea Scout troop latter on.
Like you it always amazes how many techyies can’t actually tie a Reef knot [square] & end doing weird & wondrous things from ‘Granny’ to ‘thief’ knots.
Yet they can do a ‘bow’ knot to tie up their shoe laces. Then become amazed when the two bows are pulled through, leaving a perfect Reef knot. Which is easier to explain to them then doing ‘left over right & under, then right over left & under’, as most tend to get mixed up with their Port & Starboard sides { "There is a little Red Port Left in the bottle"}.
Backstage the ones I use regularly are the Reef, Bow, Bowline, Sheet bend, Round turn & two half Hitches, Larks Head, Rolling Hitch, Prussic, Truckies hitch, Timber hitch & clove hitch {only with a bite [half hitch to it lock off ]}.
Occasionally a ‘Turks Head & ‘Crown Knot’ & ‘Fisher mans’. Along with the odd ‘Short’ & ‘Eye’ splice, the same with ‘whipping’, ‘Square’ [although I prefer the ‘Chinese Lashing], ‘Shear’ & ‘Diagonal’ Lashings which is a rarity, but lashings do come in handy with spars & battens when running short of ‘gaffer’ tape on occasions.

Sorry to bring up this topic again.