Recently we had a rather funny incident: suddenly about a liter of water ran from above the drop ceiling right in the middle of the office. Turned out it was some pipe in the air conditioning that detached and released condensed water.
Obviously that's not what anyone wants to happen above a server equipment rack. What is the easiest (I mean that materials should be easily accessible and the consrtcution should be easy to manufacture) reliable way to shield a rack from above to protect it against minor water leaks?
I take it the rack isn't an enclosure? That would protect from most drips like that.
You could try running plastic over the tops of the racks...but I don't know what you'll do to ventilation or what happens if the heat warms the plastic up and releases some kind of gas.
Not to mention if you have fire suppression...blocking things like that from reaching the equipment, what would that do?
I'd probably look at finding a way to run a trough around where the AC leak was and re-routing it. Anything you do near the equipment will affect heating/airflow/fire suppression/safety, and putting things in the ceiling that isn't made for it can be a safety/fire hazard as well. Regular maintenance of the AC unit/inspection of the unit may help mitigate the risk.
Rerouting and stopping at the source would help. Maybe find a way to run a remote water sensor that can alert you if there's an issue?
Also..insurance. Insurance to cover damage. Along with spare equipment should there be an accident.
Summary...get enclosure racks and/or minimize the risk at the source, the AC unit.
Big sheet of polystyrene.
Get a sheet of rigid 1" insulating foam or something and make a cut-to-fit lid for your rack with at least enough overhang so that drips off of it don't splash on your equipment. I used to have a rack in an office below a deli. Seriously bad land-use planning, but whatever.
You can hang foam with wires or screw it from one side without compromising it's being water proof, and being rigid you can give it a little slope to divert disasters off to the least damaging side. Being light it's not hard to position where it will do the most good.
I don't mean to write useless rows: I do understand you have a specific question and need a specific answer but I am writing just to push you not to have production system in a similar situation. Fight your way to normalize your rack instead of "legalizing" something absurd: buy a good closed rack and repair the leak seriously. Working hard in your server room is also pure fun if you have nice colleagues :)
Good luck!
use the big metal sides from one of the racks as a cheap shelter. This has been done successfully here in the past to protect from rain through a hole in the roof - long story.
This shouldn't make anybodies best practices list, but you could always use an umbrella.
They make ceiling leak diversion kits that you can set on top of the tiles and drains through a hose to a location you choose. They are around $150.
I personally would recommend something like: http://turtleshellproducts.com/
Of course you could make your own tent like product.. But that's a very expensive risk, the stuff in your racks could cost in the millions to replace.