This frozen water thing was too much for me to keep up with & had to be done too often (while I'm at work 5 days a week feeling sorry for them), the new electric heated dog water bowl is perfect in the roofed area for my chickens.  The best one I found is by SmartPet & has a non-chew cord covering, I put a fitting stainless steal dog pan dish in it for easier cleaning.  The SmartPet one is navy blue, but it is not thermostaticly controlled; so, to that I have plugged into one of those thermostaticly controlled plugin that are on at 36 degrees & off at 45 degrees to save on electricity.  It worked just fine without the controled plugins to.  It works so well that I've put one inside the coop too with the stainless steel pan & into that I put a small enough chick waterer with the quart tall plastic (about size of qt glass canning jar) with the screw on bottom (this waterer was put in coop to keep the water surface smaller as to not add too much humidity to my small coop, but needed the whole electric heated bowl to keep the water from freezing inside coop).  Wow, what a "can of worms," sounds so complicated, but it's easy & actually works well.
But alas, this topic I see is about "without using a heated waterer."  I do look forward to seeing ideas that I haven't tried yet, as my "xtra rooster pen" doesn't have an electricity option; so it's the old fashioned way of going out with hot tap water as often as possible.  Will look into the posts that mention hot water freezing faster than cold/cool, fasinating.  
In addition, I have been taking out to my consistant laying hens a warm wet mix of soaked cracked corn, chicken ration, squash/veggies/fruits, dried chilli peppers, alfalfa meal, a little grit & homemade kefir (like yogurt), etc.  It's been frozen solid here at our mountain home in Oregon for several days & I discovered last weekend that that hot food was frozen within 30 minutes, what a waist.  So lately, I've been keeping the mix dryer by only adding the kefir, so they are able to eat it thru the day.  The milk fat in the kefir doesn't seem to freeze so solid.  What next!