We don't get feet at a time here like Florida and Louisiana, nor is the annual total that high. I don't think you're going to be flooded, the overflow system the pdf at your link describes seems quite adequate.

I would have done something like this years ago except toxic products have been used periodically on the roofs here to control moss.

I have a single 60 gallon rain barrel set up to collect rainfall from my detached garage (a small 1920's era garage). That sucker will fill up completely in, like, half an hour during our heavy winter rains. At first this overwhelmed me, but I found that with a hose I can divert the overflow to areas, that, well, drain better. I've had no problems with the hose, but must remember to detach it during freezing temperatures.

In short, I've found the single rain barrel to be not so great as an irrigation tool, but really useful as a water management tool.

Yes, I guess it would fill up and spill over multiple times per year - unless the spigot was left open, allowing the water to run out and thereby defeating the purpose. Just yesterday I saw water sheeting across pavement during a downpour near here.

Would need one at every corner to get much of a quantity for watering.

Thanks! That helps. The half-hour fillup is disappointing, but expected. That won't work well for the gutter I wanted it for (no good place to direct the overflow), but that still leaves two good candidates for collecting some water for later. Not sure when that "later" will be with the current weather, but one can hope ;-)

I have rain barrels at the four corners of my house. I use a system that diverts the water out of the downspout using a hose. When the barrel fills up, the hose fills up also and forces the water back down the spout. One one corner - the least visible - I put a 275 gallon tank - which is what I use to water my potager garden. I have yet to use any municipal water so far this year. An added plus with rainwater is its ph rating of 5.5-6, vs 7 for municipal water. Lower ph facilitates plants' uptake of nutrients, which make for healthier plants. Now I'm thinking about a 5,000 gallon buried cistern, which will ensure year long water availability.

The new Puget Consumer Co-op store being built in Edmonds (on the site of the former Albertson's) is supposed to incorporate a water collection system using a 5000 gallon tank that will store water coming off the roof, to be used to flush the toilets.

I am planning on doing this at my home in Hillsboro, OR, but have yet to find affordable barrels, any suggestions?? Thanks, J

I used the instructions below. The 3/4" overflow is totally insufficient during a good rain - I keep meaning to enlarge it. I used an oak wine barrel. I think I saw them a few months ago on the King County materials exchange web site for $20 from St. Michelle winery.
I have it raised on some stacked stone- looks nice and rustic. I use a watering can with a wire hook on it so I can hook it over the spigot and let it fill without holding it.

Here is a link that might be useful: Make a rain barrel

As regards to affordable rain barrels, I would suggest looking to your local water utility. My utility, Northshore Utility district has rain barrels for sale at reduced prices, all fitted out with spigots, screened inlets and outflow pipes. Otherwise, check out craigslist or post a message on freecycle.org. I've also been lucky checking out factories that deal with food grade barrels. Often they're looking for someone to take the barrels off their hands.
If you have the space, bigger tanks equal less cost per gallon. The thingamajig I use to divert the water is called "Garden Watersaver", I've attached a youtube clip that explains it.

Here is a link that might be useful: Garden Watersaver