Poo will not go through 1/2" mesh. Thus it would be somewhat pointless, unless you have a hose that you will be able to hose the poo thru every day or few days AND don't mind the resulting pooey swampy spots all over your yard.
If you wanted something poo can fall thru, you need to go more to a 1x1 mesh; but this is harder for the chickens to walk on, and harder on their feet.
ANY kind of mesh probably ought to be covered with something solid during your winter months (I am assuming Portland gets close to and below freezing on a fairly regular basis?) and it will make wintertime ventilation difficult.
Honestly, IMHO it offers ZERO benefits (except *possibly* in some very-hot-climate situations) and a large number of downsides, and I would suggest forgetting about it. A solid floor with a bit of shavings on it will be easier to clean, better for the chickens to walk on, and warmer and less drafty in winter.
JMHO, good luck, have fun,
Pat