When I had our little 4x6 open wire coop I bought a couple plywood sheets and Lowe's cut them to size for me. Fastened them to the long sides of the little coop and then double tarped over the coop. Then I put a pop-up canopy over the coop and buried the canopy legs in the soil so it didn't para-sail away in the wind - wasn't the most pleasing aesthetics but it kept the chickens dry! However, this year we got a new Barn Coop that is a lot sturdier and heavier and hopefully our contractor will get our patio roof built over it before the rains hit. Otherwise I'll use a LARGE handy-dandy tarp again to prevent leaks into the coop.
With our old little coop i used treated 2x4s around the coop base to divert as much water as possible from the coop dirt floor. A couple areas along the base were problematic when I water-tested with our garden hose and a couple paver stones and/or smooth fireplace bricks worked to plug the seepage. The canopy was a tremendous help. Just do as much as you can think of to prevent water entering the coop from the top as well as at the coop base - it pays off in the end! Water test with a hose to see where you need to concentrate your efforts. Our worst rain poured like El Nino for 2 days straight last season and just a little bit of water seeped into the coop so I swear by water diverting at the base.
