I live in Houston and my husband and I are worried about the same question.
We are finishing out the inside of the coop to include the ceiling. Yesterday was extremely hot... with a heat index, it felt like a 102+.
Although the coop was warm inside, it did feel cooler inside than outside. We have two big windows in front where the sun will hit mid-afternoon and a row of smaller windows that will be over to nesting boxes. We are also going to put some sort window shade that will double as door for winter conditions (like Houston has a winter) to help keep the direct sun from hitting the inside of the coop.
DH has put two electrical boxes to plug fans into that will sit in the two big front windows hoping that it will create a cross breeze from the back windows.
Our run is going to be 16x12 and we are going to put solar netting across the far end for shade during this time of the year.
With fall just around the corner, August being the turning point, we will know more next summer when it really gets hot.
Other than having plenty of water in shaded areas is about all you can do... I understand that they are pretty hardy and will be ok...
