they can take quite a bit of heat as long as its not sun, and they have plenty of water, preferrably ankle deep water they can wade in, in the shade. i keep a good sized pan of water up under the bushes behind the garage, thats their favorite hangout during the middle of the day ..
Chickens can start struggling in as low as 85F. High 90s can be fatal for birds in poor health.
Providing deep shade with ground that they can dig into, to cool themselves in layers of earth, is ideal. Putting out pans of cool (not cold) water to stand in helps as well.