The most vulnerable period during a heat wave is when nights stay hot. Birds tend to build up body heat during extended periods of hot weather. If their body temperature can be reduced during the evening, they will have a far greater ability to survive periods of extreme heat.
Assuming you lock the birds in their house at night for added predator protection, there must be enough space in the house to allow the birds to cool down in the evening, (approx.10 birds per square metre floor space for young birds, 5 for adult stock, and perching space allows an increase in this stocking density).
As well as the house being the right size: the house site, orientation, height, flooring, insulation and roof also affect the temperature inside the poultry house.