if the temps are 40 celcius, you may want to consider a location where it gets less direct sunlight.
also, i would imagine winds are stronger on a roof than on the ground, which will also dry it out quicker.
in 40 degree temps, you need partial shade and less wind.
for all you fahrenheitphiliacs, 40 celcius = 104 fahrenheit ((celcius * 1.8 ) +32)=fahrenheit