We take filtered water, (but not chlorinated water) and feed it up to the roof using a solar pump. Water then runs at approx 2m per second down the line and returns.
The ideal length is 12m. This means the water is in the line for 12 seconds(Up 12m + back 12m = total 24m / 2m per sec) = 12 seconds.
Water is then heated and returned back to the pool.
Roof spaces will hold some serious heat. Even in winter a roof can be mid 30 degrees. The shoulder months really kicks in using solar.
The ideal combination is to use a thermal blanket, to retain the heat and reduce evaporation and running your solar to pull as much heat into the pool as possible.