I think i would have just rinsed it , and left it without worrying, it wiil have to run off ?
but two hours on a con roof ?
large dirty ones are around an hour ish ,
once you get the pattern of where you start first, how you will do it,... you will get faster,
i price most of them up as a two hour job, and they all nearly take an hour,
oh and you will learn not to take them all on..... but if you must take really awckward ones on...... make sure you price it for half a days work.... and with a bit of luck , they will refuse your price...
if they accept... you will not mind doing it so much, as its well overpriced. !