smurfchops
I also use Sodium Hypochlorite (I believe it's what farmers use to clean milking parlours) but I always hose it off. I dilute it 1:3. It's just fantastic - my yard doesn't get the sun for most of the winter, and by spring is covered in algae (and the dog pe*s there sometimes). I hose the yard, brush of the excess, sprinkle, wait ten minutes while I have a cup of coffee, and bingo!, hose off. It cleans the drains as it runs away.
I use it on wood, concrete, cement and pebbledash and could admire the result for days (what a sad life I have!!). Beats hosing any time.