All depends on how dirty it is. You will use a lot of pure water though, so it would be best to use tap water first.
Little tip, fill the backpack up with tap water (hosepipe), and add the correct ratio of TFR. Scrub whilst spraying it on, to really agitate it, leave for around 10-15min. Then wash as normal.
Depending if it's clear glass or polycarbonate and depending on how quickly the water dries, you might have to use just pure. If it's a cloudy day and small enough roof I would just use tap water, and then go for a final pass/rinse with pure. Ideally you wouldn't want the tap water to dry because that will leave spots and would again require another pass. It would be fine for polycarbonate roofs though. Just a quick rinse after with pure.