Roof cleaning with a rainwater tank: divert first, then wash
Divert or disconnect the tank inlet before any product touches the roof — not after. Wash, rinse thoroughly, then leave the tank isolated until the first decent rain has flushed the roof, and reconnect after that. A first-flush diverter helps but is not a substitute for isolating. If the tank feeds anything indoors, tell us before the day. Ten minutes at the downpipe saves draining a full tank.
Your roof is a funnel that ends at the tank
A rainwater tank is plumbed to collect everything the roof sheds. Rain lands, runs down the tiles or sheets, gathers in the gutter, and leaves through the downpipe into your tank. That is the whole design. It also means anything else on the roof takes the same route: treatment solution, rinse water, dead moss and lichen, the silt and bird droppings the wash lifts off. There is no clever filter in that path. Whatever leaves the roof arrives at the inlet a few seconds later.
This is not a reason to avoid cleaning the roof. It is a reason to sequence it. Mains water is a moving supply — a tank is a closed reservoir with no flow and no dilution. If something unwanted goes in, the fix is draining and refilling, and sometimes having the tank professionally cleaned out. That is a real bill and a lot of wasted water for a problem that ten minutes of work at the downpipe prevents entirely.
A first-flush diverter is a helper, not a plan
First-flush diverters are good devices doing a specific job. They hold back the opening portion of a rain event — the dusty, droppings-laden runoff that comes off a roof after a dry spell — then fill up and let everything after that pass through to the tank. They are sized for normal rainfall on a normal roof. A roof clean is not normal rainfall. It puts a concentrated volume of water and lifted debris down the pipe in one session, far more than a diverter chamber can hold.
So the diverter fills early and then passes the rest straight through, which is the opposite of what you want on wash day. Treat it as your second line of defence, not your first. Real isolation means closing the tank's inlet valve if it has one, physically swinging or unclipping the downpipe so it discharges to ground, or capping the inlet. Whichever method the plumbing allows, it happens before the first drop of anything hits the roof.
Wash, rinse, then let the weather finish the job
The clean itself does not change because there is a tank. A tiled or Colorbond roof still gets a soft-wash treatment at low pressure, growth still gets killed at the root rather than blasted off, and the surfaces still get rinsed down properly with the runoff going to ground or stormwater rather than the inlet. A thorough rinse carries the bulk of the treatment and loosened debris off the roof and out of the gutters on the day.
The bulk is not all of it. Residue sits in tile laps, valleys and gutter joins, and dead moss keeps shedding for weeks afterwards as it dries and loses its grip. That is why the tank stays isolated until the first decent rain has run across the whole roof and out the downpipe — a proper soaking, not a passing five-minute shower. Depending on the season in Sydney, that is anywhere from a few days to a few weeks. Reconnect after it.
Tell us before the day if the tank feeds anything indoors
A garden-only tank is straightforward. Plenty of homes across the North Shore and Eastern Suburbs go further — the tank is plumbed to toilets, to the laundry, occasionally to more than that. That changes how we sequence the work and how long the isolation stays in place, and it is a conversation worth having at booking rather than while a crew is setting up ladders. Mention it when you book, along with any pump that auto-switches to mains, and we plan the day around it.
Here is the honest limit. We isolate the inlet, wash the roof and tell you when it is sensible to reconnect. We do not test water quality, and we cannot vouch for what went into a tank that was left connected during someone else's clean years ago. Testing, emptying and internal tank cleaning are a plumber's or a tank specialist's job. If you have any doubt about what is already in there, sort that out separately — cleaning the roof will not clean the tank.
Getting a real number on it
Roof work does not price off a flat menu. Size, pitch, how established the growth is, access and the roof material all move the labour, so Schmick quotes roof cleaning per job rather than pretending one figure fits every house. The tank isolation is part of doing the job properly, not a line item you have to ask for. It is also worth doing the gutters on the same visit, since they inherit the residue anyway — bundling services on one visit saves 5-15%.
Per-job pricing does not mean waiting on a callback. Answer a few questions about your place in Schmick's instant quote and it returns a firm, bookable price in about 60 seconds, and it holds unless the house turns out to be materially different from what you described. Flag the tank in the booking notes and we will handle the downpipe before anything else happens. Get the number first, then decide with it in front of you.