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I was thinking of building a rainwater catchment system at my place in the boonies, but ...

I want to make sure -- priority one -- I'm not going to get toxic stuff in my water.

I looked at the galvanized corrugated roofing (impossible/difficult to walk on it) I looked at the steel sheets (what's in that paint?) Finally I looked at just plain aluminum sheets laid out in a shingle type pattern.

Aluminum doesn't seem too bad at first glance. Acidic soda beverages are shipped in them. Many water bottles & thermoses are made from them. Cookware is made from it.

Is there any proven scientific basis for not using aluminum on the roof? Am I overlooking a more obvious cheaper material?

Oh, and ... if I do choose aluminum, what's the best alloy choice? 6061, etc?

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    It's turning out to be more work than I thought it would be to make a quality answer so I will provide you with a few links if you want to do the research, the first one goes in a lot of detail and has further reading on the subject: 1, 2, 3.
    – Dispenser
    Apr 6 '18 at 18:49
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    What are you planning to do with the water you collect? What climate are you in?
    – LShaver
    Apr 6 '18 at 18:58
  • @LShaver I plan to use it for drinking and watering garden (on days it doesn't rain). I am in north central Texas, rainfall varies per month from 1.4" to 5". Apr 6 '18 at 19:40
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    Don't forget about the bugs, birds, trees, etc, that will leave "deposits" on the roof -- many of those won't be food grade...
    – LShaver
    Apr 6 '18 at 21:59
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    level.org.nz/water/water-supply/mains-or-rainwater/… says aluminium okay. Still you can use a water filter for the drinking water. Apr 8 '18 at 0:24
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You've raised a few issues in your posts, so having just built a new house I'll address those I'm able to:

To start with, I live in a country/area where corrugated steel roofing is the absolute norm and has been for over half a century. In my town less than 1% of roofs are clad in anything else. Everyone collects rainwater. Everyone uses that rainwater. Most people drink that rainwater. There are no health issues associated with properly built and serviced rainwater harvesting systems.

The most common rainwater tank material around here has historically been zincalume (combination of zinc and aluminium). So drinking water has been stored for years in containers significantly made of aluminium. Once again, no negative health effects.

Brand new zincalume tanks do leach zinc into the water intially, and this is what gives water held in those tanks its 'metallic' taste. But zinc is actually a mineral that humans need, so (apart from the taste) that's not a problem. Every season the amount of leached zinc decreases. Whilst the impact on taste is quite noticeable in the first year, you can barely taste it after about five years.

Aluminium does not leach into water like zinc does. The amount of aluminium that will end up in your water if you have aluminium roof sheeting, gutters, downpipes, or tanks is negligible providing the aluminium is not subject to mechanical abrasion. The most likely cause of abrasion is the impact of falling branches.

If you live in an area surrounded by trees, and the wind blows small branches onto your roof on a regular basis, then that's a problem. The kinetic energy of a falling branch is sufficient to break through the aluminium oxide layer upon impact and gouge out a small amount of the soft metal. These tiny specs invariably make their way into your water tank and ultimately into you. If you're always clearing twigs/sticks out of your gutters, I'd either clear away the trees causing the problem or go with (hard) steel roof cladding instead of (soft) aluminium roof cladding.

Price is also an issue. Local economics have a huge influence on the (relative) price of different types of metals, and here aluminium roof sheeting is 3x as expensive as steel. In other areas both are about the same. YMMV.

I don't know why you say that corrugated roof sheeting is "impossible/difficult to walk on". I've never had a problem and I've spent countless hours up on corrugated roofs over the last 40 years or so. If that view is gathered from third-parties, I'd advise not paying them much more attention. Continuous corrugations make for a uniform surface that is much less prone to tripping than the type of surface depicted in the image in your second post.

On the subject of profiles, the profile of that Fabral panel has large amounts of flat surface. Flat surfaces tend to accumulate clumps of debris (e.g. dust/dirt/pollen/leaves) because water always takes the path of least resistance and tends to flow around obstructions rather than trying to push through them. Thus if your area has long periods of no rainfall, debris will build up and the first (usually light) rains will not wash off the debris. The intensity/duration of the rainfall needs to increase before flat surface debris is dislodged — and by that time your first flush diverter is already full and the clumps of debris go straight into your tank > tap > you.

Corrugated profiles are half valley, half ridge. Debris naturally ends up in the valleys and, because that's where all the water is forced to go, is pushed downstream immediately — even in light rains. Corrugated profiles are thus the best profile to have if you want to have a 'self-cleaning' roof.

Corrugated profiles complement first-flush diverters. Flat profiles undermine first-flush diverters.

Moving on, we have the issue of gutters. Steel gutters rust out (and begin leaking) in as little as seven years around here. They are terrible. That's partly due to the material, but mainly to do with how downpipes are connected to the gutters. Local tradesman cut a hole in the gutter, pop in an insert, rivet it in place and put a bead of silicone around it. The downpipe is then friction-fit (or riveted) to the part of the pop that extends below/outside the gutter. The pop/silicone creates a ridge inside the gutter that allows water to pool and debris to collect, which fosters microbial activity, turns the water acidic and rusts out the gutter.

Going with aluminium or plastic gutters is a good idea as it mitigates what would otherwise typically be the first point of failure in your average roof-based rainwater harvesting system. (Note, however, that plastic may not be allowed, and isn't a good idea, if your house is located in a bushfire-prone region.)

The same 'self-cleaning' logic that applies to roof cladding applies to gutters as well. A gutter with a square profile (flat bottom with sharp corners) will accumulate debris and undermine your first-flush diverter. A 'half-round' gutter focuses everything to the bottom, self-cleans, and complements your first-flush diverter.

A small detail that most people don't pay much attention to is the type of bracket used to connect the gutter to the fascia. It is — unfortunately — normal around here to 'hide the brackets' by using a type that fits inside the gutter. Such internal brackets may make the gutter look more streamlined, but they block the gutter and force you to either climb up on a ladder a few times a year to clean the gutters out by hand, or precariously walk/crawl along the edge of your roof with a brush/scoop. Both are dangerous and/or dirty jobs.

External brackets hold the gutter in place from the outside. They do not obstruct the gutter. That means you can attach a brush to the end of a pole and (assuming you have a single-storey house) sweep clean your gutters from ground level. As far as I'm concerned, that's pure awesome.

My house has corrugated steel roofing and half-round aluminium gutters with external brackets because that combination was the most self-cleaning I could come up with. I clean my gutters from ground level. Apart from having to maintain the wood heater chimney/cowl, I have no need to go up on the roof. Rainwater quality is excellent and I will be long dead before either my roof or gutters fail and need to be replaced.

Sorry about the length of this response. Hopefully there's something of value in amongst it all for you to ponder.

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    Outstanding answer, thanks ... what I’m worried about is when I walk on the corrugated roof, that I will kink and dent the wavy corrugations. I plan on keeping the system at least 30ft away from the trees Apr 9 '18 at 5:53
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    Roof frames have trusses/rafters that go from ridge to gutter, and then purlins that run at 90⁰ to those. Metal roof sheeting is affixed to the purlins... so you'll see a horizontal line of screws wherever there is a purlin underneath. If you walk where you see screws, you know you'll be well supported (and won't be stepping onto an unsupported void). I don't know how much you weigh, but I weigh over 130kg and don't dent the roofing even when carrying tools. Wear rubber sneakers, walk along the screw lines, and you'll be fine.
    – Tim
    Apr 9 '18 at 9:04
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    Just to clarify: I can walk on steel corrugated roof cladding and not dent it. Given how much softer aluminium is, my weight would probably cause an issue with that. Steel is a lot stronger than aluminium. If you want to make sure, you can order thicker ('industrial-grade') steel sheeting that will happily support even the heaviest apes without denting. Suggestion: Buy a single 'normal' sheet, screw it down to a few pieces of lumber, then walk/jump on it. A few bucks spent on a stress-test might save you hundreds/thousands on over-engineered roofing.
    – Tim
    Apr 9 '18 at 9:35
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    Correct. 762mm wide sheets of 0.42mm thick corrugated steel with 16mm deep ribs. It's standard practice around here to fix sheets with either 3 or 5 screws per purlin — which translates to either one screw every 3/4 ribs, or 1 screw every 2 ribs. The more screws you use, the less deflection you get and the more weight can be supported. As long as you place each foot down evenly on (spread your weight over) at least two ribs there's no denting. Of course if you put all your weight on one heel and put that on one rib you'll dent it... but that's an obvious no-no.
    – Tim
    Apr 9 '18 at 15:23
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    For reference, here is a link to the actual product that is on my roof: lysaght.com/products/custom-orb
    – Tim
    Apr 9 '18 at 16:39
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As it turns there are several solutions. And as I've discovered, the headline is: aluminum is probably fine. HOWEVER ... in my case I think it will be cost-prohibitive, since a 4x8 sheet costs ~$100.

I found the documents that @Dispenser referred to, and luckily, they were commissioned by the Texas Water Development Board.

TWDB Document 1

TWDB Document 2

Here they state that most roofing materials are fine, with the exception of anything containing lead, and that the green roofing runoff should not be treated with chlorine. But also, the results from regular galvanized roofing turned in pretty good results, except perhaps for lead. In one graph I think, the lead was around 5ug/L, which is about 1/4 the EPA action level after the first-flush.

EPA Lead Action Level

As I've researched, I've found Home Depot sells these panels: Fabral 26 in. x 8 ft. Galvanized Steel 5V Crimp Roof Panel Galvanized Roofing

These panels will cost about $18 after taxes and cover 24 sq ft, at about $0.75/sq ft. So I think that my solution will be to use these panels (or another panel variety coated with Galvalume or Kydar), but not the wavy corrugated stuff, simply because it's difficult to walk on.

1) Build the roof with galvanized roofing

1a) Build a small portion of the roof and send a sample (after first-flush) off for testing before going full scale

2) Aluminum or plastic gutters to an FDA approved potable water container

3) Run the water through a high-quality filter (like a Big Berkey) before drinking, cooking or washing dishes

4) Bathe and water garden with unfiltered (or minimally filtered) water from the tank (depending on test results)

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