Arapiles says:
Our roof is very steep so the actual surface area is more than the ground area covered. Hence why I think that if I caculate on the basis of ground area the efficiency losses will be cancelled out.

Bob Gross says:
If the rain is falling vertically, then a steeply pitched roof will gather less rain.

Maybe a sketch will help:

.

Roof 1 has more area because of it's steeper pitch.

Which roof will intercept more of the rain represented by the thin vertical lines?

Think carefully guys, take your time. And don't try to tell me roof 2 will gather less because it's leaking like a sieve. ;-)

Sounds like you got your system installed at the right time anyway Arapiles, well done!

Edited by tallbloke on 10/31/2010 01:55:18 MDT.

"Arapiles, how does the pitch of the roof affect the area of collection? The same amount of rain is going to fall on a 40M^2 area no matter what the roof angle under it is n'est pas?"

Ahh, you did not define where exactly the 40 square meters were found! I assumed that it was pitched roof area. Others may have assumed that it was horizontal ground area.

--B.G.--

Ah, Bob, we had been discussing our respective roofed areas previously. :-)

"Think carefully guys, take your time. And don't try to tell me roof 2 will gather less because it's leaking like a sieve. ;-)"

I know what you're saying - interestingly the potable water calculator used in the Australian Green Star rating tool gives pitched steel roofs (> 30o angle) a run-off coefficient of 0.8 and a flat smooth roof (< 30o angle) a coefficient of 0.5. So they do presume more run-off from a pitched roof.

In any case, and to my genuine amazement, my water tanks now appear to be full.

Arapiles, quick, dig a hole! ;-)

I suspect that in a hot country the increased capture from steep roofs has more to do with less losses from evaporation, as the water runs into the tanks quicker.

Which roof will intercept more of the rain represented by the thin vertical lines?
-------------------------------------------------------------

If the rain is falling vertically, both will capture the same amount of rain. This is because both roofs cover the same ground area. Take either roof off, and assume the walls and floor do not leak, then the vertical capture has nothing to do with the roof.

"If the rain is falling vertically, both will capture the same amount of rain. This is because both roofs cover the same ground area."

Is it that or surface area? Think of a bucket of water: which will absorb more water when pushed into the bucket, a flat board covered with sponge lying on the surface or an inverted triangle (like an upside down roof) with a base the same size as the flat board and with the exact same sponge material but pushed into the bucket? Wouldn't it be a question of the greater surface area of the inverted triangle rather than the nominally identical "ground" area both have?

Arapiles,

Nick is exactly correct.

Sponge? I think you are making this more complicated than you need to. ;-)

However, consider a hot sun and a roof with semi absorbant surface like slate or concrete, or just a hot surface like steel. A shallow pitch means the water travels off it more slowly, and has more time to evaporate. That's why the coefficients af collection show a difference. A fair percentage of rain is light and gentle and the shower short lived.

The volume of rain falling does not care about the slope of a roof. The volume does not change due to roof configuration.

Evaporation only occurs on the top surface of the water, and needs solar heat to turn into vapor. While it is raining, normally the water cannot evaporate because there is little solar heat. During a rain the air is saturated with water (humidity), which slows the evaporation process. Rog's example above does show potential higher evaporation at the onset of a rain, when the roof is already hot. Water will evaporate faster if it is stored in a larger vessel with a larger surface area, but we are not storing the water on the roof. I would guess measurements would show minimal differences over a long period of time between the two types of roofs, assuming the same roofing material. But then, I am not a scientist.

If every last drop of water is important to capture, then roof design can have a decent effect. If you have a single sloped roof (not gabled) angled into the direction of the prevailing wind/rainfall (here that would be to the south), then the greater surface area will collect more water than a flat roof. Otherwise the difference will be minimal. However, having a single sloped roof means less guttering to funnel the rainfall, unless you have over-sized gutters.

...I think.

Seems like the gutter area is relevant. No matter how large the roof, it all must channel through the gutter.

After I looked at my annual water consumption by totaling my last year of water bills (CCF), I calculated how much the payback would be on various rain collection set ups including filtering for drinking water. As a result, I plan to collect water in a rain barrel or two from one of my gutter downspouts and use for gardening. Water is just too cheap where I live.

However, looking at the numbers was an enlightening exercise.

For me, it's not so much the economics as the satisfaction of being 'off-grid' in terms of water. Plus when we get a drought the authorities ban the use of hosepipes, and I need to irrigate my crops. Also, I don't like the taste of beer made with chlorinated water.

"Also, I don't like the taste of beer made with chlorinated water."

Ouch, that sucks. According to Wikipedia (so it must be true ;)

"Christchurch has one of the highest-quality water supplies in the world, rated one of the purest and cleanest water in the world. Untreated, naturally filtered water is sourced, via more than 50 pumping stations surrounding the city, from aquifers emanating from the foothills of the Southern Alps." It's not even fluoridated...

Residential water is covered by our property taxes, no matter how much we use, but I feel it's a shame to use artisian water on the garden, laundry etc...

I think the optimal roof design, at least in our area, is a gabled one with one side facing south to collect maximum rain from the southerlies, and the other facing north to accommodate solar panels. I imagine you could have the entire roof slanted to the south to collect rain, with raised panels to collect solar. I suspect this might be true in areas around the southern coast of Australia as well...I assume you get most of your rain off the Antarctic like we do??

We have lovely ground water in Yorkshire too, and many of the breweries here have their own wells. For us home brewers though, it's a choice between collecting from a clear stream above the top field boundary, or using tapwater. Rainwater doesn't make the best beer, but better than chlorinated tapwater. It can be hardened slightly with a small amount of limestone.

Our rain comes mainly from the southwest across the Atlantic, though Britains weather is a lot more unpredictable than continental weather is, being an Island situated well north for it's mean temperature.

My home is an old stone built workers terrace cottage. I'm on the end, so I have windows in the gable wall, with great veiws on to the hills nearby. The main roof faces south, with a low window which stretches right across the attic workoom. I have a hot water solar panel, and I'm trying to get a system which I can run our electrical needs with. Pricey though.

Ahh, I didn't realise you are from Yorkshire. My partner (and close family) are all from Yorkshire (Bradford unfortunately, so not such nice views). Needless to say we go through a lot of mugs of tea and milk at our house :)

I doubt that you would have much problems with rain water shortage in that part of the world! Shame about the chlorination though. Yuck. It's one of the worst aspects of traveling for me. Every where I go it seems the water is chlorinated :(

I'm up near Ilkley Moor and Otley Chevin. We get around 700mm rain per year arond here on average. It does vary a lot though.

The tapwater isn't bad, and not heavily chlorinated, but I can taste anything over around 3ppm and prefer the pure mountain spring as we all do!

I quite like Bradford, some good curry houses there!

"We get around 700mm rain per year arond here on average."

That little? I'm surprised. I thought all of the UK was just one big rainstorm :) That's about the same as Christchurch, which is considered a dry spot by NZ standards (range 300-1000).

"I quite like Bradford, some good curry houses there!"

LOL

We are just to the East of the Pennine hills which for a vertical spine down northern England. On the other side there be damp dragons, and a rusty town called Manchester.

Bradford currys are world famous round here. ;-)

Well since I don't speak metric, I had to figure out our average rainfall. It is 144 mm per year. Some years it is zero. Also found out our average summer temperature is 42 C, and 46 C is not uncommon. The highest I have seen here is 50C. The only thing we sell in kilos are illegal drugs.

I prefer to "inch" my way through life :)

Rainfall can be measured in furlongs per fortnight.

--B.G.--