Rainfall harvesting

[redsturgeon]

I am having a flat roofed extension built of around 50 sq metres. I am looking to drain the roof into some rainwater storage tanks for use in car washing and garden watering. I am try to work out the tank sizes that would work for me. Are the following calculations in the ballpark?

If I have a 50 sqm roof area and 10mm of rain falls on it then I make that 250 litres of rainwater (5 litres per sq metre per 10mm.)

Looking at my rainfall it looks like max 50mm per month so that would be 1250 litres.

I have seen a reasonably compact system that will store 1600 litres so I think that looks like it would work.

Any thoughts?

John

[twotwo22]

I calculate differently

50 sqm x .05 m = 2.5 cubic metres
2.5 cum x 1000 = 2500 litres

10mm of water on 1 sqm would be 10 litres

If you think of one of those tanks in farmers fields they are a cubic meter and hold a 1000 litres so each mm is 1 litre

[AleisterCrowley]

If I have a 50 sqm roof area and 10mm of rain falls on it then I make that 250 litres of rainwater (5 litres per sq metre per 10mm.)

1mm of rain on 1 sq m is 1 litre I think
10mm on 50 sq m 500 litres

[modellingman]

If I have a 50 sqm roof area and 10mm of rain falls on it then I make that 250 litres of rainwater (5 litres per sq metre per 10mm.)

If you had a roof that was 5m by 5m (so 25 square metres in area) then multiplying that by 10mm would give you your 250 litres. AleisterCrowley and twotwo22 are both correct. To store 50mm of rainfall falling on 50 square metres of roof you need 2500 litres of capacity.

Any thoughts?

What are you trying to achieve? Avoid the effect of potential hosepipe bans? Minimise your water bill? Minimise the costs of a drainage connection for the new roof?

Your sizing decision at the moment appears to be based on creating capacity to store an assumed level of supply (rainfall) over what appears to be arbitrary time period (one month). Dependent on your objectives, you may also want to consider your rate and pattern of usage and the variability of the supply. On the other hand, you are not a water company trying to size a new reservoir, so maybe it is simply about what you physically have room for, cost and similar considerations, etc.

[Dod101]

And irrespective of all these considerations, what about overspill? A soakaway seems to be the thing these days rather than a connection to a sewer.

Dod

[redsturgeon]

Yes you are all correct...10 litres per 1 sqm per 10mm I'd worked that out then for some reason typed 5!

What are you trying to achieve? Avoid the effect of potential hosepipe bans? Minimise your water bill? Minimise the costs of a drainage connection for the new roof?

All of those things to some extent plus I think rainwater is better for the garden than tap water, plus the ecological benefit.

It is more difficult for me to judge my average monthly usage.

I've checked my hose flow rate and it is 10 litre per minute so on average I guess I could get 250 minutes of hose use per month or an hour or so a week which would be plenty.

My water charges are about £4 per cubic metre so I guess I could save in the region of £10 per month so not a great saving but still worth the ecological saving.

Thanks for your help

Just seen Dod's post and the overspill will go to the existing drain.

John

[twotwo22]

Just as an idea.
I use 2 x IBC containers joined together to give me 2000 litres. I use a submersible pump in the top of one to either pump water directly to the garden or to top up rain butts that we use for watering can dipping.
The overflow from the 2 IBC containers, when full, goes back to the drain.
They are around the back of outbuildings, though, as they would not be pleasing to the eye !

[Meatyfool]

All you need to know: 1 cubic centimetre = 1 millilitre.

From the base point that your water bill indicates 1 cubic metre = 1000 litres

gives

100X100X100 cubic centimetres = 1000x1000 millilitres

or
1,000,000 cubic centimetres = 1,000,000 millilitres.

Voila!

Meatyfool..

[Howard]

I have a friend who collects water from his (large) house roof and stores in underground tanks. He is a keen gardener and uses most of the water for his vegetables and plants.

I wish my house had a similar arrangement. However, starting from scratch, I'm not sure that it would be that ecological. My friend's arrangement requires large plastic tanks and uses a powerful pump run on electricity.

Cheeky question: If you really want to care for the environment, do you really need an extension? Building this will use a lot of resources.

If you want to save money, given the cost of tanks, pump, running costs etc, it might be better just to use less water by converting the garden to need less water?

As a keen gardener, I am gradually reducing water usage, but this year has been a challenge!

regards

Howard

[JohnB]

When I looked at waterbutts I found that 400l (2 big butts) capacity could be bought for £50. But my water supply and sewage cost is £2 per 1000l, so that was worth 80p. But you'd only probably get 4 refills used for gardening in a season (after all, after significant rain you don't need to water), and its a lot of water to lug around in watering cans, as the head is not sufficient to drive a hose. Rainwater is better for plants than treated water, but its a lot more faff.

It would make much more sense to use the water for toilet flushing, where there is a year-round demand of 70l per person per day, some 30% of your water bill. But you'd need to cost the automated recycling system, as I doubt you'd want to be carrying buckets around.

[Imbiber]

I took an alternative route and built a 3 metre high timber tower with a 1000 litre IBC on the top. I pump up the water from the four water butts on two large sheds and rely on gravity to feed the hose and lance. Three metres provides enough head for a reasonable flow. I grow a lot of veg and SWMBO does the flowers. Struggling a bit this year, have had to top up with mains water (£1-60 / 1000 litres ). I do have to pay the cesspool emptying charge, about £240 per year.