rainfall and water saving
Ref: E08c

Rain falling on tarmac

Collecting and using roof water

When rain falls on hard surfaces such as tarmac, concrete or tiles for example it does not soak in, but remains on the surface or runs off the surface if it is sloping.

When water falls onto a rooftop it runs down the sloping sides of the roof into the gutters, then into the drains where it is carried by a series of pipes to a sewage treatment works. This rainwater could be used before it goes to be treated, if it could be diverted and stored.

In Cornwall, and indeed the whole of the South West region we have a lot of rain!

Cornish Weather Stations

The bar chart below is produced by the Meteorological Office (click here to visit the Meteorological Office website) and shows monthly rainfall for four major cities across Britain. You can see that the amount for Plymouth is quite a bit higher than the other British cities.

Rainfall Bar Chart

Plymouth has around 900 mm of rainfall each year.

Watering can

• Using a ruler measure out how much water this is.

• If all this water fell from the clouds at the same time would it reach your ankles, your waist or your eyes?

The total amount of water that falls in Plymouth takes some calculating. You would need to know the area of Plymouth in square metres and multiply this figure by 0.9m (900 mm = 0.9 m) to get the total volume in cubic metres of rainfall each year in Plymouth!

If more people in Plymouth could collect rainwater before it enters the river Tamar or the local sewage system they could save a lot of money and be more water efficient at the same time.

• You are now going to work out how much water your family could have saved last month if you had stored the water falling on your roof and used it for things like: washing the car, cleaning the windows and watering the garden.

1. To do this you will need to know how much rain fell in your area last month, look on the rainfall data page (E08c2) and choose data from a town near you or use the average data for Cornwall.

Alternatively you can look on the Met Office web site for recent data.

How much rain falls each year in your area? …………………

2. You will also need to know the area of your roof. You will be able to work this out by measuring the length and width of your home.

Length of house (in metres) X width of house (in metres) = area of roof over which rain falls in M2 (metres squared)

Tip ~ If you live in a detached or semi-detached house it might be easier to measure the area of the ground floor by going outside and measuring the length of the walls.

Tip ~ Measure out a 1 metre length of string or rope and use this as a guide to measure the walls of your home.

What is the area of your roof (M2 )? ……………………….

3. You can now work out the volume of rain falling on your roof every year, by doing the calculation below:

Rainfall (answer to question 1) X roof area (answer to question 2) = Volume of water falling onto your roof last month in M3 (cubic metres).

4. You can now work out how much money your family could save by using the water collected from the roof last month instead of water supplied by the Water Company.

If water costs 86 pence per cubic metre.
Do the sum below using either monthly or annual figures to work out the value of the water falling on your roof in pounds Sterling (£) per month or year:

Volume of water falling onto your roof each year (cubic metres) X 0.86 = amount of money saved in £ each year by using rainwater collected from the roof!

Summary