We’re English and we love a drop of rain

Have you ever wondered about how much rain runs off your roof, into the gutter and down the pipe? Your answer is probably “no”. I don’t spend my days considering it either. But heavy downpours used to worry me – the guttering at the front of our house didn’t seem to be able to cope well with torrential rain and the result was that a small area near our front door used to fill up. The soil there is so clay-like that you could probably grab a bucket-full, put it straight on a potter’s wheel and knock up a vase. Even digging two big holes and filling them with gravel didn’t really help. On a couple of occasions I’ve worked for an hour scooping up water, fearful that the level could rise above the damp course (usually at night, in the rain). The solution was quite simple in the end – clear out the gutter, remove the clog of leaves and mud from the down-pipe and fix the leaking joint.

Regular dadams.co.uk readers will remember that last year we had our kitchen renovated. One of the major changes was to remove a window and install french doors, and as a result of this change Mrs A wanted the 50 gallon water butt re-positioned. So I emptied it, did some restructuring of the down pipes, and placed the water butt at the other end of the garden. However, after a wet Summer, followed by a wet Autumn, a wet mild Winter, a quite pleasant Spring and another wet Summer, it became apparent that the water butt wasn’t actually collecting any water. My father-in-law concluded that the gutter was sloping slightly so that water was draining to where the water butt used to be.

Now add in to the story three new fir trees which Mrs A wanted in order to add some privacy to the garden. Our neighbour’s gardener got them at trade price and myself and my friend Derek spent a day planting them (and yes, it rained all day). The gardener advised a lot of watering, every day, even if it rains. It was at this point that Mrs A decided that the water butt would be useful in it’s original position, so once again I restructured the down pipes and moved the water butt.

So here’s the question again… how much rain runs off your roof, into the gutter and down the pipe in a day? When attaching the overflow into the drain I said something to the effect of “it doesn’t matter, it won’t get that full”. It did… less than a day later, after some steady but not catastrophic rain, the 50 gallon container was full and the surplus was draining out. Thinking of the price of water, I rigged up a hose attachment, opened the tap and let the trees have the full 50 gallons (which took over an hour).

Finally, you’ve probably seen the news coverage of the effects of the rain on the unfortunate inhabitants of Cockermouth in Cumbria. Living half-way up a hill I hope that the worst we’ll ever get is a flooded flowerbed. But it’s a sobering thought that this type of flooding can happen close to home, as these pictures of Lotus Park, Staines (2003) on the old dadams.co.uk site demonstrate.


  1. You should be able to calculate that pretty easily. I found http://bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT003790 where it looks like London get about 45 mm of percititation per month on average. Multiply that with the horisontal area of your roof. Let’s say your house is 150 square meters. 150 m^2 x 0.045 m will give you a volume of 6,75 cubic meter. Since one litre is one cubic decimeter (and there are 1000 litres per cubic meter), this means that you get about 6750 litres of water per month on your roof, if I calculated everything right.
    If you use non-SI measurements , it seems like we are talking about rought 1485 imperial gallons/month.

  2. I reckon Karl-Henry is right but what is even more impressive is that equates to 6.75 tonnes of water. I don’t want to worry you but my Boss lives on a hill and he got flooded a couple of weeks ago. The council had resurfaced the road and there was no kerb height to keep the water out. It poured down the street and made a right into his property as well as continuing straight down the hill. Straight into his kitchen and Living room. Even more unfortunately the living room is on two levels with a step down so he had a paddling pool until the water drained into the soil under the house.

  3. I installed a large capacity water butt in 2005 for water harvesting from the roof run off from just 2 downpipes, the 700 litre water butt fills up in a day with persistant rain, or a couple of days with light rain. With my garden expansion plans for next year I hope to have a larger capacity water butt installed and may even plumb the system into the house for flushing the toilets.

    Add a Hozelock water pump that gives the same pressure as a tap and I can water the entire garden and run a lawn sprinkler for free all summer long, I even wash the car with it sometimes – no streaks as the chalk content is minimal.


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