Help with Calculating Required Gutter Capacity

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Hey there. Wondering if you have any rules of thumb / methods of determining required gutter size for a lower smaller roof that has to cope with the rainwater from a higher larger roof as well as its own?

My main roof (one 'pitch') is about 33sqm, being 10.5m wide. It has standard 112mm half round guttering so that should be alright. I believe a 100mm gutter meets the regs for this roof area(?).

However the downpipe serving this roof takes the rainwater onto a smaller, lower pitched roof of around 10.5sqm.

If the lower roof was the same width as the higher roof, I believe I could simply add the 10.5sqm to the 33sqm to arrive at an effective roof area of 43.5sqm. From a table I have seen from the building regs this would suggest I needed guttering on the lower roof of 115mm diameter. Right? (The table in question does not seem to have an entry for the max roof area to be supported by 112mm guttering, but there you go - anyone know?)

However the lower roof is an extension and only 4m wide. So I am guessing I cannot use this simple sum to determine the required capacity of the lower guttering. My schoolboy math says I need to take into account the reduced length of guttering on the lower roof something along the lines of the following:-

( width of upper roof / width of lower roof ) * recommended guttering diameter for total area.

( 10.5 / 4 ) * 115mm = 302mm

This would suggest I needed a gully with the same capacity of a half round 30cm diameter pipe.

In other words, using more basic math, a gully with a cross sectional area of 3.14 * 15cm * 15cm ("pie r squared"!) divided by 2 = 354sqcm

This is around 19cm by 19cm gully which, if I could not buy, I could construct myself and line with lead or a polymer of some sort (I have seen another post on repairing an existing gully).

Does the above sound right, or is it (i) simpler than this or is it (ii) more complicated than this, requiring me to calculate flow rate more precisely?

Your help would be much appreciated, thanks :D
 
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Thanks guys. I am inching forward but I am still unsure of one thing.

If I look at required flow capacity using the 75mm/hr constant, the required flow capacity of the main roof is 0.75litres/sec. The capacity of the lower extension roof onto which the main roof currently drains is 0.35litres/sec. If I add them together I get 1.1litres/sec.

So I could look for a rainwater gutter/pipe for the lower extension roof of 1.1litres/sec. IanB's link suggests a standard 112mm half round gutter would cover it.

However the crux of my question is this. When the lower gutter length (on my extension) is significantly less than the higher gutter (on my main roof) which also drains into it, do I have to increase the capacity of the lower gutter or not??

I kinda understand that as long as the capacity of the downpipe into the gully from the lower roof can handle 1.1litres/sec (no big deal), then it won't back up in the pipe. But I cannot help feeling the short length of gutter might overflow if it is not increased.

The cost of gutter, pipe and particularly fittings all adds up. I would like to avoid throwing away my money when the guttering system can't cope!

:confused:
 
purplechap said:
However the crux of my question is this. When the lower gutter length (on my extension) is significantly less than the higher gutter (on my main roof) which also drains into it, do I have to increase the capacity of the lower gutter or not??

The man from Geberit (www.geberit.co.uk) says NO!

Length of guttering does not appear to be part of the equation after all when it comes to determining flow capacity. The guttering is just there to transfer the water collected from the roof to the all important downpipe, regardless of the length of the roof/guttering, it does not increase flow capacity in itself (but obviously it must not be less than required). So it turns out I got my knickers in a twist after all and things are a bit simpler than I had feared!

For instance, an effective roof area of 52sqm could be (4m x 13m) or (7.2m x 7.2m) or a couple of roofs like mine: main roof (10m x 3.6m) dropping onto a extension roof (4m x 4m). This is about 1.1 litres/sec and I just need to ensure my downpipe and gutter combination supports this flow capacity, regardless of the length of pipe and guttering used to collect the water (give or take the odd adjustment factor for long runs).

Taking it further, if a roof was a strange triangular shape that fed all of its water directly into the downpipe, then we just need a downpipe capacity to deal with the given N litres/second that the roof sheds into the pipe. The guttering to match the downpipe is then chosen, which must also take into account any fall on the guttering and the position of the downpipe(s).

In my case a pretty standard 68mm downpipe should handle 1.1 litres/second. Because of my unusual arrangement I am going for a larger rather than smaller round gutter (eg see Geberit's 2400 rapidflow).

This seems to reflect the "gut feel" of breezer and masona, so thanks guys. However I now know why I am about to do what I am going to do! If I got the theory wrong I'll let you know after the next storm!

PS: Geberit also have an installation and product selection guide some of you might find useful
http://www.geberit.co.uk/gb/Webgb.nsf/files/pdf-GTRAININST.pdf/$file/GTRAININST.pdf
 
Sometime if there's a heavy down pour the rain come over the gutter on the back of my extension, so I put in another down pipe on the other side and the gutter fall from the middle to take the rain away both side, haven't has any problem yet so far !
 
I guess adding another downpipe essentially doubles the flow capacity of that section. So right on.

If I have any problems I guess I could add another downpipe somewhere too. Thanks.
 

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