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- 8 Sep 2004
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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
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