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TaniaB
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I am trying to figure out what size eaves we need for our house extension.
I've tried a few calculations and now I'm posting here in the hope that a logical thinker will be able to tell me what I'm doing wrong.
We live in Adelaide, Australia, and we want to put in glass double-doors on a north-facing wall.
The glass doors are 2.7 metres high. The height from the floor to the underside of the eaves is 3.6 metres.
I've found 3 ways to calculate the desired eave length.
Method 1.
To use the sun angles for particular dates.
I've only been able to find 3 relevant sun angles - the elevation angles of the sun at a). the summer solstice (Dec 22), b). the equinoxes (March 21 and September 23), and c). the winter solstice (June 21).
The angles are a). 79deg (approx only, this is for Sydney), b.) 56deg (again, approx only, this is for Sydney) and c.) 31deg (apparently this is correct for Adelaide).
If we want to block the sun between the equinoxes, from Sept 23 until March 21, then the
eave length = 3.6/tan56 = 2.43 metres.
but this seems awfully long.
I can also calculate the penetration of winter sun if the eave was 2.43 metres, and that would be = ( 3.6 - (tan31*2.43) )/tan 31 = 3.56m
which = 3.56m
Method 2.
To use a rule of thumb a found on the internet.
This rule says that the eave length should equal 45% of the height from the bottom of the glass (in our case, the ground) to the underside of the eave
So this would be 45% of 3.6 = 1.62 metres.
This sounds closer, but I have no ideas what dates the sun would be blocked.
Method 3.
I found a chart used for calculating eave lengths here
http://www.concrete.net.au/publications/pdf/briefing09.pdf
The chart is Figure 15. It is towards the end.
I can calculate the eave length if we wanted to block the sun at 7 dates throughout the year.
If we look at blocking the sun between the equinoxes, the eaves length should be 2.52.
This matches closely with my calculations using sun angles, but again, it sounds huge.
I'm a bit stumped.
Thanks for reading this.
Tania
I've tried a few calculations and now I'm posting here in the hope that a logical thinker will be able to tell me what I'm doing wrong.
We live in Adelaide, Australia, and we want to put in glass double-doors on a north-facing wall.
The glass doors are 2.7 metres high. The height from the floor to the underside of the eaves is 3.6 metres.
I've found 3 ways to calculate the desired eave length.
Method 1.
To use the sun angles for particular dates.
I've only been able to find 3 relevant sun angles - the elevation angles of the sun at a). the summer solstice (Dec 22), b). the equinoxes (March 21 and September 23), and c). the winter solstice (June 21).
The angles are a). 79deg (approx only, this is for Sydney), b.) 56deg (again, approx only, this is for Sydney) and c.) 31deg (apparently this is correct for Adelaide).
If we want to block the sun between the equinoxes, from Sept 23 until March 21, then the
eave length = 3.6/tan56 = 2.43 metres.
but this seems awfully long.
I can also calculate the penetration of winter sun if the eave was 2.43 metres, and that would be = ( 3.6 - (tan31*2.43) )/tan 31 = 3.56m
which = 3.56m
Method 2.
To use a rule of thumb a found on the internet.
This rule says that the eave length should equal 45% of the height from the bottom of the glass (in our case, the ground) to the underside of the eave
So this would be 45% of 3.6 = 1.62 metres.
This sounds closer, but I have no ideas what dates the sun would be blocked.
Method 3.
I found a chart used for calculating eave lengths here
http://www.concrete.net.au/publications/pdf/briefing09.pdf
The chart is Figure 15. It is towards the end.
I can calculate the eave length if we wanted to block the sun at 7 dates throughout the year.
If we look at blocking the sun between the equinoxes, the eaves length should be 2.52.
This matches closely with my calculations using sun angles, but again, it sounds huge.
I'm a bit stumped.
Thanks for reading this.
Tania
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