Thread: Calculating the area of a roof

Originally Posted by LandyAndy

If you want to know how much rain you collect off the roof the conversion factor is 1L of rainwater from every 1m2 of roof per mm of rainfall.
So a 1000m2 roof will collect 1000L water from a 1mm shower if my sums are correct.
Andrew

Spot on Andy

Cheers

Originally Posted by Meken

Be mindful that the limiting factor in heavy rain may be your downpipes. ......

That is more or less what I meant by "Depending on how the downpipes are plumbed". Also, in very light rain, depending on the roof material, there may be a lot lost by absorption into the roof material (or the lichen/mould, leaves etc there and in the gutters) and subsequent evaporation.

John

Originally Posted by DiscoMick

You could ask if your local council can supply your house plan. Or just guess...

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Thanks - I did think of that but they don't have anything pre 1986 and this was built 1980. I'll get the measure out.
cheers,
D

If you have a first flush device dont forget to take that into account. It should be sized to take the first 25L of every 100 square metres (ie for a 300 square metre roof your first flush would be 75L in size. This is to remove pollutants from the water that makes it to your tank.

Also keep in mind that gutters and downpipes are usually designed for a 5% storm (1 in 20yr ARI in the old way). Thats a storm that has a 5% chance to occur once a year. If the downpipes catch more water than that its a bonus (box gutters hould be designed for the 1% [1 in 100yr] storm)

If you want to find the design rainfall for your area you can use the tool at
http://www.bom.gov.au/hydro/has/cdir...dirswebx.shtml (you will need flash installed)
You will need the latitude and longitude of your site.

As far as area goes just the plan area of the roof is used, with typically about 5% loss to account for absorption into the materials and whatnot.

Cheers

Dan

Originally Posted by Disco-tastic

If you have a first flush device dont forget to take that into account. It should be sized to take the first 25L of every 100 square metres (ie for a 300 square metre roof your first flush would be 75L in size. This is to remove pollutants from the water that makes it to your tank.

Also keep in mind that gutters and downpipes are usually designed for a 5% storm (1 in 20yr ARI in the old way). Thats a storm that has a 5% chance to occur once a year. If the downpipes catch more water than that its a bonus (box gutters hould be designed for the 1% [1 in 100yr] storm)

If you want to find the design rainfall for your area you can use the tool at
http://www.bom.gov.au/hydro/has/cdir...dirswebx.shtml (you will need flash installed)
You will need the latitude and longitude of your site.

As far as area goes just the plan area of the roof is used, with typically about 5% loss to account for absorption into the materials and whatnot.

Cheers

Dan

But the design rainfall rarely occurs - and is only for a 5min duration for design purposes - so in brisvegas it's 251mm/hr for a 1/100 storm but you will rarely see a storm like that and if you do it's probably only for max 10mins so you would be looking at a bit over 40mm of actual fall

Originally Posted by Meken

But the design rainfall rarely occurs - and is only for a 5min duration for design purposes - so in brisvegas it's 251mm/hr for a 1/100 storm but you will rarely see a storm like that and if you do it's probably only for max 10mins so you would be looking at a bit over 40mm of actual fall

The design storms go up to something like 72 hours. The smallest is 5 min and, if you can be bothered, you can calculate the rainfall for each storm up to 72hrs, at minute intervals.

For example the 1/100yr 5min storm might be 251mm/hr, but the 1/100yr 60min (1hr) storm might be 140mm/hr. Higher intensity, less rain in the 5 min, lower intensity, but more rainfall in the second storm.

The page i linked to earlier has a summary table that has about 15 storm durations for each design period.

If you're designing a typical domestic system (up to 10kL) in heavy rainfall it will probably be full after 5-10 mins anyway.

Cheers

Dan

If you wanna know exact area use a trigonometry calculator app on your phone. All you need to input is 2 numbers. The width of your roof and the pitch (angle) it will calculate the rest.

Is it a Skillion roof?

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I should mention then u multiply that number the calculator gave u by your shed length to get the total

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