Building a sloped roof on a trapezoid shaped pergola?

[Angus!]

Hi all hope someone can help me with my calculations,

I'd sunk the posts for a pergola but as I've gone all grand designs I decided this shape would look good, especially with the polycarbonate tinted roof, however the conundrum is what is the minimum difference in height for a&b to c&d for the correct rain water run off?


Sorry for the terrible drawing.

Thanks for your help

 

[Notch7]

Is the fall perpendicular to the line A to C?

And what is the minimum pitch angle for your roof?

Is it a right angled trapezoid with A and C 90 degs (as viewed on plan).

 

[Angus!]

hi Notch

Thanks for your reply,

Yes fall is perpendicular,

Minimum angle is say 3 degrees, its just really the minimum for water to run off Plastic sheet roof I to the gutter,

Yes A and C are 90 degrees,

Thanks for your help

 

[big-all]

is this one single length off over 4m without a join ??
i would suggest if they say 3% the minimum will be nearer 10% to allow for the roof to sag a bit unless well supported over the 4m run

 

[scbk]

My way to do it would be fix a rail between C - D at whatever slope you want/looks right.

Then measure 0.8m out from C to give you your height for the other end (post B)


edit: oops the roof is running the other way

 

[Angus!]

You think 10%, I'm using 4x4 posts with 6m lengths cut down for 6x2 beams all round, then 4x2 rafters I'm having the rafters spaced at around 500mm with least amould of fixed sheets of 5mm polycarbonate for roof and small gutter length C & D , thanks for your help, the structure is free standing over my patio for a shade / rain sheild .

 

[big-all]

the simple fact is i dont know but you have to decide how much deflection is likely
you may be fine with say 6 or 8% but if you get a sag it means the first part may be 3 times as steep meaning the last bit will be perhaps half as steep
you may find fixing a molding or a small bead off mastic applied along the edges to direct the water to the end will help if the angle becomes too shallow
the main reason is to avoid water shedding to front or back but towards the end

 

[tony1851]

I was away when we did maths.

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[^woody^]

I was away when we did maths.

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Locked away?

 

[Tigercubrider]

Being picky, it isn't a pergola if it has a roof

 

[big-all]

I was away when we did maths.

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If you think my posts are helpful, please hit the 'thanks' button.

if you are referring to this comment
"the first part may be 3 times as steep meaning the last bit will be perhaps half as steep"
and would look better as 3 times as steep meaning the last bit will be perhaps 1/3 as steep
whilst the comment does seem contradictory its all approximate "guesses" and as it is likely to be an arc between the shallowest and steepest point the angle will be constantly changing around the transition