Why are shade sail posts often installed out of plumb ?

[Blr243]

I have to install two of them in a bare yard for a customer not useing house at all strictly posts. I watched an instructional video and they said if useing steel posts install them ten degrees out of plumb. Stupid. Why should I not just install them plumb?

[bladeracer]

I have to install two of them in a bare yard for a customer not useing house at all strictly posts. I watched an instructional video and they said if useing steel posts install them ten degrees out of plumb. Stupid. Why should I not just install them plumb?

I would assume so they don't pull in due to the weight of the sail.

[TassieTiger]

You know the direction of strain so installing them on a 10 degree angle, opposing the direction of pull makes the sail more secure over time - you have built in tension adjustment on sail - twist tie.
I’ve installed a few over the years and you would be extremely surprised how much force can be induced by wind / shade sail...

[Stix]

Id hazzard a guess its to do with a combination of the tension they're under as well as uplift...

The posts would be under varying degrees of tension in windy conditions, right through to no tension, & would be nearly impossible to pull out vertically (from uplift) if set in ground at an angle.

Whereas if set plumb & having suffered the effects of that tension over years (effectively "loosening" the footings hold to ground), the resistance against uplift would be significantly less than if set at an angle...

Just a guess though...im not an engineer...

And ive never put any in...lol

Actually...reading it all back...i think thats what Tassie said too...

[flutch]

yeah set them in plumb if you want, go right ahead, but in my experience i have always installed them closer to 13* off plumb... depending on the size of the sail and the strength of winds in that area, you would need to put very substantial footings on those posts the other issue is if you dont put them over at least a bit off plumb when you adjust the tension/turnbuckles you will end up off plumb in the opposite direction anyway. In any case I think youre better off installing them as instructed.

[Die Judicii]

What has NOT been mentioned is that presuming the sail is square or rectangular,, two posts should be higher than the others.

Put two extra high ones diagonally opposite each other, and the two shorter ones diagonally opposite each other.

Then when the sail is pulled up to tension it won't hold leaves or water in the middle of the sail.

(of course putting them in "off plumb" is still required)

[Blr243]

I was aware of the PUT THEM IN OUT OF PLUMB SO IN TIME THEY CAN NEVER BE PULLED OUT OF PLUMB LEANING INWARDS SCENARIO, But I just thought 10 degrees was excessive ie ten degrees is approx 400 mm out of plumb over 2400 mm height. But after reading all your posts I have become aware of things a bit more Thank you for the input , very useful info you have supplied. I can draw pics for my customer prior to installation to ensure she is content with the finished product ... then I am sure to get paid And I can use that money to go pig and fox hunting again. life goes on