How high can I install an aluminium sign?

No reason at all you can't install aluminum that high. Skyscrapers have aluminum on them. As long as it's secure to the wall, why not? I'm not quite following your terminology with 'stretched' and 'coach screws' though..

 

Yea, the lingo isn't familiar, but no reason a sign that is constructed and installed correctly can't go at any height

 

the location tag list South Africa, so I am assuming it is a language difference
http://blog.mutualscrew.com/2015/07/07/what-are-coach-screws/

the mention of stretched PVC, makes me think of banner.

as mentioned the frame MUST BE SECURE
if you are using a "banner" material again make sure it is secure, also make sure the wind cannot get behind the material creating a sail with wind drag

 

Just curious what the rationale was for not putting aluminum at a certain height. Is it that the air is too thin and the aluminum gets hypoxic?

 

6m is only about 20' so putting a aluminum frame with a banner on the wall would not be a problem.

 

coach screws. lol.
that's a first for me, but my first guess was that it was another term for Carriage Screw/Bolt. ...and I was right!

 

When I first saw coach screw my mind went to Roy Rogers and Dale Evens getting it on in the stagecoach, or Matt Dillon and Kitty.

 

Lag screw

Lag screws, also called lag bolts
Lag screws (US) or coach screws (UK, Australia, and New Zealand) (also referred to as lag bolts or coach bolts,

 

How high can an aluminum sign be? How many gummies can it eat? Then you'll get your answer.

 

Beat me to it!
Though I was going to recommend avoiding installing aluminum signs if you're high enough to see sounds and taste colors...

I can see Cheech and Chong working just like my guys on a bucket truck. One yelling from the ground, 'It's me, Dave", the other answering without looking, "Dave's not here man..."

 

I'm surprised at the amount of ppl here that do that stuff...then again they are "sign artists" and artists have been known to be a little dopey.