I would like to know if I can make a complaint to the developer regarding the design of the roof. The roof dont have a proper rain water piping and causing the rain water to fall directly to the ground instead to longkang. This causing a very bad erosion to the ground just beside the drain.

Clearly this is a design flaw but can i ask them to built a proper piping?
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My suggestion is put it down in your defect list and see if your developer will willing remedy

If they won't do it, demand a written explenation why?

After which if the developers reply is not to your satisfaction you could try using the stakeholders sum to rectify your erosion problem 

Witholding Stakeholders Sum
http://forum.lowyat.net/index.php?showtopic=423171&hl=

I don't guarantee any of these measures will work but do give it a try... any other forum members with better ideas or advice please help?? 
Thanks mate...