Exterior of my house rendered and after 5 days the render developed hairline cracks. Builder says it's nothing to worry about, it's still sound. Could this be okay in the long run?

Hi one of two things could be wrong with it its either dried to quick or there is no scratch coat to prevent cracking only one coat applied either way there shouldnt be cracking

If its cracking after 5 days what will it be like after 5 weeks/months/years.If your builder is telling you it is fine,get him to back it up with a written guarantee of a minimum of 12 months.If he refuses dont pay him.

You shouldnt have cracking in your render after only 5days, or at all. Sounds like the render has dried out to quickly, high suction on the wall or to thin a coat, if the original wall was high suction, best to give it a coat of sbr slurry, then scratch coat it while still tacky with water proofer in mix. Second coat/float coat should be slightly weaker mix, if it was a hot day it needs to be dampened down. Best to get them back, he will tell you its nothing to worry about as you have probably paid him.

high suction and possibly to much cement in mix or wrong sand will create cracking...

Wrong sand and probably didn't use lime in the mix