Hello i'm Azdaroth and i'm going to tell you how i play my jayce.
This bluid is for TOP only, don't try to do this build for a jayce ADC for exemple.
I'm not top elo but trust me, i never loose a lane with this build.

And please don't blame me for my bad english i'm french.

With jayce i take classic AD runes, armor magic resist physical damages.
This runes will give you nice early AD and will make you able to harass the enemy this nice damages with the E + Q combo.

For this jayce i make 9/21/0 runes, 9 in attack cause you just need the armor penetration to deal noce damages with jayce and 21 in defence cause i play my jayce verry tanky, to able to tower dive early and go in the team fight to attack the ADC enemy.

In early game don't try to kill just farm ( verry important ) and harass the enemy with auto attack or E + Q combo to make him back. By doing this you will be able to outfarm you enemy and take adventage on him.
Try to don't back before you have enouth gold to buy your boots and your phage.
Once you get this items you will be able to take a kill, keep farming and harassing the enemy and when you think it's time do E + Q combo in cannon then press R and rush him with Q, use your W and run after him doing hit and run ( Don't use your E in hammer if you're not sure to kill him with, otherwise you'll just bump him and make him safe ). If it's not enouth to take the kill press R again to use mercury cannon and try to finish with E + Q combo.

I hope for you that you'll have at least your trinity force and frozen heart before the team fight.
If you have those, you're role will be to harass the enemy team with the combo E + Q then jump in the melee to attack the ADC you can also try to bump him to your team with your E ( with hammer ).
Once the adc is dead, just do the same thing to the AP.

Well this is the end guys, i told you all about how to play this jayce.
i'll update if i found new op build for him or if there is any change on jayce during the next patches.