Maximum space between ballisters is 4" according to residential code. The problems we've seen with shingle roofs is how there are nailed is the biggest problem.
either not having the proper number of nails, six required on three tab shingles, nailed in the wrong locations, either too high or low, the bundles show the proper location and the biggest problem recently is improper adjust of air pressure with power nailers causing over tears in the shingles. We also are recommending sealing down the last row of shingles with a construction adhesive.