Dimensional shingle seems should be offset and staggard to prevent the shingles from leaking.  If they are lined up, the shingles can potentially leak per the shingle manufacturers.  Seems should have a 4 inch offset to prevent this and be lined up in a pyramid effect.  This is an example of an incorrect installation where the seems are lined up vertically.  This can cause the seems to leak into the attic below. It is normal for the older 3 tab asphalt seems to be lined up vertically and they do not leak.  Click here for illustrations of this from the Journal of Light Construction. For more information or to schedule an inspection, visit cornerstonehi.com.