When reshingling a roof, shingle nails need to be placed appropriately in order to keep elements out and the shingles on.
Standard roof installation requires four nails per shingle. The rationale behind the number is that in most cases four nails are the right number of nails to do a quality job. However, placement of the nails is the most important in any roofing job. A "nail line" is indicated on the shingles and locates the correct places to put nails. Nails placed above the nail line endanger the shingle of being ripped off during high winds or extreme weather.
Steep Slope or High Wind
Very steep slopes, also called Mansard roofs, as well as areas with extremely high winds, require six nails per shingle to allow enough hold against the elements. The extreme slope means that not only is there a larger pull from gravity on each shingle, but also substances are going to flow faster across the surface. The added friction requires more nails in order to adequately hold the shingle in place. There is no reason six nails cannot be used on a standard roof, but with the extra money and time necessary to add the extra nails, it is often considered unnecessary.