The roofers lead (leed) is the pattern of shingles that the roofer lays down to form the start of each course. This relates mostly to asphalt shingles, the most common variety being the 3 tab, although it works for the heavier architectural grade. The wrapper on the shingle bundle has instructions for laying 3 tab shingles, but the pattern produced by the alignment of the spaces between the tabs and the ends of the shingles produce an unsatisfactory finished product unless considerable care is give to align each course with the one below. That pattern has each course offset to the one below by a half a tab's width. The effect is that in every other course the slots line up and the visual effect is lines straight up the roof from eave to ridge. If one course is misaligned it stands out like a sore thumb and even small variations of butting adjoining shingles in a row will alter the spacing and hence the pattern produced. Pros call that lead the homeowners lead.

Since time is money in construction, a pro roofer wants to attach the most amount of material with the least amount of moving around on the roof. A pro roofer will start the shingle rows so that the alignment is offset to the row below by 1/3 of a tab. That doesn't sound like much difference, but the effect is that now the roof surface appears to have diagional lines going from eave to ridge, and small variations in the spacing don't show up near as much. Also with the smaller offset the roofer can do more work each time he repositions himself, therefore working faster. Check it out for yourself, look up at some roofs. There is also roofers lead (led), but that's another node.