Understanding headlap

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Author Message   Old_school (Old_school) Senior Member Username: Old_school Post Number: 563 Registered: 01-2009 Posted on Tuesday, December 21, 2010 - 09:20 pm:        The headlap is just at the head of the slate, compard to the top of the slate beneath it. Cutting scallops or diamonds on the bottoms of the slate doesn't affect the headlap; as long as you don't cut anything off from the bottom of the slate. If you are doing that however, you will want to lay the slates straight and not random so that the side laps don't leak from being too close together.   Johann (Johann) New member Username: Johann Post Number: 1 Registered: 12-2010 Posted on Tuesday, December 21, 2010 - 04:12 pm:        Headlap - Do I need to increase the length of the slate or reduce the exposure to maintain a 3" headlap if the butt of the slate is "pointed" or "deep scallop" instead of square?   Bud (Bud) Intermediate Member Username: Bud Post Number: 33 Registered: 02-2010 Posted on Tuesday, November 02, 2010 - 09:50 am:        That answers my next question about the geese (love the sarcasm). Just thought the 2 inch headlap was small when compared to what is needed for slate and other such materials.   Old_school (Old_school) Senior Member Username: Old_school Post Number: 537 Registered: 01-2009 Posted on Monday, November 01, 2010 - 09:46 pm:        Because of the 2" headlap! The same reason that one side of the "v" you see when geese are flying is longer than the other side...There are more geese on that side!   Bud (Bud) Intermediate Member Username: Bud Post Number: 32 Registered: 02-2010 Posted on Monday, November 01, 2010 - 04:29 pm:        Okay, here is what has been running through my head lately... If I have a 20 inch slate installed at 8 inches of exposure I will have a head lap of 4 inches. This head lap is viable at slopes as low as 6/12 (maybe lower?). If I have that correct then here is what I am wondering about. An asphalt shingle with a full height of 12 inches and an exposure of 5 inches gives me a headlap of 2 inches. Which is acceptable for application to a pitch as low as 3/12. A 2 inch headlap on slate is good if used at very steep pitches as I understand however 3 inches would be better. The capillary action of water is 2 inches, which further compounds this concern. So why don't asphalt shingles leak?