What is measured to determine head lap in roofing?

Head lap in roofing is determined by measuring the distance of overlap between ply layers. This measurement is critical because it ensures that water runs off the roof appropriately and does not penetrate through the seams between the layers. Adequate head lap is essential for preventing leaks and maintaining the integrity of the roofing system, particularly in multi-layer applications.

By having a proper measurement of head lap, roofers can guarantee that the top layer effectively covers the underlying layer, directing water away from vulnerable spots. The overlap prevents water from seeping into the lower layers during heavy rain and is particularly important for built-up roofs and certain types of shingles where layering is used to enhance waterproofing.

The other options relate to different aspects of roofing and do not impact head lap measurement: the width of the roof material pertains to the overall size of the tiles or sheets, the height of the roof relates to its pitch or angle, and the length of the gutter deals with drainage rather than the overlap of the roofing layers.