17 November 2009
The hip roof is an extremely common type of roof which slopes down to meet the walls on all four sides of a building. Essentially, it's a roof with sloped, rather than vertical, ends. Clearly, for this to occur each pair of adjacent sides must meet. These intersections are known as hips, and are covered by special hip shingles.
The size of the flat roof is defined by the primary hip truss, which is set back from the walls by a specified distance, often a distance of seven feet. This primary hip truss forms an elongated letter A. From the upper corners of this primary truss to the edge of the roof a downward-sloping shape is formed from hip rafters. These unbroken members intersect the 90 degree corners of the building, forming two sections of 45 degrees.
Having defined the basic structure of a hip roof, we are now in a position to understand the function of the hip jack. At some point our roof must meet the edges of the building - the rafter plate. Simply put, any part of the roof frame which travels from the hip to the rafter plate is known as a hip jack. At the wall end these rafters are cut perpendicular to the wall, and meet with the hip itself with a 45 degree cut.
For comparison, the hip jack's counterpart, the valley jack, connects the roof ridge to the valley rafter in a similar fashion while the cripple jack, on the other hand, is placed between valley and hip - essential for joining up those awkward non-rectangular roofs.