Roof Installation
Typically, a roof is put together pretty much like the floor or walls of your house. It has a framework of rafters to support a roof deck (sometimes called a "subroof"), which consists of sheathing and underlayment; the roof deck, in turn, provides a nailing base for the roof surface.
The type of roof deck a house has depends primarily on the nature of the roof surface material, but most decks have both sheathing and underlayment.
Sheathing is the material that provides the nailing base for the roof surface. Sloping roofs with asphalt shingles usually have solid plywood sheathing-4 by 8- foot panels nailed directly to the rafters (though in older homes you may find 1 by 6 or 1 by 8-inch boards): Homes with open-beam construction may reveal fiberboard sheathing that has been manufactured so one side forms the nailing base for the roof, the other the finished ceiling of the house.
Roofs of thin wood shingles or thick wood shakes are often laid over "open" sheathing 1 by 4-inch lumber spaced evenly over the rafters to permit air circulation. In some instances, particularly where wood shingles or shakes are put down over an existing roof, you'll see horizontal batten boards nailed directly onto the old roof surface.
Sandwiched between the sheathing and the roof surface is the underlayment, usually roofing felt. Roofing felt is a thick, fibrous black paper made from wood chips and recycled paper, then saturated with asphalt. It is thick enough to resist water penetration from the outside, thin enough to allow moisture from within the house to escape.
To produce a more fire-resistant roof system, some wood shake roofs have underlayments of both roofing felt and metal sheeting. Where extra waterproofing is necessary, such as in hurricane zones of the Southeast, tile roofs often have underlayments made with built-up layers of roofing felt and hot-mopped asphalt.
Most sloping roofs are covered with overlapping layers of asphalt shingles, wood shingles or shakes, or tile, though the possibilities range from sod at one extreme to molded plastics at the other.
Flat - built-up - roofs are most frequently covered with alternating layers of roofing felt and asphalt, with a layer of gravel on top. In some cases, though, they're surfaced with an insulating polyurethane foam that's sprayed on and painted with a protective coating.
The principle underlying the function of the shingle roof is simple: to shed a drop of water that falls on a sloping roof by drawing the drop gradually down the side, over layer upon layer of lapped material, until it falls to the ground.
Wherever water is likely to collect and penetrate the roof surface - along the joints around a chimney, at the edges of roof vents, or in the "valleys" where two roof planes meet at an angle - at all those places, protective flashing is necessary.
Made of malleable metal or plastic, flashing appears as the drip edge collars around ventilation pipes, and the "steps" along the chimney. Less obvious flashing also protects the points where solar panels and television antennas are connected to the roof.
The slope, or pitch, of a roof refers to the vertical rise measured against a standard horizontal distance of 12 inches. The term "4 in 12," applied to a roof, tells you that the roof rises vertically 4 inches for every 12 horizontal inches. Very low-sloped roofs measure only 1 in 12 or 2 in 12; steeply sloped roofs range from 12 in 12 (a 45 degree angle) up to 20 in 12. Ordinarily, home crafts persons should be able to work safely on roofs with slopes no steeper than 6 in 12.
Determining roof slope. To find out the slope of your roof, all you need is a carpenter's level, a metal tape measure, and a flat piece of wood. Working either on the roof or from a ladder at the house eave, form a right triangle with the level, board, and tape measure; the tape and level should intersect 12 inches from the point where level and board meet. Then measure the vertical distance face to the bottom of the level. The number of inches, followed by "in 12," gives you the slope.