|
How To Measure Your Roof's Pitch And Square Footage
In the attic, place the level against a rafter with a 12-inch mark on the bottom of the rafter. Measure from the end of the level up to the bottom edge of the rafter. That is the roof's rise.
How To Calculate Square Feet:
If your roof is flat or no more than a 3-in-12 pitch, you can calculate its total square footage by simply multiplying length times width.
Measure your house at ground level, then add in the roof's overhang for more accurate dimensions. As an example, if the roof overhang is 12 inches, that would add 2 feet to the overall length and 2 feet to the overall width of the house. This nominal square footage amount must then be multiplied by a factor associated with the roof's pitch.
Roof Sheathing: The skeleton of a roof is formed with rafters or trusses, generally spaced 16 inches or 24 inches on center (from the center of one rafter to the center of the next) apart. To stabilize the rafters and support the roofing material, rafters are covered with sheathing, or decking.
Common Roof Sheathing Materials Include The Following:
Plywood: Comes in standard 4 x 8-foot sheets, is strong, durable, and relatively light. It also holds roofing nails well. The common grade of plywood for roof sheathing is CDX. As a quick primer in plywood, each side is rated from A to D. A is smooth and knot-free while D, the lowest quality, has missing knots and cracks. One side is usually better quality than the other—C in this case. It should be placed up on the roof. X stands for exterior use. For rafters spaced 16 inches on center, 1/2 inch plywood is standard. For rafters spaced 24 inches on center, 5/8 inch plywood is preferred to minimize sagging.
OSB: Oriented Strand board: Is made up of multiple layers of wood strands compressed at odd angles to each other and bound with water-resistant adhesives. It comes in 4 x 8 foot panels, as does plywood, and is commonly used interchangeably with plywood, such as for roof decking. It is a little cheaper than plywood, which is why it is popular on housing tracts as a cost-saving measure. But many roofers believe that it is not as strong as plywood and in particular does not have equal nail-holding power. If using OSB, note that one side has a slip-resistant coating that should be placed facing up.
Tongue And Groove 2 x 6: These boards are commonly used as roof sheathing where a ceiling will not be installed and they will be visible from inside the house, such as across beams in a vaulted ceiling. Wood is an excellent insulator and in moderate climates no additional rigid insulation on the roof is necessary. The boards can be painted, stained, or coated with polyurethane to retain their natural color.
Step Sheathing: Until recently step, skip or spaced sheathing was commonly used for shingle, shake or tile roofs. The boards were normally 1 x 6 or 2 x 6. The spacing was to permit air to circulate under the shakes and shingles, particularly shingles. Tiles were designed so that lugs, or short protrusions, on the top backside of each tile would hook over the step sheathing. More recently, however, most roofers recommend solid sheathing on the roof to give the house additional resistance to movement, or shear strength. Step sheathing can then be applied over the solid sheathing as needed. Shingles, which must permit air to circulate under them, are now often placed on a special membrane that permits circulation.
V-rustic: These 1 x 8 boards are not roof sheathing but are commonly used on the eave and rake overhangs when soffits will not be installed. The boards are more attractive when viewed from underneath than rough plywood. The 5/8 inch thick boards extend from the eaves just past the exterior walls, where they butt up against the plywood roof sheathing. Roof material then covers all.
Flashing: Which comes in a variety of materials and shapes, is used to prevent water from working it’s way under the roofing material in specific areas, such as valleys, around chimneys, vent pipes, skylights, and where roof planes change. Primary types of flashing include the following:
Valley Flashing: Used in an open valley, this type is usually the most visible flashing on a roof and is often a source of leaks. The most common valley flashing is called W-flashing for its raised center to prevent water from sloshing back and forth and under shingles. It comes in 10-foot lengths and ranges from 16 x 24 inches wide. Problems can develop in regions of extreme temperature variations if the valley flashing is nailed so tightly to the roofing deck that it cannot expand without buckling. In moderate climates this is not a problem. Some roofers nail only the top edge of the flashing, then apply tabs over the sides that allow it to expand and contract without buckling. Shingles that overlap the edges of the flashing must be trimmed back far enough that runoff water will not be constricted and thus diverted under the shingles.
Plumbing Vent Flashing: Plumbing vent pipes can penetrate all roofs and must be flashed to prevent rain from running down between the pipe and the opening cut in the roof. The most common type is a square of galvanized tin with a metal cone in the middle that is rimmed with a rubber gasket. This type of flashing is sold by the size of the vent pipe and the angle of the roof. The cone, which is angled according to the approximate pitch of your roof, slips over the vent pipe and the rubber gasket seals around it. Roofing material is then applied over the flashing.
Lead Flashing: Is most commonly used on tile roofs, both concrete and clay, where the flashing must be formed around the tile. One example is plumbing vent flashing. First, a hole is cut in the tile that fits over the pipe and that tile is fitted in place. The hole is filled with concrete around the pipe. Next, the flashing and cone are slipped over the pipe and the soft lead flashing is molded to fit the tile's surface shape. The next course of tile covers the top edge of the flashing so water cannot get beneath it.
Step Flashing: Step flashing is standard where the flashing must be carried up the roof, like a series of steps. This type is regularly used next to chimneys or beside dormer walls that intersect with a sloping roof. The step flashing is aluminum or galvanized tin about 6 or 8 inches wide and a foot long. It is bent in the middle so that one end lies flat on the roof and the other fits snugly against the vertical wall and is covered by the siding. For chimney flashing, another piece of flashing called counter flashing, is also used. The top of the counter flashing is embedded in the mortar between the bricks and the remainder is bent down over the step flashing
|
 |
| Rafter Pitch |
|
|
