overlaps a shingle next to it in the same course. The area that one shingle overlaps a shingle two courses below it is known as head lap. Head lap is measured from the bottom edge of an overlapping shingle to the nearest top edge  of  an  overlapped  shingle.  “Exposure”  is  the  area that is exposed (not overlapped) in a shingle. For the best protection against leakage, shingles (or shakes) should be applied only on roofs with a unit rise of 4 inches or more.  A  lesser  slope  creates  slower  water  runoff,  which increases  the  possibility  of  leakage  as  a  result  of windblown  rain  or  snow  being  driven  underneath  the butt ends of the shingles. Slope “Slope”  and  “pitch”  are  often  incorrectly  used synonymously when referring to the incline of a sloped roof. View B of figure 3-13 shows some common roof slopes with their corresponding roof pitches. “Slope” refers to the incline of a roof as a ratio of vertical rise to horizontal run. It is expressed sometimes as a fraction but typically as X-in-12; for example, a 4-in-12 slope for a roof that rises at the rate of 4 inches for each foot (12 inches) of run. The triangular symbol above  the  roof  in  figure  3-13,  view  B,  conveys  this information. Pitch “Pitch” is the incline of a roof as a ratio of the vertical rise to twice the horizontal run. It is expressed as a fraction. For example, if the rise of a roof is 4 feet and the run 12 feet, the roof is designated as having a pitch of 1/6 (4/24= 1/6). MATERIALS In completing roofing projects, you will be working with a number of different materials. In the following section,  we  will  discuss  the  most  common  types  of underlayments, flashing, roofing cements, and exterior materials  you  will  encounter.  We  will  also  talk  about built-up  roofing. Materials  used  for  pitched  roofs  include  shingles  of asphalt,  fiberglass,  and  wood.  Shingles  add  color, texture, and pattern to the roof surface. To shed water, all  shingles  are  applied  to  roof  surfaces  in  some overlapping fashion. They are suitable for any roof with enough slope to ensure good drainage. Tile and date are also  popular.  Sheet  materials,  such  as  roll  roofing, galvanized  steel,  aluminum,  copper,  and  tin,  are sometimes  used.  For  flat  or  low-pitched  roofs, composition or built-up roofing with a gravel topping or cap  sheet  are  frequent  combinations.  Built-up  roofing consists of a number of layers of asphalt-saturated felt mopped down with hot asphalt or tar. Metal roofs are sometimes used on flat decks of dormers, porches, or entryways. The  choice  of  materials  and  the  method  of application are influenced by cost, roof slope, expected service   life   of   the   roofing,   wind   resistance,   fire resistance,  and  local  climate.  Because  of  the  large amount  of  exposed  surface  of  pitched  roofs,  appearance is also important. Underlayments There are basically four types of underlayments you will be working with as a Builder: asphalt felt, organic, glass fiber, and tarred. Once the roof sheathing is in place, it is covered with an asphalt felt underpayment commonly called roofing felt. Roofing felt is asphalt-saturated and serves three basic purposes. First, it keeps the roof sheathing dry until the shingles can be applied. Second, after the shingles have been laid, it acts as a secondary barrier against wind-driven rain and snow. Finally, it also protects the shingles from any resinous materials, which could be released from the sheathing. Roofing felt is designated by the weight per square. As we mentioned earlier, a square is equal to 100 square feet and is the common unit to describe the amount of roofing material. Roofing felt is commonly available in rolls of 15 and 30 pounds per square. The rolls are usually 36 inches wide. A roll of 15-pound felt is 144 feet long, whereas a roll of 30-pound felt is 72 feet long. After you allow for a 2-inch top lap, a roll of 15-pound felt will cover 4 squares; a roll of 30-pound felt will cover  2  squares. Underpayment should be a material with low vapor resistance,  such  as  asphalt-saturated  felt.  Do  not  use materials,  such  as  coated  felts  or  laminated  waterproof papers,  which  act  as  a  vapor  barrier.  These  allow moisture   or   frost   to   accumulate   between   the underlayment  and  the  roof  sheathing.  Underlayment requirements for different kinds of shingles and various roof slopes are shown in table 3-5. Apply  the  underpayment  as  soon  as  the  roof sheathing  has  been  completed.  For  single  underpayment, start at the cave line with the 15-pound felt. Roll across 3-12