Simply put a roof must shed water it does this through proper installation of multiple materials not just shingles. Below we listed some common materials that might be needed for a standard re-roof job.  It is important to recognize that what is under the shingles effects the total performance of the roof as much as the shingles themselves.

Shingles
Hip & Ridge Shingles
Starter Strip
Felt
Ice & Water Shield
Drip Edge
Gutter Apron
Galvanized steel sheet metal
4x4 frontwall flashing
4x4 sidewall or Step flashing
PVC pipe boots
1 ¼” Nails
Roof Louver / Ridge Vents
Roofing Cement
Polymer sealant

Ice and water shield is a rubber like membrane that may be installed at the eaves (lower edge) and in valleys (two slopes meeting).  It is installed under the shingles and sticks to the wood surface of the roof.  Its purpose is to add extra protection and seal the nails that penetrate the roof.  In winter as snow melts off the roof it tends to refreeze at the eaves (lower edges) because they are colder. When water freezes at the edge of the roof it tends to push up and under the shingles, ice and water shield helps prevent damage from this.  Adding ice and water shield to your roof is added insurance to prevent leaks. Aten Construction will install this on every roofing job as part of our standard installation service.

Flashing is made to flash water where two components come together. There are many different metal flashings that can be used on a roof here are the most common.  
D-Style (drip edge) - This flashing come in 10’ sections and is installed up the gable or “rake” edges of the roof.
Gutter Apron - This flashing also comes in 10’ sections and is installed at the bottom edge or “eaves” of the roof.  Where the gutter is tucked under.
Step Flashing - This is flashing that flashes shingles as they go along side walls and are in the shape of an “L”