What You Should Know About Shingle Roofing Systems

Asphalt roofing shingles are by far the most popular materials for residential roofs because they are relatively inexpensive, easy to install and come in a variety of colors and textures to suit most house designs.

The term asphalt shingles covers two different, but similar, products. Organic asphalt shingles have a paper or cardboard base, or mat, that is covered with asphalt for waterproofing and ceramic granules for protection from the sun. The other type has an inorganic fiberglass mat that supports the asphalt and granules. Fiberglass-based asphalt shingles have now outpaced the use of organic asphalt shingles.

The Shingle Roofing System

Asphalt shingle roofs are really a group of components with the shingles being the most visible part. Their goal is to help the roof shed water. Asphalt roofs need to be installed on a slope in order to work properly. A roof's slope is its vertical rise for every 12 inches of horizontal run. So if a roof rises 4 inches for every 12 inches of run, the slope is 4 in 12, sometimes written as 4:12. Shingle manufacturers specify the slope for their products, but usually, asphalt shingles work best on slopes of 4:12 and steeper. Roofs with less slope should be protected with different materials.

Asphalt roofs are installed starting at the eaves, the horizontal lowest level of the roof, and working up to the roof ridge—from the bottom up. That way every shingle is overlapped by the shingle above it to keep the water flowing down the roof and into the gutters.Here's a list of typical roofing components in the order they are usually installed.

Drip Edge

This L-shape piece of metal is attached to the edge of the roof deck along the eave. Its purpose is to protect the exposed edges of the roof deck, which is usually plywood or another sheathing material, and to help to direct water from the roof into the gutter.

Ice Dam Membrane

In cold or even temperate climates, snow that melts on the roof can refreeze once it hits the exposed gutter system. This dam forces the water behind it to work its way under the roof shingles. The ice dam membrane is a rubberized material that adheres tightly to the roof deck. It also seals any holes caused by nails from the shingles above. Building codes require the membrane be installed three to six feet up from the edge of the eaves. Ice dams are formed because of improper ventilation and insulation inside the attic and can be reduced or eliminated by improving ventilation, but the membrane on the roof deck is the best defense against water leaking into the house.

Valleys

Valleys are not so much roofing components as they are conditions. Where two roof sections meet at an angle, they form a valley. There are different ways to handle valleys, but most require a combination of metal flashing and waterproof membrane installed in the crease of the valley. The valley can be left open, without shingles; or closed, where the shingles are woven across the face of the valley.

Underlayment

The rest of the roof deck is covered with asphalt saturated felt, a material that comes in rolls that some people call tar paper. Starting at the lower portion of the roof, the underlayment material is rolled out horizontally. The edge of each roll overlaps the previous row by two or three inches. Roofers use staples or nails to attach the felt to the roof deck. When the underlayment is in place, it is good practice to install a drip edge along the rake of the roof—the edge that slopes upward. Nailing the drip edge over the underlayment protects it from wind uplift.

Carpenter uses nail gun
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A roofer nails up a course of shingles.

Starter Shingles

There are products called starter strips, but many roofers simply cut the tabs off of regular shingles and nail the remaining section to the eave edge of the roof, making sure the strip overhangs the edge slightly. The adhesive on the starter strip gives the first full row, or course, of shingles something to grab onto, and the strip fills the spaces under the openings and cutouts of the first full course. Some roofers like to install a starter strip along the rake because it provides a straight edge for lining up the shingles, and it aids in wind resistance.

Field Shingles

The rest of the roof is covered with shingles starting from the lower portion and working up to the ridge. There are different ways to lay out these field shingles, but the roofer will overlap the course below the one he is working on by a specific amount specified by the shingle manufacturer. This determines the shingles' exposure, the part you can see once the entire roof is installed. Openings or cutouts in the shingle will be placed over solid portions of the shingle below. Shingle manufacturers set the nailing schedule, but most roofers apply four nails for each shingle. Six nails are used in high wind areas. Shingles have an adhesive strip that bonds to the shingle installed above it. The sun helps melt the adhesive to create the bond.

Flashing

Anything that interrupts the field of shingles, such as a dormer, skylight or plumbing stack, needs flashing to divert water from the interruption onto the surface of the shingles. Flashings can be metal, a waterproof membrane, or sometimes an asphaltic mastic material. There are different methods used for different types of penetrations,but they are all important. In fact, if you have a roof leak, suspect the flashing first. It is usually the weak link in the system.

Roofer builder worker installing shingles on a new wooden roof
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A roofer measures a section of metal flashing.

Ridge Shingles

These are special shingles installed along the ridges of a roof. Often they cover a ventilation system that helps ventilate the attic.

A good roofing installation takes attention to detail to ensure its success. That's why it is important to hire a qualified roofing contractor for roofing repairs and installations.