Anatomy of a Roof
Ever wonder what's underneath the shingles on your roof? A well built roof should have rafters and decking boards, flashing and a moisture barrier of felt underlayment.
Here's a breakdown of all of the parts that go into a residential roof, some of which can no longer be seen when the job is done.
What's under roofing shingles
On the surface, a roof looks like a simple layering of shingles, but it's actually much more complex than the average homeowner may realize. The roof is a "system" composed of layers of different materials that work together to keep wind and weather out of your home. Each layer has a specific purpose that, when combined with the other components, provides protection to your home:
Decking (Wood Sheathing)
The decking is the base on which everything else is laid. The decking material varies, but most homes have OSB (oriented strand board) or plywood, but other materials, such as polystyrene, concrete and metal decking is also used. The decking is attached with nails onto the rafters, creating the foundation of the roof system.
Underlayment
Underlayment is installed onto the decking, before the installation of the asphalt shingles. The most common type used today is felt, a durable paper which is saturated with asphalt. It provides a temporary barrier against rain and wind before the shingles are installed and provides an extra layer of protection in the case of broken or torn shingles. The underlayment is rolled onto the roof horizontally. The installation begins at the bottom edge of the roof, and each layer of underlayment is rolled with a significant overlap, which helps water to shed downward without seeping through the seams.
Drip Edge
The drip edge (also called eave flashing) consists of angled aluminum strips nailed into place over the underlayment on the sides of the roof at the edges and beneath the underlayment at the eaves. The drip edge prevents water from building up, guiding it over the eaves and into the gutters.
The shingles, whether they are asphalt, metal, clay or other material, make up the "outer shell" of your home's roofing system. This system, when installed and working properly, provides your home with the protection it needs to withstand all but the worst weather.
Flashing along the base of a chimney where it meets with the roofing prevents water leakage and extends the overall life of the roof. (Photo courtesy of Brian Robinson)
The purpose of roof flashing
Since moisture can be a roof's worst enemy, roof flashing is intended to prevent water leakage and extend the overall life of the roof. Although it sounds high-tech, roof flashing is nothing more than a piece of sheet metal that is usually made from aluminum or galvanized steel. It is often strategically placed over joints in the roof and wall construction in an attempt to further prevent water from entering the home. This is especially important considering that one of the main causes of roof leaks is a fault in the roof flashing.
Roof flashing is based on a simple engineering concept that relies largely on the force of gravity. Assuming the roof flashing has been properly installed, gravity will help the flashing material direct water down the roof and then to the ground. Most roofs have roof flashing around the chimney, plumbing vent stack, fan vent, or other item that sticks out from the roof.
Although there are some environmental concerns about the bulk use of heavy metal required for roof flashing, there are several qualities about the material that are environmentally-friendly. According to the Lead Sheet Association, lead roof flashing is both recyclable and extremely durable with a lifespan of 500 years. Considering the fact that many modern materials have a lifespan of up to 20 years, roof flashing appears to be an environmentally-efficient selection.
For the most effective use of roof flashing, the material must be weather resistant, highly durable, low maintenance and sturdy. The latter quality is especially important because many roofing materials regularly expand and contract on a daily basis, and more dramatically during seasonal weather changes. During the day, the sunlight can raise the surface temperature of the roof, which results in expansion. However, this temperature drops significantly at night, which results in a contraction of the materials. More significant expansions and contractions during seasonal changes can have an effect on the framing lumber, which means that roof flashing must withstand all of these changes to effectively prevent leaks.
Because roof flashing can often endure several stresses from extreme weather conditions, extra care must be taken during installation to keep the flashing material in good condition and capable of withstanding damage. Many professional roofers will cut and shape their flashing materials from sheet metal; however, homeowners can also choose to have the flashing preformed. It's very important to use flashing materials that aren't prone to rust, such as stainless steel.
How to put on roof shingles
Shingle installation should be done in accordance with local building code. However, other factors, such as local weather conditions and the shingle type plays a part in how shingles are installed.
Asphalt shingles are most commonly available in strips of 36 inches long by 12 inches wide. Each strip has a set of three "tabs," which provide the visual "shingle" effect when installed correctly. Each 3-tab strip has a layer of adhesive just above the tabs. The heat of the sun activates the adhesive, which binds to the layer below and creates a seal that augments the nails when installed.
A starter course (layer) of shingle strips with the exposed tab portion cut off is installed on the eave edges of the roof. The starter course's vertical ad horizontal edges should extend between 1/4 inch to 3/4 inch beyond the edges of the rake and eave. The starter course should be secured with roofing nails applied 2-1/2 inches to 4 inches above the edge of the eave, and between 6 inches to 9 inches apart over the entire length.
After the starter course, the shingles are installed in an offset pattern of a consistent measurement, usually 4 or 5 inches. This ensures complete coverage and gives the finished roof an attractive visual pattern.
The pattern is begun in this manner:
1. Starting on the left side of the roof at the eave, place the first full shingle strip directly over the starter course at the eave and rake edges. Each complete strip should be secured by a minimum of four roofing nails, spaced evenly (high wind areas may require six nails per strip).
2. Cut 4 inches from the left side of the next tab strip, and install directly in line with the left edge of the previous strip.
3. The next strip is cut 8 inches and is installed in the same manner.
4. Continue shortening each strip by an additional 4 inches for every subsequent row until the last row is a 4-inch tab.
5. Continue the pattern to the right of the first set, using whole 3-tab strips.
Soffits help provide ventilation for attic space. (Photo by Eldon Lindsay)
Soffit and fascia boards
Soffit is the board that bridges the gap between a home's siding and its roofline. It's located under the horizontal fascia board, which holds the gutters. Soffits help provide ventilation for attic space.
The fascia is attached to the edge of the roof and stretches horizontally from one end of the rafters to the other.
The purpose of the fascia board is to prevent water and wind from damaging the interior of the house and the roof. It also supports the gutters and drainage pipes as well as to help enhance the appearance of the house.
Failing to clean gutters on a regular basis can cause water to pool, which can damage the fascia and soffit.