It's a common misconception that the roof over your head is made up of just shingles nailed into plywood. In reality, your roof is a system of components, 5 to be exact, that work together in unison to create a complete roofing system. BUT - The roof system is only as good as its weakest link.
There are 5 Major Components that make up a Roofing System. Lets take a look at them one by one.
The foundation of any roofing system is the roof deck. The roof deck is made up of plywood or lumber planks that run perpendicular to each roof truss (or rafter).
The roof deck is a key component to the roofing system because this is what the shingles anchor into. Many times the plywood can come apart (de-laminate) or rot due to excessive moisture building up in the attic. It can also deteriorate from a roof leak that went unnoticed or ignored. Rotted wood should always be replaced before installing a new roofing system. The reason behind this - you must create a reliable nailing surface to anchor the shingles into. Nailing shingles into rotted plywood is like nailing them into bologna. It's not very effective. So creating a reliable nailing surface from the start will ensure the new roofing system will last for it's expected lifetime.
Most shingle manufacturers and local building codes recommend the roof deck be made up of at least 15/32" OSB plywood or 1" lumber no wider than 6" planks. It is important that the 1-1/4" roofing nail is able to sink through the width of the wood on the roof deck.
During the demo process, after the shingles are removed from the roof, the crew will then strip the deck of nails and staples. During this process, they inspect the roof deck, looking for deficiencies. Rotted wood is removed and replaced before the first shingle is installed.
The product that goes under the shingle is called underlayment. Underlayment has several functions and is one component that is typically skipped when contractors try to cut corners.
There are several types of underlayment for an asphalt roofing system:
15 # Felt, 30 # Felt, Synthetic and Ice and Water Shield
15# Felt and 30# Felt are an asphalt fiberglass based paper that your typically see get rolled out on the roof deck and stapled or button cap nailed down. The fiberglass in this product upgrades the fire rating on the roof (also known as the ASTM rating).
Synthetic Felt is relatively new to the roofing world. It's tear resistant properties make it easier for roofing crews to walk on and its less likely to wrinkle, whereas felt paper typically tears and wrinkles fairly easily. Some synthetic felts offer traction for the roofers working on steep slopes. In addition, they can also offer a water resistant protection while the roof is being installed.
Ice and Water Shield is a self-adhering polymer modified bitumen - which translated into "sticky rubber sheet". Ice and Water Shield is used around roof penetrations, valleys, roof edges, and other vulnerable areas of water intrusion. When used on the gutter ends of the roof it provides protection against ice damming. Ice and Water Shield is required as a building code in some municipalities here in St. Louis.
The purpose of underlament is to provide a secondary layer of protection against roof leaks. In the case of a shingle blow off, the underlayment protects the roof deck until a repair is made. It can also provide a barrier between the shingle and the plywood, thus preventing the asphalt properties of the shingles from leaching into the wood.
Not all asphalt shingles are created equal. Ask any veteran roofer. High quality shingles last longer on the roof and are less prone to premature deterioration. There are many types of shingles available on the market. The most common asphalt shingles are 3 Tab Shingles and Architectural Shingles.
3 Tab Shingles are just as they sound; a 3 foot sheet with three 12" tabs or teeth. 3 Tab Shingles have been around since the 1920's. Today's 3 Tab Shingles carry a 25 Year Life Expectancy. They are designed to shed water from the roof. The wind resistance rating on three tab shingles is approx. 68 mph for most brands, although it can vary. They come in basic colors and patterns.
Architectural Shingles were developed in the 1970's and consist of a three tab shingle overlayed with an additional layer or teeth. The added strength prevents architectural shingles from blow offs and the wind resistance rating, if installed according to the manufacturer's requirements, is 130 mph. Architectural shingles weigh quite a bit more than three tab shingles which assists new shingled roofs in sealing faster. They have a 30 Year Lifetime Expectancy. Because of the dimensional design of architectural shingles, they also help hide deflections from the plywood and other imperfections in the roof structure.
Anywhere that the roof breaks or there is a penetration, there should be a flashing. There are many different kinds of flashing, but they all have the same purpose - to keep the water out. The most common flashing on a roof consist of Pipe Jack Flashing, Drip Edge, Roof-to-wall flashing, or apron flashing, step flashing, skylight flashing, chimney flashing and counterflashing.
Flashing can be made up of non-corrosive metal or copper. The flashing on the roofing system should be inspected regularly - as they are prone to damage from animals, storms and debris. The #1 most common cause of roof leaks are due to failed flashings.
Flashings should be replaced with new flashings when a new roofing system is installed. Here is another Pete with a Pickup trick to cutting corners on a roof. The cheap contractor will eliminate the flashings from the bid to be price competitive, or try to reuse old deteriorating flashings instead of replacing them with new flashings. Flashings are vitally important to the longevity and integrity of the roofing system.
An asphalt shingle is designed to absorb heat, rather than reflect it. The heat is then transferred into the attic space. Ventilation of that attic space is critical in allowing the trapped air to escape through a hole or a vent of some kind.
Many roofing contractors don't fully understand roof ventilation or how to calculate proper ventilation, so ventilation gets ignored. The building code requires a 1/300 ratio of roof ventilation, which means for every 300 sf of attic space there should be 1 sf of ventilation. Now to further complicate this, once you have determined the amount of ventilation needed, then you divide it 60/40 for intake and outtake. I won't get into the specifics of roof ventilation because of the length it would take. But you can read more about ventilation here.
Ventilation becomes important in regulating your heating and cooling bills. When trapped heat cannot escape the attic, it is forced to radiate down into your living space, the space you pay good money to keep cool in the summertime and warm and cozy in the winter time.
There are many different ways to ventilate your roof. Some of the most popular roof vents are box vents (commonly referred to as "turtle vents"), and ridge vents.
Box vents, or turtle vents, are the most common. A 12" x 12" hole (1 sf) is cut in the back slope of the roof just below the peak. A Box Vent is them installed over the hole. As heat from the attic rises, it is allowed to escape through the box vent. You will typically see several of these box vents in a row on the back side of a roof.
Ridge ventilation is a hole cut along the peak of the roof (we call the "ridge") and capped with a special roof vent that is then shingled over. Ridge ventilation provides continuous ventilation at the highest point on the roof, and is the most effective ventilation system. You can watch a ridge vent installed here
The Weakest Link
A roof is only as good as it's weakest link. Each component of the roof system is just as important as the other. Saving money by cutting corners, using low quality material, and improper installation techniques are the most common causes of premature roof failure. (Failure of a roof system before the end of it's expected service life).
Your roof is the most important investment you will ever make because it protects everything that is important to you underneath it. Investing in a good roof that is installed correctly should last you 25 years or more.