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There are several components to a roof system. At the base, the structural members are referred to as the trusses, or in older homes, rafters. These are the lumber pieces that make up the skeleton of your roof.
On top of the structure, sheets of wood are usually placed as a base. Then, a moisture barrier like tarpaper is stapled down to the wood sheathing.
The most common roof covering in Western Washington State is asphalt composition shingle. It's made from a mixture of petroleum products and rock. It's those grains of rock you see atop your shingles that give your roof its life. Over time (20-30 years) that layer of rock granules wear away, revealing the less durable materials beneath, and that's when it's time to replace your roof.
So, when a roof is replaced, what actually gets replaced? This depends greatly on the roofing contractor you choose and your budget.
ROOF-OVER
Some homeowners opt to install brand new shingles on top of the old. This reduces the labor and disposal costs of removing the old roof covering material. However, it is generally not recommended as it will void warranty of the new roofing material because, without a smooth surface on which to lay the new material, there will be contours on the new surface which could reduce the flow of water of the roof making it susceptible to early failure.
REPLACEMENT
In a full replacement, all of the roof covering is removed, down to the wood sheathing. This give the opportunity for a thorough inspection of the structure of the roof. In some cases, leaks through the shingles can occur without the homeowner even realizing. However, it will be apparent in the wood and a good roofing contractor will have been prepared to replace any wood sheathing that has been compromised.
There are other components of your roof system you may wish to ask your roofer to replace with new at the time they are replacing your roof covering. They are relatively inexpensive and usually recommended. Here's a list:
FLASHING
These are sheets of metal placed strategically in places more prone to water runoff and where your roof meets wood, walls, chimneys and trim. It has also become common to use flashing in the valleys of your roof, rather than just shingles, as the metal is more durable. A valley is where to angles of your roof come together to meet in a low spot.
ATTIC VENTS
Attics are vented to allow air to flow through and out of your home, helping prevent moisture capture and to keep your home cooler in the summer. Vent boxes are usually square, made of plastic or steal, and sit atop your roof in a row. A newer method of attic ventilation is a ridge vent, hidden under shingles all along the peak of your roof.
EXHAUST VENTS
There are several things in your home that create moist air, including bathrooms, stovetops, and clothes dryers. These things must be exhausted to the outside of your home, and they may terminate out your roof. If they do, there will be vents with flappers installed on your roof.
PIPE BOOTS
Your plumbing system's pipes that serve your sinks, shower, and toilets draining to your sewer or septic have air vent pipes that penetrate through and above your roof and draw air in allowing your drain lines to flow smoothly, rather than getting stuck in a vacuum (picture the Thanksgiving cranberry sauce can). Around these pipes are rubber boots which act as a seal around the penetration. Over time, the rubber will dry and crack.
FLUES AND CHIMNEYS
It is not common for a roofer to replace chimney and flues serving gas and wood appliances. However, they should inspect and replace any seals around them where they penetrate the roof. Also, if a chimney structure is compromised, water may find it way through. A qualified chimney expert should address any issues with your chimney.
BOTTOM LINE
If you plan and budget for a full roof replacement and choose your roofing contractor carefully so that a thorough examination and replacement is done, one of the most important systems of your home will protect your investment for decades.