When it comes to protecting the home, it's what's on top that really counts. On top, as in the roof.

It's the home's first level of protection -- against sunlight, rain, snow and wind -- but the last thing a homeowner usually thinks about.

"No one thinks anything of the roof until it leaks," says Bill Good, executive vice president of the National Roofing Contractors Association, an Illinois trade group.

Unfortunately, when a roof goes bad, it can be one of the homeowner's biggest -- and most expensive -- headaches. A leaky roof often means interior water damage, so there are phone calls to the insurance company and a restoration contractor.

A typical roofing replacement job includes tearing off the old roof and installing a new layer of asphalt shingles, said Keith Kaval, co-owner of two companies that specialize in roof replacement. It can also include replacing sheathing, fascia boards or soffits, all areas that might have rotted away, or new gutters.

Hold on. What do sheathing (the wood underneath the shingles) and fascia boards (where gutters hang) have to do with roofing? A new roof means simply replacing shingles, right?

Wrong. The roof is one of the home's most important "systems." It's how an exterior shell sheds moisture. Home systems, such as plumbing and electrical, are made up of several components working together to perform a larger series of similar tasks. A roofing system primarily consists of the following:

· Wood sheathing:This is also called the deck.

· Underlayment:This is typically a fiberglass-reinforced paper that helps shingles lie flatter and keeps the wood deck drier.

· Leak-barrier membranes:These prohibit water intrusion, including ice-damming, at critical spots, such as the roof's outer and inner edges and in valleys.

· Flashing:Metal material formed to stop leaks at vents, pipes and other objects that extend through the deck.

· Ventilation:Fresh attic air prevents mold and moisture from damaging the underside of the sheathing and keeps homes cooler during warm weather.

· Shingles:The exposed layer that battles the elements, especially wind and water.

So, to say roofing is just shingles is like saying plumbing is just water or electrical is just outlets. Yet, far too many homeowners consider a new roof simply a new layer of asphalt shingles. In fact, most think it's all right to cover an existing layer with a new layer of shingles.

In their lifetimes, homeowners might purchase just one new roof. Making poor decisions on such an important protective barrier can lead to moisture problems, either inside walls or above ceilings, and structural damage.

Here are some popular -- and false -- myths about roofs:

· It's fine to cover an existing layer of shingles with a new layer.

Don't do it, even though many a building code says it's acceptable. Think of shingles as the skin of an apple. Ever bite into that bright red skin only to get a mouthful of a soft, bruised, mushy interior?

Underneath shingles is a layer of wood sheathing, usually plywood. Sheathing can rot because of leaks, inadequate attic ventilation or just age. The best way to inspect this critical layer is to strip away the old shingles.

· All asphalt shingles are basically the same.

Today's laminated and fiberglass-reinforced products are better than ever. Shingles are rated for durability -- some are warranted to last 50 years -- and wind resistance (up to 110 mph). Manufacturers, such as GAF and CertainTeed, offer limited lifetime warranties.

And, boy, are there choices. Some shingles are made to combat specific problems. For example, in humid areas, some roofs, over time, show a black mold. Algae-resistant shingles contain imbedded granules of zinc and copper, which, when mixed with roof water, are natural algaecides. Other shingles are manufactured to look like wood shakes or slate tiles.

· Flashing needs to be replaced only when a new roof is being installed.

Flashing, the metal material fabricated to divert water away from vents, pipes and other roof openings, including chimneys, is its own, separate animal. It can lay in place for years and can protect even longer than other components of the roof. Or it can fail in months and otherwise ruin a perfectly sound roofing installation.

As a rule, flashing should be checked every six months. A visual inspection can be made with binoculars. Look for dried caulking or sealant, cracked or broken flashing pieces and damaged shingles in contact with the flashing.

If a visual inspection turns up potential problems, have a roofing contractor climb the ladder for a better look or to make repairs.

· Attic insulation saves energy and helps roof performance.

Adding more than the required insulation can block ventilation openings at the soffits and eaves and might trap moisture. Trapped moisture can warp and rot sheathing from the attic interior and also can be a source for mold. Actually, there is a special relationship between insulation and ventilation.

Insulation is rated by thermal resistance, called R-value, which indicates the resistance to heat flow. The Energy Department recommends that attic insulation in the Northeast have a minimum R-value of 49. The higher the R-value, the greater the insulating effectiveness.

Determining existing R-value is based on the type of insulation material and its thickness. For example, the most common attic insulation in new homes is fiberglass blankets or batts that are about 16 to 17 inches thick. The fiberglass has an R-value per inch of 3.2; the result is an insulation layer with an R-value of between 51 and 54. Increasing the R-value with another layer of fiberglass would seem logical. However, adding insulation can vary the temperature at the wall, possibly creating condensation inside the attic.

Proper ventilation eliminates moisture buildup and maintains steady attic temperatures in both cold and warm seasons. The important concept is that ventilation requirements are related to the area of the attic floor.

Ventilation devices, including fans, vents, louvers and ridge venting, are rated in square inches for net free area (NFA).

Signs of inadequate ventilation are ice-damming, mold on the underside of the sheathing and excessive frost accumulation on the roof deck or in the attic.

· Gutters are separate from the roofing system.

Roofs are designed to divert water from the structure. The gutters are the final piece of the process; they prevent water from dripping down exterior walls and move it from the foundation. It's why many roofing installations can include new gutters and downspouts.

At the very least, clean gutters of debris, sediment and leaves regularly. Repairing damaged runs might be as easy as tapping in a few gutter spikes with a hammer or reattaching downspouts with a riveting tool. Clogged and sagging gutters can cause leaks behind exterior walls and contribute to ice-damming.

Over an extended period, a faulty gutter system can rot away fascia boards, soffits and the sheathing at the roof's edge.