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Many assume that a leaky roof means replacement is imminent.

Often that's the case, but leaks could be caused by a flashing problem. Flashings are a part of every roof. They keep water out at certain locations where the roofing itself can't.

For example, most types of roofing aren't capable of sealing themselves to pipes and ducts, or to skylights, or to intersecting roofs or intersecting walls. Flashings make watertight connections possible at these locations.

A flashing is a shingle of sorts. When a vent pipe goes through a roof, a vent-pipe flashing is used to seal the penetration. The flashing is a large, flat metal shingle that contains a piece of rubber at its center. The rubber portion seals itself tightly around the pipe and the large, flat metal portion laces into surrounding shingles.

On a flat roof the same rubber grommet also is used to seal itself to the pipe while the large, flat portion provides ample surface area to facilitate a good connection to tar and other types of flat and low-slope roofing materials.

Flashings are made from a variety of materials — galvanized sheet metal, fiberglass or plastic, lead and copper.

Heavy-gauge galvanized sheet metal is the most common flashing in use today. It is relatively inexpensive and easy to install. If properly maintained, it can last a lifetime.

Lead is used almost exclusively on certain tile roofs, and, although copper flashings can last longer than galvanized sheet metal, the cost generally is not worth the extra life.

By telling you a little bit about how roof flashings are connected, we hope to give you an idea about how they can leak and how they can be repaired or replaced.

- Wall-to-roof connections on flat roofs. In places where the roof intersects with a wall, the flashing is L-shaped and travels the entire length of the connection. It's placed behind the wall covering (siding or stucco) and beneath the roof covering and, therefore, it isn't obvious.

The flashing travels up and behind the lower edge of the exterior wall covering at least 6 inches and prevents leaks from even the most torrential of downpours. The roof side of the L-flashing is connected to the roof material with hot tar or some other sealant.

- Wall-to-roof connections on pitched roofs. On pitched roofs, wall-to-roof connections are slightly different. Where the roof and wall are parallel, the flashing resides behind the wall covering and above the shingles. Where the roof slopes along a wall, the flashings still are L-shaped, but not continuous.

At pitched areas the flashings are shingle length and overlap in the same way that shingles do — one end lacing into the shingles and the other side fitting behind the exterior siding.

- Chimneys and masonry walls. A slightly different technique is used to connect the flashing to masonry. The connection to the roofing is the same. However, with stone and brick, the flashing is laced into a mortar joint and mortared in place.

A second flashing is used to cover the point where the first one is mortared into the masonry. It also is tied into a mortar joint — 6- or 7 inches farther up the masonry wall.

The upper flashing is known as counter flashing or cover flashing. With two flashings together — both mortared in place — watertight connections can be made between the roof cover and a vertical masonry surface.

- Valley flashings. The point at which two pitched roofs join (at an inside corner) is known as a valley and is where most shingle materials rely on a piece of metal for a watertight connection to each other. This one is known as valley flashing.

With all the shingle nailing that goes on along the valley flashing, there is a good possibility that a leak can result. This occurs occasionally in new roof installations.

- Edge flashings. Flashings also exist at the roof's edge to protect damage to barges, fascias, rafter tails and other wood parts that readily can be damaged by watershed.

Edge flashings travel under the roof covering and over the wood in question. Generally these flashings would not be a culprit in the case of a roof leak because they normally are outside the leak area, at the outermost edges of the roof.

- Walls and decks have flashings, too. Off the roof, many other connections depend of flashings for water-leak prevention.

Horizontal joints between stacked sheets of plywood siding use something called Z flashing to keep water out. The flashing starts on the face of the lower sheet, bends over to cap the top of the lower sheet and then travels up behind the sheet above (sort of in the shape of a backward Z). Other Z flashings are used at horizontal connections between exterior siding and penetrations such as doors and windows.

A similar flashing is used at cantilevered wood decks and at waterproof decks, whether cantilevered or not.

What you might have thought was a roof leak could be a leaking deck flashing letting water into a wall from one or more stories above. Water can travel down as many walls as exist.

Flashings of all types play a big role in preventing water from getting into your home through the roof and walls. Because they are metal and-or rubber (and generally not regularly maintained), metal flashings corrode, rust and otherwise deteriorate to a point where they can leak — sometimes long before the roof or siding material is even close to failing.

The rubber section of a pipe flashing usually fails long before the metal portion does.

Whether you have a roof or siding leak, check the flashings first. Regardless of where they are, they usually are easier and less-expensive to repair than alternative patching, or replacement.

When "drip, drip, drip" is what you're hearing, it's time to check flashings.

And don't just look for a leak in a flashing. Sometimes the hole is hidden under roofing, counter flashing or mortar. Test flashings in the area of the leak.

If rubber is near the leak, test that part first.

Flashings of all types last longer if a fresh coat of paint is applied every several years.

For more home-improve-ment tips and information visit our Web site at www.onthehouse.com.

Readers can mail ques-

tions to: On the House, APNewsFeatures, 50 Rockefeller Plaza, New York, NY 10020,

To receive a copy of On the House booklets on plumbing, painting, heating/cooling or decks/patios, send a check or money order payable to The Associated Press for $6.95 per booklet and mail to: On the House; P.O. Box 1562; New York, NY 10016-1562.

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