Repairing and Replacing Flashing

Think of flashing as a special-purpose shingle. Like a shingle, flashing overlaps and interweaves with other roofing materials to shed water. Flashing is made of thin-gauge metal that is bent and formed to fit angled joints where two or more surfaces abut. Because these intersections are vulnerable to leakage, flashing deserves closer scrutiny than the rest of your roof. Look for flashing that has pulled away from adjoining surfaces and for roofing cement or caulk that has dried and cracked. Even tiny holes can leak; when in doubt, apply new cement or caulk.

Rusted, cracked, or corroded flashing around chimneys, dormers, and plumbing vents will last for a few more years if you trowel on a coat of fibered asphalt-aluminum roof paint. If there is widespread deterioration or valley-flashing failure, call in a roofer or sheet-metal specialist to replace these sections entirely. You can replace small flashing pieces as long as you buy precise duplicates and replace them in their exact original positions.

Take care that two types of metal do not come in contact with each other, or corrosion could result. For durability at a reasonable price, choose aluminum flashing. Vent flashing comes as a single molded piece of metal or plastic. Simply fit the piece over the pipe and cover with roofing on the uphill side.

Tools: Tin snips, putty knife, hammer, cold chisel, trowel, joint strike, caulking gun, ladder, rope.

Open- and closed-valley flashing. Because it is visible, open-valley flashing is easy to inspect. Cement down any shingles that are even slightly curled or loose. Closed-valley flashing hides beneath the roofing. In some cases shingles are so interlaced on top that it is impossible to check the flashing.

Flashing for a dormer. To reflash dormers buy pieces of flashing bent at 90 degrees. Using the old flashing as a guide, tuck the flashing under the siding on the dormer. Use valley flashing along the peak. For brick use step flashing capped with counter flashing let into mortar joints.

Drip cap. Drip cap flashing keeps water from seeping under the frames over windows and doors. The drip cap should be several inches high so that water cannot work its way up and around it. Check drip caps periodically for damage.

Replace a vent. Don’t bother repairing faulty vent flashing. Just install a new neoprene and aluminum replacement. You’ll need to replace only a few shingles.

Flashing a Chimney

Most chimneys have a two-part / VI flashing system to ride out minor structural shifting. Step flashing and base flashing fit under shingles along the sides of the chimney and lie on top of the shingles below the chimney. Counter-flashing is applied over the base and step flashing and serves as a cap to keep water out. Its top edge, bent into an L shape, is mortared into the chimney’s mortar joints to hold it securely. Replacing chimney flashing calls for time and patience. If you are unsure how the old flashing was installed, or if the old flashing leaked because it was not installed properly (rather than because of metal failure), call in a pro. Whenever possible buy flashing pieces already cut and bent to fit. If necessary, form sheet metal by clamping it between two pieces of wood and bending it.

1. Install the base flashing. To avoid confusion, work by removing one or two pieces and replacing them; then move on to the next pieces. To replace base flashing, apply asphalt primer to the bricks and install the flashing to the front. It should overlap the roof shingles by 4 inches.

2. Add the step flashing. To remove a piece of old step flashing, gently pry up a shingle and pry out the nail. Working from the bottom up, slip each piece of new flashing under a shingle, work roofing cement under the flashing, and drive a nail. Also use roofing cement to join the flashing pieces to each other.

3. Flash the cricket. If there is a cricket on the up-roof side and it is rotted, build a replacement out of plywood. You may need to cut away some roofing to do so. Use roofing cement to embed the rear corner flashings and base flashing that covers the cricket. Nail the flashing to the deck only.

4. Set the counter flashing. Set the counter flashing on the front and sides into raked-out mortar joints. Refill the joints with masonry caulk.

5. Flash the corners. Make and install counter flashing suitable to the situation on the up-roof corners and side of the chimney. Install it with as few nails as possible; setting metal in a thick layer of roofing cement provides a firm installation. Where you must drive nails, cover the heads with roofing cement.

Cleaning and Repairing a Masonry Chimney

Chimneys have two enemies: heat and water. The crackling fire you enjoy on winter evenings subjects masonry to temperature extremes that can chip out mortar, especially at the top where the flue penetrates the cap. Most chimneys have a ceramic flue liner running up through the center. Concrete blocks surround the liner, and bricks cover the blocks. Other chimneys are all brick; a few use firebrick instead of a ceramic tile flue liner. Many also include a chimney cap to keep out rain, nesting birds, and downdrafts. Regardless of your chimneys construction, it pays to inspect it every fall. Examine every visible surface, including the attic. Look for cracks and deteriorated mortar.

Occasionally test for hot spots by feeling with your hand. These may indicate a broken flue—a definite fire hazard that a mason should fix before you use the fireplace again. How often a fireplace flue needs cleaning depends on how much you use it and the type of wood you burn. Pine and other sappy species produce creosote, which cakes the flue and constricts the opening. The result: smoking and a possible chimney fire. Hire a chimney sweep to clean your chimney, or do the job yourself, as shown here. A faulty firebox design or downdrafts also cause smoking.

Tools: Hammer, cold chisel, caulking gun, sweep tools.

1. Seal the fireplace. Before brushing, open the damper and seal the fireplace opening with a wet sheet, canvas, or polyethylene sheeting. Be sure the opening is sealed very tightly and firmly. Measure the diameter and length of your chimney, and buy chimney brushes and extension handles to fit.

2. Brush the chimney. From the top, insert the brush, moving it up and down to dislodge debris from the flue liner. Add an extension and repeat until you reach the damper at the bottom. Wait approximately a half hour for the dust to settle. Slowly remove the sheeting. Wet down the soot before you clean out the firebox. Vacuum around the damper before closing it.

3. Repoint and apply caulk. Rain erodes mortar joints. Chip away loose material, then repoint. Apply a generous bead of masonry or butyl caulk around the flue for a flexible seal that rides out expansion and contraction. If the area is badly damaged with large, loose pieces, call in a pro.

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