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Flashing - What is it and why is it important?

Scott Gibson, Contributing Editor

What can be done to prevent long-term water damage to my home's siding and framing where my exterior deck meets the side of the house?

Many a homeowner has pulled off an aging outdoor deck to discover siding, sheathing and framing that's the consistency of oatmeal and home to a thriving colony of carpenter ants. More often than not, a major contributor is worn or missing flashing. Without flashing, there's nothing to prevent water from seeping behind the ledger board that connects deck framing to the house.

Over time, trapped water leads to decay-not only in wood building materials but even in metal fasteners that hold the deck together. The result is never good, and often expensive and time-consuming to repair.

Flashing is layered protection

So what is flashing? It's a thin layer of waterproof material that keeps water from getting into places it doesn't belong. (Caulk is fine for filling small gaps around windows and doors, but it shouldn't be confused with flashing.)

Deck Flashing Details

You can count on one thing-water flows unerringly toward the ground. And as it travels, flashing carries it safely over the inevitable crevices, cracks and gaps in the building's exterior.

If it's going to work, flashing must be installed so no seams face uphill. It's usually layered with other building materials. The upper edge of flashing, for instance, is protected by house wrap or tarpaper, not installed over it.

If you wonder whether flashing has been installed the way it's supposed to be, follow a drop of rain water as it moves down the side of your house. It should be carried from surface to surface all the way down, never encountering an open seam or an upturned lip that blocks its progress.

When it comes to the deck, a strip of flashing is brought down the wall of the house and over the top of the ledger (the piece of dimensional lumber attached to the house at the edge of the deck). This all-important water barrier should extend up the wall at least 4 in.

Many types to choose from

Lots of materials can be used for flashing. As long as it won't degrade from contact with incompatible materials, and is impermeable to water, it should work.

While repairing his 18th century farm house, my neighbor discovered that an enterprising carpenter had used folded lengths of birch bark to waterproof the intersection of a roof and adjacent wall. There wasn't a hint of water damage.

Today, builders often use one of these standards:

  • Aluminum. Easy to form, durable and relatively inexpensive. Aluminum will-corrode if it's left in contact with alkaline materials like concrete or fiber-cement siding. It also will degrade from contact with the copper-rich-preservatives that have replaced chromated copper arsenate in treated lumber-commonly used in deck framing (these new preservatives also are a problem for-fasteners).
  • Copper and lead-coated copper. Expensive and harder to form than aluminum-but extremely durable. Copper is compatible with newer types of wood-preservatives.
  • Lead. Very soft and easy to bend. Durable and a favorite of masons for-chimney flashing. Carries some health risks.
  • Galvanized steel. Inexpensive but not as durable as other choices.
  • PVC. Easy to work with and inert. Polyvinyl chloride is not affected by-treated wood, an advantage in deck building.

These materials come in rolls of different widths that can be cut as needed and bent right at the job site.

More recently, another type of flashing has been developed that's extremely effective. It's bituminous (tar like) material with a very sticky backing-the same stuff used along roof eaves to prevent leaks from ice dams. It's sold by different companies under different brand names but the one I've used is called Vycor, made by Grace Construction Products.

It's wonderful stuff-tough and pliable. It also seals around fasteners that are driven through it. Just remember to protect it from sunlight with another material-aluminum, for instance. Used together, these two flashing layers should keep your house dry and safe for many years.

About the Author
An accomplished woodworker and carpenter, Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He also is former managing editor of the Kennebec Journal, a daily newspaper in Maine.

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