My builder is talking about installing metal flashing as he’s putting up my vinyl siding. What in the world is this material, and why is it necessary? I don’t want to waste money. Would you use it when building homes?

— Tara P. Oak Harbor, Wash.

Flashings have been around for hundreds of years, and if I were in charge of the building code and the best-building-practices manuals, these materials would be here for a millennium or more.

Flashings are used on exterior walls but primarily on roofs. They can be made from many materials, including tin, copper, galvanized metal, plastic, wood and lead.

I describe them as a transitional material. When a flashing is used on a roof, it typically is used to transition from the roofing material to something that is not a roof. For example, you need flashing where a skylight, chimney, dormer, plumbing vent or air vent extends through a roof. The roofing material stops where these elements go through the roof, and something must connect them to the roof so there is no leak. That’s the job of a flashing.

In your case, with the vinyl siding, your exterior trim work has horizontal fascia boards at the bottom of the house and at each story line. Where the vinyl siding rests on top of these boards, a piece of flashing is needed to create a leakproof conversion between the first piece of vinyl siding and the horizontal fascia board.

Here’s why the metal flashing is so important in your case. You have to think about how roof shingles — or feathers on a duck — work to get a full understanding.

The reason ducks and birds stay dry is because of the way their feathers overlap. Water can’t get under the feathers to the duck’s skin. That keeps them warm and dry. That’s where the phrase “like water off a duck’s back” comes from. Water readily flows off the duck because of the composition of the feathers and the overlapping layering.

To keep your house dry so it doesn’t rot, you need to build it the same way. This goes for exterior siding as well as roofing. Each time you transition between materials — be they foundation, siding, brick, windows, doors or what have you — you need to make sure each material sheds water over the top of the material below and doesn’t let it behind or under it.

In your case, there is a great need for a flashing that runs along the top of any of the horizontal fascia boards. Metal flashing comes in many different shapes, but in your case the metal overlaps the fascia board perhaps 1 / 4 inch, then is bent at a 90-degree angle to pass over the top of the fascia board, then is bent again at a 90-degree angle to go up the wall so that it extends behind the first row of vinyl siding.

You should be able to see how this would work to shed water. Without the flashing in place, water would roll down the vinyl siding and hit the top of the fascia board. It would run along the top until it found a tiny crack, and then it might flow behind the fascia. This could cause a leak indoors or start to rot the wood fascia board, the wall sheathing or other wood indoors.

With the flashing in place, the water runs down the siding, and when it reaches the bottom of the first row, it drips off. The water contacts the metal flashing that extends across the top of the fascia board and then runs off the downturned edge of the flashing on its way to the ground.

— Tribune Media