If you've ever noticed a damp spot on your interior drywall after a heavy rain, there's a good chance something went wrong with your flashing masonry during the construction process. It isn't exactly the most glamorous part of a house—you usually can't even see it once the building is finished—but it's arguably the most important defense your home has against the elements. Without it, water would find its way into your wall cavities, rot your framing, and turn your insulation into a soggy, moldy mess.
Most people think of brick or stone as a solid, impenetrable barrier, but the truth is a bit more complicated. Masonry is actually porous; it breathes, it absorbs moisture, and it lets water seep through over time. That's why we build "cavity walls" with a gap between the brick and the inner structure. The goal isn't to keep every single drop of water out, but rather to manage the water that does get in. That's where the flashing comes into play.
The Hidden Barrier That Keeps Water Out
Think of your home's exterior like a raincoat. If you have a tear in the fabric, you're going to get wet. In a brick home, flashing masonry acts as that waterproof layer that redirects water back to the outside. It's usually a thin sheet of material installed at strategic points—like above windows, at the base of walls, and where the roof meets the chimney.
When rain hits a brick wall, some of it runs off the surface, but a surprising amount actually soaks into the brick. Once it passes through the brick, it hits the air space in the cavity and starts to trickle down the back side of the masonry. Without flashing, that water would just pool at the bottom of the wall or sit on top of window frames, eventually soaking into the wood or steel. The flashing is bent in a way that catches this water and funnels it through "weep holes" so it can escape safely.
It's one of those things where if it's done right, you never think about it. But if it's done wrong? You'll be thinking about it every time the clouds turn gray. Fixing failed flashing after the fact is a nightmare because it usually involves tearing out sections of brick, which is why getting it right the first time is so vital.
Choosing the Right Materials for the Job
Not all materials are created equal when it's time to install your flashing masonry. Back in the day, builders often used basic felt paper or thin plastic, but we've learned the hard way that those materials don't always stand the test of time. They can crack, rot, or get eaten away by the high alkaline content in mortar.
Metal Flashing Options
If you want something that's going to last as long as the house itself, metal is usually the way to go. Copper is the gold standard. It's incredibly durable, it doesn't corrode easily, and it's flexible enough to be shaped into complex corners. The only downside is the price; it's definitely not the cheapest option on the shelf.
Stainless steel is another heavy hitter. It's tough as nails and won't react with the chemicals in the mortar. Some builders also use lead-coated copper, though that's becoming less common for environmental reasons. Aluminum is out there too, but you have to be careful with it; if it touches wet mortar, it can corrode pretty quickly, so it usually needs a protective coating.
Flexible or Membrane Flashing
On the more modern side of things, we have rubberized asphalt and EPDM membranes. These are popular because they're easy to work with—they often have a "peel and stick" backing that makes installation a breeze. They're also great for sealing around nails or screws because the material "heals" around the puncture.
The catch with these flexible membranes is that they can degrade if they're exposed to too much UV light. Since flashing often peeks out just a tiny bit at the edge of the brick, you have to make sure the material is rated for sun exposure or that it's tucked away properly.
Where Most People Mess Up
You'd be surprised how often flashing masonry is installed backward or upside down. It seems simple, but it's a game of detail. One of the most common mistakes is not "lapping" the materials correctly. Just like shingles on a roof, the piece on top needs to overlap the piece below it. If you do it the other way around, you're basically creating a funnel that leads water right into your house.
Another big issue is the "end dam." When you install flashing over a window, you can't just lay a flat strip of metal and call it a day. The water will just run off the ends of the flashing and into the wall. You have to turn the ends up to create a little "dam" that forces the water to stay on the flashing until it exits through the weep holes. It's a tiny detail that takes maybe two minutes to do, but skipping it causes thousands of dollars in damage over time.
Then there's the issue of cleanliness. If a mason is messy with their mortar, they can accidentally clog the flashing or the weep holes. If the "bed" of the flashing is covered in mortar droppings, the water won't flow; it'll just sit there and eventually find a path inward.
Don't Forget the Weep Holes
You can't talk about flashing masonry without mentioning weep holes. These are the little gaps you see in the bottom row of bricks or above windows. They are the "exit doors" for the water that the flashing has collected.
Sometimes homeowners see these holes and think, "Hey, the builder forgot some mortar here," and they fill them in with caulk or cement. Don't do that. If you plug those holes, you're essentially trapping water inside your walls. The flashing will catch the water, but with nowhere to go, it'll just rise until it spills over the top of the flashing and into your home.
Nowadays, many builders use small plastic vents or "rope" wicks in the weep holes to keep bugs out while still letting water drain. It keeps the house looking clean and prevents wasps from deciding your wall cavity is a great place for a nest.
Spotting Trouble Before It Gets Expensive
So, how do you know if your flashing masonry is failing? Aside from the obvious sign of water dripping down your interior walls, there are a few "tells" on the outside.
Keep an eye out for "efflorescence." That's the fancy word for those white, powdery salt stains you see on brick. While some efflorescence is normal on a new house, a heavy buildup in one specific spot—like under a window or near the base of a wall—usually means there's a lot of water trapped behind the brick that can't get out.
You should also look for cracked or deteriorating mortar joints right above the flashing lines. If the water can't drain properly, it'll freeze and thaw inside the wall, which eventually blows the face off the bricks or crumbles the mortar. If you see bricks that look like they're "spalling" (peeling or flaking), you've got a moisture problem that needs to be addressed before your structural lintels start to rust and sag.
Final Thoughts on Keeping Your Walls Dry
At the end of the day, flashing masonry is all about respecting the power of water. Water is patient, and it's always looking for a way in. By installing high-quality flashing and making sure those weep holes stay clear, you're giving that water a clear, easy path back to the outdoors.
It's not the most exciting part of a home renovation or build, but it's the one that lets you sleep soundly during a thunderstorm. If you're building a new place or fixing up an old one, don't cut corners here. Use the good materials, pay attention to the overlaps, and make sure those end dams are in place. Your future self (and your dry carpets) will definitely thank you for it.
It's a lot easier to spend a little extra time on the flashing now than it is to hire a crew to tear your house apart five years down the road. Keep it simple, do it right, and let the bricks do their job while the flashing handles the rest.