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When planning a project, many people underestimate the importance of having sufficient flashing in a range of locations to make these areas weathertight. Typically sold as a roll of sheet metal or vinyl, or as small sheets of pre-bent material, it’s typically placed in valleys, at dormers, in places where there are protrusions in the roof, at the juncture of a wall and a deck, and around windows and doors. Flashing prevents water from penetrating these changes in elevation. But why is it needed in each location? Here’s a quick look at locations where flashing may be installed and why it’s important to do it right.


House roofWhen rain falls on your roof, it can build up to a strong flow of water in the valleys of your roof and wind can drive it against protrusions in your roof, such as dormer walls, vents and chimneys. By installing flashing, you’re ensuring that the rain doesn’t enter your home’s structure through these points. Valley flashing often has a ridge in the center to prevent water that is flowing quickly down one side of the roof from pushing up under shingles on the other side of the valley. Vent flashing can often be purchased in a format that wraps around the protrusion, making a tight seal, while easily incorporating into the rest of the roofing system. In dormers, you’ll need to flash the small valleys that form at the change of roof elevation and where the walls meet the roof. However, one of the trickiest spots to get right is at the corners of the dormer walls, where you’ll want to use two pieces and a bit of sealant to make sure the corner stays dry.

Windows & Doors

Blue front doorBecause windows and doors create a point where your structure’s weatherization plan changes, they create an area where water can seep into your framing members, causing rot and decay. In general, you’ll want to flash from the bottom up, starting with the horizontal surface at the base of the door or window’s rough opening, proceeding up both sides and then flashing the top. Much like shingle installation, this ensures that any moisture flowing down the wall system will be directed away from the top of the opening, down the sides and to the ground without getting into the wall cavity.


Back deckThe stretch where your ledger board ties into your home is a prime location for moisture to build up and cause serious damage to your home’s framing and structural integrity. Because a ledger board is typically fastened to the side of your home, it creates a ledge where wind-driven rain can sit, causing moisture to build up and rot the area behind it. To prevent this problem, once you’ve installed your ledger board, install the flashing behind the siding just above the ledger board by an inch or two, more if you tend to get driving rains on that side of your structure, continue over the top of the ledger board and then bend it over the face of the ledger board, ensuring that any water that flows down it will drip underneath your deck rather than onto your ledger board.

By making sure you have sufficient flashing in these areas, you can keep your structure dry, comfortable and healthy. But what if you’re not sure if you need to put flashing in a particular location? At Poulin Lumber, our job is keeping your project moving forward, and we’re happy to help you figure out what building solutions will work best. Please feel free to contact us today with any questions or for more information on our wide range of building supplies.

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