Flashings - Exterior Wall Assembly

Saturday, October 28, 2017

There’s an old saying that fits this topic very well: the devil is in the details.  at’s because it’s how we  ash windows, doors, porches, chimneys and other key areas of the exterior walls and roof that will keep the water out of your home. Flashings are one of the most important and overlooked components of the building envelope that make or break long-term resistance to damage. Make certain that your framers and roofers are using the right materials for the job and installing them in the proper fashion.  is is not the place to cut money from your budget! Flashings of all sorts are manufactured to  t every joint and intersection of your exterior building envelope, to cover all the different angles, seams, gaps and penetrations. It’s not just the materials; even more importantly, how they are installed keeps the water running downhill.

With windows, the process begins with a sill pan or a layer of waterproof material covering the bottom of the rough opening that the window will be installed in.  is material should be turned up at the corners at least six inches on the inside of the studs and then extend out on top of (not under) the house wrap below the open- ing. The house wrap has a  wrap cut above the window. The window is then installed, and the sides are  flashed with an adhesive tape, followed by a layer of  flashing tape across the top of the window  flange. The  flap of house wrap is then brought down over the top  flange, and it is taped in place with  flashing tape.  The bottom  flange of the window is not flashed.  is will allow any water that does get into the opening to drain out and  ow down the waterproof drainage plane of the house and out the weep holes or screed at the bottom of the wall. By the way, the order that these flashings are installed, from the bottom up, is critical. For curved windows and doors there are flexible flashing tapes on the market now that work very well.


The process for porches and decks is much the same.  e drain- age plane material is fully installed before the ledger board is put in place. A er attaching the ledger board, the drainage plane is cut above the ledger board to which the joists will be attached. A  ash- ing material like metal or  flexible waterproofing is attached to the sheathing, and wrapped around the front of the ledger board.  e drainage plane is then turned down over this flashing and taped in place. You can obtain detailed drawings showing exactly how these openings need to be flashed on the websites of the house wrap or window manufacturer for your project.

The addition of an insulated sheathing to the wall assembly re- duces thermal bridging. Or you can replace the rainscreen assembly that we just described with an insulated sheathing panel system that is taped at all seams (with a 50-year warranty). These types of water management systems are always employed behind masonry or stucco walls (because water penetrates those materials so rapidly) and behind wood walls, too, in rainy or marine climates.

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