Some years back I began to notice a real need for fixing rotting and decayed wood on residential homes. What I saw was everything from simple windowsill rot to major structural decay issues, usually deriving from moisture infiltration. It wasn't long before I began to see consistent common problem areas on a home, but also began to realize that many of the decay issues we were fixing could have been avoided in the first place—yes, even here in the rainy and damp Pacific Northwest. In fact, it is my opinion that most—if not all—rot issues are simply due to lack of quality materials, appropriate design, and proper installation.
Take for instance this scenario I was once called in to remedy: The absence of flashing (it should have been—but wasn't—installed on the chimney) on a re-roof on a historic home created so much internal damage we had to shore up the home and remove two floors of complete wall systems studs, including complete rebuilds of all the historic windows. That project was in the range of about $60,000—all from one piece of missing flashing!
On a more common scale, there's the myriad of wood windows, frames, and sashes rotting away within about 15 years due to use of soft woods, or bad design. And, frankly, the list goes on. There are three consistent mistakes that I see: 1) A lack of care to quality of installation. 2) Inferior materials. 3) Inappropriate design.
It's important to remember that there are quality materials, install methodology, and great designs available on the market today that, when used in tandem and chosen wisely for the unique circumstances of any project, will yield long-term use. The main thing is that contractors and tradesman need to keep up on the knowledge of all the materials available today, as well as stay up-to-date on proper install techniques.
Without desire, care, and pride in our craft, we are deemed to repeat the kind of unnecessary rebuilding repairs I've listed above. Here are some steps I follow when doing exterior trim rot replacement on windows.
We usually set up a kind of portable workstation under a canopy. This helps avoid having to make a lot of trips to and from your work truck, manufacturing windowsills, and of course, pre-priming raw material. I know firsthand how this is much more efficient because there is no need to look for materials, tools, or worry about the weather, over-spray or paint drips, disorganization, etc.
We remove the rotten trim and try to determine why it's rotten and if there is moisture infiltration getting into the sheathing, etc. it's a kind of "inspection" as we are performing demolition. Many times the trim is rotting away because of a lack of caulking and of primer on all sides of the material (including cut ends).
3. Trim Preparation
We like to take measurements and manufacture all the trim sizes first, and then sand and pre-prime all sides. I like to use a good weather-resistant wood like fir or cedar that is kiln-dried, so we can prime right away. When measuring and cutting our joints, we pre-prime our cuts after dry-fitting.
Flashing is key! Many times we are replacing trim and windows because there wasn't any head flashing installed in the first place, so we always flash. This includes installing weatherproofing around any window flanges, door rough openings, flashing under doors, or even windows before they are installed.
I have found it extremely important to back caulk all trim before installation with a high-grade adhesive caulking like Sikaflex® . This includes all cut joints, the back side of any siding joints, etc. When the trim goes in the caulking bed it will fill up the back of any voids and help further protect joints. Once we are finished with installing the trim I like to use a high-grade latex caulking and smooth that off with a damp rag.