Bad Wood?
We built our house in 2004 using the same cedar trim and installation methods I used on our garage and sheds. Unfortunately, while the shed trim is over 25 years old with no issues, several pieces of trim on the house had significant rot after just 14 years. To make matters worse some of the rot had gone deep enough to damage areas of the underlying plywood siding as well. So this time around I was determined to take every precaution I could to prevent rot in the future.I thought about using PVC lumber for the trim but read that it wasn't recommended with dark colored paints due to heat expansion and contraction. I preferred to stick with wood anyway. The new 1x cedar lumber I found at Home Depot didn't look very promising for long term durability. It looked to be mostly sap wood and didn't look like it would hold up very well. So, I opted to buy 5/4 cedar decking boards and plane them down to the 3/4 inch thickness I needed for the trim. The decking boards were much better quality and had a nicer finish once they were planed down. Once they were cut to size I applied two coats of an oil based exterior primer to all ends and sides of the trim.
Where the siding had been damaged I removed any wood that was rotten, then used a two part epoxy wood filler to fill and level the holes. The deeper areas required two applications, sanding after the first. The siding on the front corner of the house near our stairs had significant rot and would have been difficult to fill on the corner. The siding is already two layers thick on the front of the house, so I cut away the damaged wood and installed new lumber to replace the damaged siding. I used the epoxy filler to adjust for gaps and to fill the cracks around the patch.
Once the siding was repaired and repainted, I applied window flashing membrane around our front windows. Because the trim is applied on top of the siding, the membrane probably won't serve much function as flashing, but I used it mostly as an isolation membrane between the siding and trim. If the trim should rot again in the future, I'm hoping the membrane will prevent the siding from being damaged also.
I decided to build the front window trim as a single unit. This allowed me to fill any holes and gaps on the back and prime all sides of the trim before installation. I used pocket screws and polyurethane glue for the joints and the epoxy wood filler to get things smooth. I routed rabbets on the back of the trim to provide clearance around the window flanges and grooves at the bottom to allow drainage for any water that may find it's way behind the trim in the future.
I replaced the entire trim on three corners of the house, but the front corner trim near the steps is complicated. It fits behind and around the roof flashing and would cause more issues trying to replace it. The upper half was in good condition so I opted the cut the trim below the roof line and patch in new lumber. I had some flashing membrane left over so I applied some behind the trim on that corner as well. Then I used the epoxy filler where the new trim boards met the old. Once it was sanded and painted I can't even tell where the joints are in the trim.
Ironically, I have one other exterior corner on the house and the trim boards there are in perfect condition. I'm not sure why they held up better than the other corners, but I didn't need to replace that corner. The new siding stain matches fairly well, but we will probably need to restain the siding in a year or two anyway so it doesn't really matter. I did have to replace the caulking around the windows on the south side of the house and repaint that trim. I think the vinyl windows expand and contract more on the sunny south side of the house and had separated from the caulk. It hadn't caused any issues, but I caulked and repainted anyway just to be safe.