Last year, when I was shopping for a top-drawer roofing contractor, I took advantage of the time I had to do some repair work on my fascia boards and soffits. It made sense to take care of this before the new felt, shingles, and flashing went on.
The first thing I did was to walk around the house with a notebook, and do a close visual inspection to see what I was in for. For the most part it was all in reasonable shape, but a few spots were begging for attention. I even found one spot where a bird had made or enlarged a hole, and was homesteading in the soffit!
If you're planning a similar project, take your tape measure with you, in addition to a notebook. When you spot some fascia board or plywood on the bottom side of the soffit that needs replacement, take a measurement and make note of it. Also, make note of the dimensions of the existing wood.
If your fascia boards are 12' long, in most cases it's not necessary to replace the whole length. The only thing to remember is that the break should be on the center of a rafter end, for nailing purposes.
On my house, the fascia board is 1" X 6" and the plywood is 3/8". That's rather standard in my neck of the woods, but it might vary depending on where you live. Another thing to make note of is how many soffit vents you need to buy.
Soffit vents are those rectangular vents on the underside of the soffit. They're usually made of galvanized metal, and covered with a nylon mesh or netting to prevent bees and such from taking up residence in your attic. Note that Hardiplank soffit material has the vent built in. This is a great option on new construction.
When you go to the home improvement store for materials, remember to get a little extra to allow for bits that are just too small to use. There's always some scrap.
Replacing Fascia Board
When you get ready to replace fascia board, you'll have a choice of what material you want to go with. Hardiplank is a good choice since it's guaranteed not to rot, delaminate, or warp. This is because it's a fiber-cement material. Because of this, you can go a lot longer between paint jobs; expansion and contraction is minimized.
Myself, I just went with pressure-treated wood. With the re-roofing job coming up, my pockets just weren't deep enough for Mr. Hardi.
Before removing the old fascia board, I had to carefully prop up the shingles. Next, I removed the flashing. Now, to cut out the bad sections of board, I set my circular saw blade to a 45-degree angle, and set the depth to just cut through the board -- but not so deep as to cut into the rafter more than 1/16". I know, I'm picky...
At this point, I replaced any under-soffit plywood that I needed to. It made more sense to cut the holes for the soffit vents before nailing it up, so I did that.
Then I just cut the new fascia boards and nailed them up. I used my airless finish-nail gun to do this, but a hammer and nail-set would have been almost as easy.
Finishing the Job
Don't be in too much of a hurry to get the flashing back on. The first thing I did at this point was to putty all the nail holes and caulk all the joints. (Always be careful when you buy caulking. For this job, it must be labeled "paintable.") The joints were where the fascia boards mitered together. Also, where the soffit plywood met the house, other plywood, and the fascia board.
Next, I primed and painted the face of the fascia boards, which freed me to replace the flashing and let the shingles back down. Knowing that my work was now protected from any impending rain, I could tackle the job of priming and painting everything else.
The final part of the project, which I'm still working on, was to add extra soffit vents. This was recommended by my roofer, because the new ridge vents had a higher air flow capacity than my previous system. How many CFM's? I haven't a clue.
I will tell you that this wasn't be the fastest DIY project I've ever tackled. Everything is detail and it's all "cut-up." Don't attempt it without a stack of CDs.