If you want to protect wood outside, the two most important things to do
1. eliminate any places where water could accumulate in puddles on the
2. Prevent liquid water from coming into contact with the end grain of
Wood absorbs liquid water through it's end grain 15 times faster than it
does across it's grain.
So, when wood is exposed to rain water or snow melt, the water that
comes into contact with the end grain is absorbed, and that causes the
wood cell walls to swell. That water that isn't absorbed into the wood
cells right at the end grain diffuses deeper into the wood and the wood
cell walls further from the end grain swell up as their moisture content
increases as well.
But, then the rain stops, the Sun comes out, and the wood starts to dry.
The water in the wood evaporates fastest at the end grain as well, and
that results in the wood drying at it's end grain faster than it does
only a few inches in from the end grain.
The result of that differential drying is that the wood will start to
split at it's end grain.
And, wherever you have splits in the end grain of the wood, those are
places where water will be sucked in by capillary pressure, and form
places for wood rot to start.
If you ever see weathered wood outdoors, you will notice that the end
grain is all slit up on it. However, you never see that in the exposed
2X4 wall studs or roof rafters of an unfinished garage. It's absorbtion
of WATER into the end grain that causes wood to split, and those splits
then suck liquid water in by capillary pressure. When that happens, you
have all the right conditions for the wood to start to rot at those end
If you have exposed rafters that stick out from under your roof line,
the most important thing to do is prevent liquid water from coming into
contact with the end grain of the wood. You can do that by painting
that end grain, or by fitting some sheet metal over the end of the
rafter (kinda like a cover), but have that cover open at the bottom so
that any water to leak in can leak out, and allow an inch or two of
space between the end of the rafter and the end of the cover to allow
air circulation around the end grain of the lumber to keep that end
grain dry. Glue that cover on with caulk or roofing cement and paint
it with the same exterior latex you use on the rest of your exposed
rafters and trim.
Posting a picture in here of your exposed rafters would help us better
formulate a game plan for dealing with the wood rot.