Porch Rafter Beam Damage
beam damage and rafter damage is pretty common, when that wood is
exposed to the weather, like the picture above. Not only will the
rafters suffer from damage but the beam that is holding the rafters up
can also suffer.
The beam that holds the rafters will often suffer because it runs horizontal and could hold moisture for long periods of time, lying for hours on top of the beam. This moisture will in turn, often soak into the rafters, where they connect to the porch beam.
The end of this rafter is badly damaged, in comparison to the other porch rafters. This one is hard to explain, how the damage actually happened.
Look for water stains and other wood rot, while tearing a porch roof like this apart. You will have to look for all the clues, as you are dismantling the wood damage.
The first thing I would look at, would be the damaged porch rafter beam. Look for any large cracks or knots that would allow moisture to soak in faster than the other porch rafter beams. Sometimes the wood isn't painted enough and allows moisture to soak right through the paint into the wood.
Since these wood beams are exposed to the moisture, as well as the heat, these cracks could have formed after the wood was painted properly. Small cracks allow some moisture in and large cracks allow more moisture in, allowing the wood to rot faster.
All cracks and loose knots should be caulked and painted to eliminate future problems in the other beams, anyway.
Using a little common sense and practical knowledge from websites like these can prevent future problems to your home.