Here is a puzzle that comes up a few times a year. A homeowner has fascia board going soft along one side of the house. The gutter above it is clean, hung straight, and has never once overflowed that anybody has seen. There is no leak at the corners, no clog in the downspout, and no standing water in the trough. By every visible measure the gutter is doing its job. The wood underneath it is still rotting.
What is happening is that some of the water never gets into the gutter at all. It comes off the last inch of shingle, curls around the edge, and runs backward along the underside of the roof deck until it reaches the fascia board, where it runs straight down the face of the wood behind the gutter. You cannot see it from the ground because it is happening in the half inch of space between the back of the gutter and the house. During a storm that space is a private little waterfall, and the gutter in front of it is catching almost everything else and looking fine while it happens.
Surface Tension Does Not Care About Gravity
Water sticks to things. Everyone knows this from pouring from a pitcher and watching a dribble run down the outside instead of into the glass. The same effect happens at a roof edge, and asphalt shingles are especially good at causing it because the edge is rough and slightly absorbent. Water moving slowly will follow that edge around the corner rather than launching cleanly off it. A hard downpour has enough momentum to throw water out into the gutter. The steady rain Augusta gets for hours at a stretch in the winter does not.
The piece that solves this is a strip of bent metal called a drip edge. It runs along the roof edge under the shingles with a lip that kicks out and down, so water has to leave the roof at a defined point instead of wrapping around wood. It is inexpensive, and since the 2012 code cycle it has been required on new asphalt shingle roofs. That last part is the qualifier that matters. Plenty of houses in the CSRA have roofs older than the requirement, and plenty of quick reroofs skipped it, so a lot of homes here simply do not have one.
A gutter apron is the same idea with a longer bottom leg that reaches over the back edge of the gutter. Drip edge kicks water off the roof and trusts it to land in the trough. An apron carries it past the back lip and drops it inside. Where the gutter sits low relative to the roof line, the apron is the better of the two, and it is the easier retrofit, since it can usually slide up under the first course of shingles without taking the roof apart.
How to Tell Which Problem You Have
The distinction matters because overflow and behind-the-gutter water look identical after the fact. Both end in stained fascia and eventually rotten wood. They call for completely different work.
Overflow leaves evidence on the front. Dirt streaks down the outside face of the gutter, mulch scattered in a fan below it, a groove worn into the bed underneath. Water going behind the gutter leaves the front face clean and puts the staining on the fascia itself, on the corner of the soffit, and sometimes on the top edge of the siding. Behind-the-gutter water also leaves a distinct vertical dirt track on the wood, and that track will feel spongy if you press a screwdriver into it.
Timing is the other clue. If the trouble shows up during light steady rain rather than during thunderstorms, that points to surface tension rather than volume, because volume problems only appear when it is really coming down.
One more that surprises people: a house with no drip edge often has healthy fascia on three sides and one bad side, and the bad side is the one that gets the least sun. Wood that dries out in an afternoon does far better than wood that stays shaded until Thursday. North-facing runs and anything under a tree canopy go first.
What Getting It Right Involves
Adding a drip edge or a gutter apron to an existing house is not a large job, but it does mean loosening the gutter or taking it down, which is why it usually gets bundled with other gutter work rather than done on its own. If the gutters are coming off anyway for re-hanging or replacement, that is the cheapest hour you will ever spend on this problem, because the access is already paid for.
If the fascia has already gone soft, the sequence matters. New metal over rotten wood just hides it. The board comes out, the new board goes in, the drip edge or apron goes on, and the gutter goes back up with hangers screwed into wood that will hold them. Doing those steps out of order means paying for the same access three times.
None of this is exotic. It is a piece of bent aluminum at a couple of dollars a foot, protecting a board that costs a great deal more than that to replace. It is just invisible from the ground, so it is the last thing anybody thinks to check.