Every year, maybe a couple times a year, a homeowner should look for evidence of water penetration into the house. Why, you ask? Because when water gets into the structure of a house, one, you can’t see it. Two, it cannot evaporate, so it will decompose the wood structure (aka framing). Three, it will be a water source for insects and mold. You won’t know there’s serious rot happening until the collapse of the wood results in the surface of the wood being affected. That takes months usually.
How do you recognize water penetration? Where there is a crack at the junction of two house elements, for example where siding butts up against a door jamb, there will be a darkening of the caulk if water is getting in. If there’s not caulk there, then water is getting in there, and it is wetting the backside of the wood, and the wood will stay wet since the water cannot evaporate due to lack of exposure to outside air. How soon it will rot depends on how much water gets into the house at any one time. Often those little cracks don’t let in much water, but if the crack(s) is significant, the water will drip down the wall behind the wood and will accumulate at the the subfloor and floor framing.
The most common areas for water penetration are thresholds of doors, sills of both doors and windows, and siding to trim junctions. Look for cracks and darkening of the caulk. If you see both, test the wood with a screwdriver or chisel to see if it’s solid – simply press the tip into the wood. It should not penetrate at all. You can scrape off the old caulk and recaulk with a good, 40 year acrylic latex caulk plus silicone. Do not use 100% silicone, because it doesn’t work in wet areas. It will create a wick for moisture to enter at that spot. That’s experience talking. I recommend DAP Alex Plus, a 40-year caulk that goes on easy, smoothes with a moistened finger tip, cleans up great, and is under $3 a tube. acrilyc latex with silicone
As a homeowner, I believe you should check all your wood-to-wood junctions on the exterior of your house every spring. Select a warm day and spend the whole day examining every joint on the exterior of your house. Check your roof eaves, fascia boards, and roof edges. If that’s too much for you, or beyond your abilities, call a painter to do it. They have ladders, and a real painter knows how to use caulk (believe it or not, I’ve met painters here in Maine who will not use caulk – it’s mind boggling).
If you let this chore go, it’s possible that a simple water penetration problem will become a complex structural problem that will affect your budget in ways you will not like. Look at two of my jobs on this site. The door repair job was five thousand, and the 3-season repair was ten thousand. It’s much wiser to inspect every spring, and nip these problems before they develop into something more. Remember, you can’t see the development of the problem, because the water is hidden from site once it gets into the framework. It’s there, where evaporation is not possible, invisible to you.
For those who like words, it’s called hydrolysis, which is the decomposition of wood by water. The water spreads through a home’s frame by capillary action, which is how water moves up a wick in a kerosene lamp, and how sap runs up a tree’s interior, without a pump. These two links explain it.