Common Causes of Wood Rot Around Doors and Windows

Rot around an opening is never really about the wood. It is about water getting somewhere it was not meant to be and staying there. Replace the timber without finding out how the water arrived and you will be doing it again in a few years.

Why openings rot before the rest of the wall

A wall sheds water. An opening interrupts it. Every door and window is a hole cut through the weather barrier, and everything relies on the details at the edges — flashing, sealant, and the sill — working together to route water back out.

Those details have a service life shorter than the house. On the older stock around here many openings were fitted before modern flashing practice existed at all, and plenty have been replaced since by someone who sealed the outside and hoped.

The usual causes, in order

Missing or wrongly installed flashing

This is the big one. Flashing has to direct water out and down, layered so each piece laps over the one below. Fitted the wrong way round, or omitted at the head of an opening, it channels water into the wall instead. The damage then appears below the opening, which sends people looking in the wrong place.

Failed sealant treated as the fix

Sealant is a secondary defence with a service life of a few years. When it fails, water gets behind it — and running a new bead over the top traps whatever is already in there. Sealant should never be the only thing between weather and framing.

Sills that hold water instead of shedding it

An exterior sill should slope away and have a drip edge so water leaves rather than tracking back. Flat sills, sills that have settled backwards, and sills where a drip edge was filled in during painting all hold water against the bottom of the frame — which is why door frames rot from the bottom up.

Ground and splashback

Anything within a few inches of the ground stays damp. Rain hitting a hard surface splashes back onto the bottom of a frame repeatedly, and on the sloped lots here, runoff arriving at a doorway does the same on a bigger scale.

Condensation from inside

Less obvious and genuinely common. In a house with high indoor humidity and single-glazed or poorly sealed units, moisture condenses on the coldest surface in the room — the glass and the frame — and runs down into the sill. That rot starts inside and works out.

How to find the actual source

Press a screwdriver into the suspect wood, gently. Sound timber resists; rotted timber takes it with no effort. Then work outward and upward from the softest point — water travels down, so the entry is almost always above the damage.

Check the head of the opening, not just the sill. Check whether the damage is worse at one end. Look at what is above the opening entirely — a failed gutter, a roof edge dumping water, or a upper-floor window doing the same thing.

Repair or replace the unit

Rot confined to trim, casing or a section of frame is a repair — cut back to sound wood, replace with material rated for the exposure, prime the end grain before it goes in, and correct whatever let the water in.

Where the frame itself has gone at the corners, or the damage has reached the framing around the opening, replacing the unit is usually better value than repairing the same thing twice. And if the rot has reached the structural framing, that is a different job again.

What we would do

Probe the surrounding wood before quoting, because rot always runs further than the visible part. Find the source. Fix that first. Then replace what has gone with the right material and seal it properly.

Wood repairs covers rot work generally, rotten wood repair goes into the cut-back-and-replace side, and door and window installation covers replacing a unit where the frame is beyond saving.