Rot in Window Sills and Door Jambs

Window sills and door jambs are small pieces of a house, and they are where the wood on most San Diego homes fails first. On a stucco house the walls may be sound for decades while the sills under the windows quietly soften. On an older wood home in National City or the neighborhoods of San Diego near the water, they are usually the first place a careful painter looks.

The reason is the same everywhere: these are the places water sits rather than runs.

Where does rot start on a window or door?

Rot needs wood that stays damp. On windows and doors, a few spots stay damp longer than anything else on the house, and those are where it begins. Checking those spots first finds most of the rot on a home in a few minutes.

• The flat top of the sill. A sill is meant to slope outward so rain and sprinkler water run off. Many older sills have lost their slope, or were built nearly flat, and water pools in the corner against the window frame.
• The ends of the sill, where it runs into the stucco or siding. End grain soaks water up like a bundle of straws, and the caulk joint there is often the first to crack.
• The front edge and underside, where drips gather and the paint is thinnest.
• The bottom of a door jamb, the last few inches above the threshold. Water from rain, sprinklers and mopping collects there, and the end grain sits right in it.
• The brickmould or casing at the bottom corners of a door, for the same reasons.
• Any joint where two pieces of trim meet, if the joint has opened.

The marine layer adds to it. Near the coast and the bay, morning damp sits on horizontal wood surfaces on many mornings of the year, and a sill that never fully dries between mornings is a sill that will rot sooner.

How to find it before it spreads

You do not need special tools. Take a screwdriver or an awl, and a little light.

1. Look first. Paint that is cracked along the grain, bubbled, or peeling on a sill when the wall around it is fine is a sign of moisture underneath. So is a dark stain at the bottom of a jamb, or a gap opening where the sill meets the frame.
2. Press, do not stab. Push the tip into the wood at each of the spots above. Sound wood resists. Rotted wood lets the tip sink with little effort and feels soft or spongy.
3. Check under the window, too. Stains on the stucco below a sill, or efflorescence there, often mean water has been getting in behind the sill for a while.
4. Do the same on the inside. A soft interior sill or a stained wall under a window points at a leak in the window itself, which is a different conversation from paint.

If you find soft spots, make a note of where. It saves time at the estimate and helps tell a small repair from a large one.

How is it repaired?

The approach depends on how much of the piece is gone. Ebenezer looks at each sill and jamb during the repair check after the power wash, and tells you what he finds before any work begins.

Small, contained rot

Rot limited to a corner, the front edge or the bottom inch or two of a jamb is cut or dug out back to firm, dry wood. The opening is treated and filled with a material made for exterior wood, then shaped to match the profile of the piece and sanded so the repair is invisible under paint. The materials themselves are covered in epoxy filler and consolidants for exterior wood.

Larger damage

When rot runs through the thickness of a sill or along much of its length, filling it is a patch on a failing piece. The better repair is to replace the damaged section or the whole sill. On a door jamb, the bottom of the jamb can often be cut off above the rot and a new piece spliced in, rather than replacing the entire frame.

When it is more than trim

If the rot has reached the framing behind the sill, or the window itself is leaking into the wall, that belongs to a framing contractor or the window installer. Ebenezer repairs the sill, jamb or trim itself and then paints it, and he will tell you plainly when a repair belongs to another trade.

Making the repair last

A repair that only replaces wood will rot again in the same place if nothing else changes. These steps are what keep it from coming back:

• Restore the slope of a sill where possible, so water runs off rather than pooling.
• Seal the end grain before priming. It is the most absorbent part of any board.
• Prime all bare wood and every repaired area, then apply full coats of paint, paying attention to the front edge and the underside where film is usually thinnest.
• Caulk the right joints. The joint between sill and stucco, and between casing and wall, should be sealed. The drip edge under a sill should not be caulked shut.
• Look at the source. Turn sprinkler heads away from the wall, add or replace a door sweep, and check that thresholds slope out.

Why it is worth catching early

A sill with a soft corner is a small repair. The same sill after another two wet winters may need replacing, and the wall below it may need work too. That is why a repaint is a good time to deal with it: the trim is being prepared anyway, and the repair disappears under the new finish.

Ebenezer’s stucco, drywall and wood repair covers this kind of work: he repairs the wood and then paints it. Older homes in National City and San Diego tend to have the most of it, but any house with wood sills can.

If you have pressed a screwdriver into a sill and it went in further than it should, ask for a free estimate. He will check every window and door, show you what he found and tell you what each repair involves.