By EA Paint & Light Carpentry6 min read
At a glance
- Typical timeAverage
- About 5 minutes per side of the house
- Best time to checkAverage
- Late spring, after the snow is gone
- Biggest risk
- Paint over rot fails within 1–2 years
- Test first
- Pre-1978 trim: assume lead until tested
Times are rough averages for planning only. Every house is different — size, condition, repairs and weather all change it. Your written estimate gives the actual schedule for your job.
How do you tell if a window sill is rotted?
Press a screwdriver or awl into the sill at the outer edge and the corners. Sound wood resists and leaves a small dent; rotted wood is soft, sinks in easily and comes up in short, stringy fibres. Paint that keeps peeling in the same spot, dark staining and cracks along the grain are the other warning signs.
Why sills rot first
A window sill is built to shed water, which means it is the piece of trim that gets the most of it. Rain runs down the glass and the sash and lands on the sill. Snow sits on it for weeks. If the slope is shallow, the drip edge is worn, or the joint where the sill meets the casing has opened up, water sits in the end grain — and end grain drinks water the way a straw does.
The same is true of the bottom of corner boards, the ends of clapboard where they meet trim, the water table along the foundation, and the lower ends of porch columns. Anywhere wood ends close to a horizontal surface is where to look first.
The screwdriver test
You need a flat-head screwdriver or an awl, and about five minutes per side of the house. Press the tip into the wood with firm hand pressure — not a stab.
- Sound wood: the tip dents the surface and stops. You have to lean on it to go further.
- Early rot: the tip sinks in an eighth of an inch or more with ordinary pressure, and the wood feels spongy.
- Advanced rot: the tip goes in easily, the wood comes out in short fibres or crumbles, and it may be darker inside than on the surface.
Where to probe
Check the outer front edge of each sill, then both ends where the sill meets the side casings — the ends are where it usually starts. Then the bottom six inches of every corner board, the lowest course of clapboard above the foundation, and the bottom of any door casing that sits near a step or a deck.
Do the north side and any wall that a gutter overflows onto. Those stay wet longest and fail first.
The signs you can see without touching anything
Probing confirms it, but the house usually tells you where to probe.
- Paint that peels in the same spot every year, even after it has been scraped and repainted
- Paint that is blistered or lifting in a patch while the wall around it is fine
- Hairline cracks running along the grain, or a sill that has cupped or split
- Dark staining or grey, weathered wood showing through where the paint has gone
- A joint that has opened up between the sill and the casing, or caulk that has pulled away
- Moss, mildew or small mushrooms at the bottom of trim — wood that is wet enough to grow things is wet enough to rot
What the signs usually mean
A quick way to read what you find. None of these replaces probing the wood, but together they tell you where to look hardest.
Paint peeling in the same spot every year
- What it usually means
- Water is getting into or behind that board
- What to do
- Probe it; find and stop the water source before repainting
Soft, spongy wood under the screwdriver
- What it usually means
- Rot has started
- What to do
- Cut back to sound wood and consolidate, or replace the piece
Wood that crumbles or comes out in fibres
- What it usually means
- Advanced rot
- What to do
- Replace the board and check the framing behind it
Open joint between sill and casing
- What it usually means
- Failed caulk or a board that has moved
- What to do
- Re-seal now, before it lets water into the end grain
Dark stain or mildew at the bottom of trim
- What it usually means
- Wood staying wet for long periods
- What to do
- Probe it, and look for a gutter, splash or drainage problem
Why New England sills take a beating
Massachusetts weather is close to the worst case for exterior wood. Snow sits on sills and against the bottom of trim for weeks, melts in the afternoon sun and refreezes overnight. Water that gets into a crack expands when it freezes and opens the crack a little further, so a small gap in October can be a soft spot by April.
Age adds to it. In Marlborough and Hudson, about 60% of homes were built before 1980, which means a great many original sills that have been through decades of those winters and several rounds of paint. A house does not have to be antique to need sill work; it only has to have had one joint left open for a few seasons.
Tip: Check your sills in late spring, after the snow is gone and before painting season. That is when winter damage is easiest to see and there is still time to plan the repair.
Repair, consolidate or replace?
A small, shallow soft spot in an otherwise sound sill can often be cut back to good wood and rebuilt with an epoxy consolidant and filler, then primed and painted. Done properly it lasts; done as a smear of filler over wet wood it does not.
Once the rot runs deep, runs the length of the board, or reaches the end where the sill meets the casing, replacement is the honest answer. The new piece should be primed on every face — including the back and the cut ends, which is the step most often skipped — and the joints sealed before it is painted. On many houses a PVC or cellular-trim replacement is the better choice for the part that sits closest to the weather.
What does not work is painting over it. Fresh paint seals the moisture in, the rot keeps going underneath, and the new coat starts failing at that spot within a year or two — which is the peeling-in-the-same-place pattern above.
A note on older houses
If the house was built before 1978, assume the old paint on the trim contains lead until it has been tested. Scraping or cutting out painted wood makes dust and chips. Probing with a screwdriver is fine; tearing out a sill yourself is the point to stop and plan for lead-safe work practices, including containment and cleanup.
What this means for your paint job
If the test turns up soft wood, the painter you hire needs to either do the carpentry or work alongside someone who does — and the repair needs to be on the estimate, priced, before the job starts. An estimate that does not mention rot on a house that has it is not a lower price. It is a change order waiting for the ladders to go up.
Talk to a local crew
EA Paint & Light Carpentry is a painting and light carpentry company based in Northborough, MA. If you would like the house looked at, call (508) 466-5846 or request a free written estimate. Related services: Exterior Painting, Light Carpentry & Wood Repair.
