The screwdriver test takes about four seconds and it settles most of the argument before anyone gets a quote.
A sill at Thirroul, north-facing bedroom window, looked terrible from the driveway. Grey, cracked across the grain, paint hanging off it in strips. The owner had been told by someone that the whole window needed replacing. I put a screwdriver into the middle of it with thumb pressure and it stopped dead after about a millimetre. Went to the left-hand end, where the sill runs into the jamb, and the blade went in to the handle.
Two different problems on one piece of timber, a metre apart. That is the thing about rot. It does not spread evenly and it almost never starts where it looks worst.
Weathered is not rotten
Bare or failing paint on the western and southern faces down here goes grey and fibrous within a couple of years. Salt and UV break the surface fibres down and the timber looks dreadful. Underneath, it is usually fine.
Sound timber stops a screwdriver. That is the whole test. If you can push a blade in with thumb pressure and it keeps going, the wood has gone soft, and soft means decay rather than weathering. Dark patches that stay damp days after rain are the other tell.
Sand a weathered sill back and you find clean timber under a few tenths of a millimetre of grey. Sand a rotten one and you find more rot.
Worth doing the test in three or four spots per sill. People test the middle of the face, which looks the worst and is usually the soundest part of the whole thing.
Where it actually starts
Water has to sit somewhere and get in somewhere, and on a window that is nearly always the same two places.
The joint where the sill meets the jamb is first. Everything sheds water into that corner, the joint opens up a fraction as the timber moves, paint bridges the gap and hides it, and then it wicks. By the time it shows on the surface the end of the sill has been wet for a couple of years.
The second one is the underside. A sill should have a drip groove routed into the underside near the front edge, a shallow channel whose only job is to stop water tracking back along the bottom face and into the wall. Four coats of paint over a few decades fills it in. Once it is bridged, water runs back underneath and sits against the frame where nobody ever looks. If you can get a finger under a sill, run it along and feel for the groove. A lot of them have not existed for years.
Fascia is the same story in a different spot. The failure is the end grain, at the mitre on a corner or wherever two boards butt. End grain drinks water about ten times faster than the face does, and the ends are the one part nobody paints properly because you have to be up there with the gutter in the way.
Which brings up the other half of it. A gutter that overflows dumps water straight down the back of the fascia, every time it rains, on a surface nobody can see. If your gutters have not been cleared in a few years, the fascia behind them is where the bill turns up, and the gutter clearing post covers how often that actually needs doing around here.
When filler is a real repair
Two-part epoxy repair systems are genuinely good and they have a bad name because people use them for the wrong thing.
They work when the decay is a pocket with sound timber around it. Cut or scrape every soft fibre out until you are into wood that stops a screwdriver, and go further than feels necessary, because decay runs ahead of what you can see. Let it dry properly. Treat the exposed timber with a wood hardener or preservative. Fill in layers rather than one lump, shape it slightly proud, cut it back once it has gone off, then prime it the same day. Epoxy left bare picks up UV and chalks.
Done that way, a filled sill end lasts years and you cannot find it once it is painted.
Where it fails is when it is used as a cover. Fill over soft timber and you have sealed moisture into the middle of a piece of wood and given it a lid. It comes back, and it comes back worse, because now it is wet all year instead of drying out between showers. I have cut out filled sills where the epoxy was the only solid thing left in the section.
The rough line I work to: if more than about a third of the section is gone, or the soft stuff runs the length of the piece rather than sitting in a pocket, it is a replacement. Filler is for pockets.
The bit where a fascia job stops being ours
Sill replacement is usually straightforward. Cut the old one out, match the profile, prime all six faces before it goes in including the two ends, bed it, seal the joints, paint it.
Fascia is not, and the reason is nothing to do with the timber. The gutter is fixed to the fascia. To replace a fascia board, the gutter comes off first and goes back on afterwards, and guttering is roof plumbing in NSW. That is a separate trade with its own qualification and it is not one we hold, so a full fascia replacement is a job we hand to a roof plumber rather than quote. Same answer we give on gutters, downpipes and metal roofing generally, and the roof repairs page sets out where that line sits.
What we can do is fascia end repairs where the gutter stays put, plus all of the sill, architrave, barge board and trim work that goes with them.
One more thing on older houses. Eaves and soffit linings on anything built before about 1990 can be asbestos cement sheet, and rotten fascia often means somebody wants the lining pulled down to get at it. Nobody should be cutting or drilling that sheet on a hunch. It gets identified first.
Licensing, since it comes up
Repairing and replacing timber trim is carpentry work in NSW, which the government defines as cutting and placing timber for on-site construction work to put up framing or fix components, "such as stairs, windows, doors, floors and wallboards". It is trade work, so it sits under the usual test: a contractor licence is needed once a job is valued at more than $5,000 in labour and materials including GST, and not below that.
The painting afterwards has a wrinkle worth knowing. NSW dropped the licence requirement for stand-alone contracts for internal paintwork back in 2015, but the painting work page is clear that the exemption is for internal paint on its own, and that painting is otherwise trade work needing a contractor licence above $5,000. Exterior work, and paint that comes as part of other building work, sits under the threshold like everything else. Sill and fascia repairs are exterior, and the repainting is part of the repair, so that is the rule that applies to them.
None of which changes what we take on. It is all repair-scale work well under the figure.
Do the boring thing instead
Almost every rotten sill I cut out was preventable with a brush.
Paint the end grain. Every cut end, every mitre, every joint, primed before it goes in and sealed after. It is the single highest-value five minutes on the whole job and it is the one that gets skipped because the ends are not visible.
Keep the drip groove open. If you are repainting sills and you can feel a filled groove, scrape it out before you put more paint over it.
Check the corners after a wet spell rather than a dry one. Damp shows up where the water is sitting, and in three months of dry it shows up nowhere.
And get on top of the exterior paint before it fails rather than after. Once the film breaks and the timber is drinking, the clock has started, and the repaint that would have cost a weekend becomes carpentry. The same logic runs through the decking post, which is the same problem lying flat and getting rained on harder.
The Thirroul sill got one end cut out and spliced, the rest sanded back and repainted, and the window stayed where it was. Somewhere between "it is fine" and "replace the whole window" there is usually an afternoon's work that fixes it, and the screwdriver tells you which.
If you want a look at yours, send a few photos with a close-up of each end of the sill and one along the fascia line. Sill and trim repairs run in with the painting work we do, and it is usually the same visit.
Rot and repair questions
How do I tell if a window sill is rotten or just weathered?
Push a screwdriver into it at an angle, using thumb pressure only. Weathered timber is grey and fibrous on the surface but stops the blade almost straight away, and it sands back to sound timber. Rotten timber lets the blade sink in, comes away soft and crumbly, and is usually darker and damper than the wood around it. Test the ends and the joint where the sill meets the jamb, not the middle of the face, because that is the last part to go.
Can you fill a rotten window sill instead of replacing it?
Sometimes, and only with a two-part epoxy repair system rather than ordinary putty or builder's bog. It works when the decay is a pocket surrounded by sound timber, once all the soft material is cut right back and the exposed wood is treated and dry. It is not a fix for a sill that is soft the whole way along or has lost a meaningful part of its section. Filling that is hiding it, and it comes back.
Who removes the gutter to replace a fascia board?
A roof plumber. Guttering, downpipes and roof flashings are roof plumbing in NSW, which is its own trade category, and the gutter has to come off before a fascia board can. We do not take that work on at any value, so a full fascia replacement is a job we hand over rather than quote. Fascia end repairs that can be done with the gutter left in place are a different matter.
Want a hand with this at your place? Get a free quote or call 0450 959 936.