Repairing a Rotten Window Frame: Splicing In New Timber

Cut the decay out of a London sash cill and you are usually left holding a piece of timber about the length of your hand. Either side of the cut the wood is as sound as the day it was machined. That is the awkward fact under most replacement quotes: the failure is local and the price is total.

Rot travels along the grain until it reaches timber dry enough to stop it, and then it stops. So the standard remedy is not a new window. It is a splice: the decayed length cut out on a long slope, a matching piece of new timber glued and pinned into the gap, and the whole element repainted. Joiners were doing this long before there was an alternative, and it survives because a well-cut splice in dry wood lasts as long as the timber around it. It rarely reaches a homeowner because a replacement is one line and one number, whereas a splice needs somebody to survey the window first and say, member by member, what has actually gone.

Rot lands in the same four places

Decay in window joinery is not random. It appears in the same four spots on almost every London sash, each with a mechanism behind it.

The cill. Horizontal, lowest, and it catches everything that runs off the glass. Lose the weathering fall under a century of repainting, or let the drip groove on the underside clog with gloss, and the cill stops shedding water and starts holding it. That is why the cill is usually the first casualty on an otherwise sound window.

The bottom rail of the lower sash. Two things attack it at once. Water sits in the glazing rebate behind failed putty and soaks in from inside. Meanwhile condensation runs down the glass every winter morning onto the same rail. The mortice-and-tenon joints at each end open slightly as the timber moves, and water wicks straight into the exposed end grain of the tenon.

The foot of each frame stile, where it meets the cill. End grain again, sitting on a wet horizontal surface.

The bottom of the pulley stile inside the box. Rarely seen until the beads come off, because it rots from behind.

The common thread is end grain and standing water. End grain absorbs moisture many times faster than the face of a board, which is why the ends of things rot and the middles do not. Timber decay fungi need a moisture content above roughly 20 per cent to get going at all; keep the wood drier than that and nothing grows, whatever species it is. Everything below is really about getting the timber back under that line and keeping it there.

Our guides to wet rot in sash windows and telling wet rot from dry rot cover identification.

Ten minutes with a bradawl, before anyone quotes

Work the whole window, not just the bit that looks bad.

1. Tap along the cill and the bottom rail with a screwdriver handle. Sound timber rings. Decayed timber returns a dull, dead knock. Mark every dull spot in pencil.
2. Press a bradawl or slim screwdriver into each dull spot at about 45 degrees. Sound softwood resists and the point stops within 2 or 3mm. If it sinks 10mm under thumb pressure, that timber is gone.
3. Check the joints, not just the faces. Push the awl into the shoulder line where the bottom rail meets the stile. This is where repairs get skimped, and where a surface repair cannot work.
4. Read the paint. Blistering and flaking in a patch, with a slightly sunken profile underneath, almost always means wet timber below. Crazed, chalky paint over a firm surface is just age.
5. If you have a pin-type moisture meter, take the bottom rail and the cill. Expect a seasonal swing rather than one correct number. The British Woodworking Federation puts equilibrium moisture content in the UK climate at roughly 8 per cent in July and around 20 per cent in November, and specifies that joinery timber should not exceed 13 per cent going into a heated building or 16 per cent into an unheated one. What matters diagnostically is the outlier: a bottom rail reading over 20 per cent after a dry week is not seasonal. It is getting wet from somewhere.
6. Find the water source. Failed putty, a cracked cill, a bridged drip groove, a defective sill below, a leaking gutter, or condensation. Skip this step and the repair fails in the same place.

Filler, resin or new timber

Three different jobs get called "rot repair" and they are not interchangeable.

The professional epoxy systems are genuinely good and are not the same thing as a tub of two-pack filler from a DIY shed. They stay slightly elastic, which matters because a window moves seasonally and a rigid filler cracks at the perimeter and lets water in behind itself. But they have a limit. Once you are replacing more than about 50mm of section, or the decay has reached a tenon, resin is the wrong tool and the answer is timber.

Filler over a rotten tenon is the classic bad repair. It looks perfect for a year, then the sash drops out of square, the joint opens and the rail lets go.

How a splice is actually cut

Remove all the decay plus a margin, cut a long sloping joint so the new piece is mechanically held and sheds water outwards, then glue in matching timber.

1. Cut back beyond the decay. Sound timber only, and take at least 25mm past the last stained or softened fibre. Decay runs further along the grain than it looks from the surface.
2. Cut a splayed or scarf joint, never a butt joint. A shallow slope, in the order of one in six, gives a long glue area and no vertical shoulder for water to stand in. On a cill the slope is always cut so the joint sheds outward and downward. A square butt joint on an external cill is a rain trap.
3. Treat the exposed sound timber. A boron-based preservative, or boron rods inserted near the joints, applied to dry timber. Let it dry before gluing.
4. Machine the new piece to match. Same species, same profile, grain running the same way, moisture content close to the existing frame. Matching the moulding matters more than people expect, because paint does not hide a profile that is 2mm out.
5. Glue with a gap-filling adhesive. Epoxy or polyurethane, not PVA. PVA wants a tight, clean, dry joint and does not tolerate the real tolerances of an in-situ repair or any later wetting.
6. Fix with non-ferrous screws or hardwood dowels. Stainless steel, silicon bronze or brass. Never plain steel: ferrous fixings in damp timber rust, stain the wood black, blow the paint above them and can start the next round of decay themselves.
7. Prime the end grain twice before final assembly. End grain drinks primer and it drinks water. Two coats there is worth ten on the face.
8. Make good, sand flush and paint the whole element, not just the patch.

Where a sash has come out of the frame, a decayed rail end can be renewed complete with its tenon and the joint re-wedged and re-glued, which is a far better result than any surface repair. SPAB takes the same line on wood windows: a skilled carpenter letting in well-seasoned matching new timber, a rotten cill section cut back in situ and held with glue and non-ferrous screws, and replacement of the whole window only as a last resort. That is what a sash window restoration amounts to in practice. Keep the box, the pulleys and the old glass, and renew only the timber that has failed. Across the London boroughs that is the bulk of what our sash window repair service gets called out to do.

What to splice in

The original sashes in most London houses are slow-grown Baltic redwood, imported through the 18th and 19th centuries. Look at the end grain of a Victorian sash and you will often count fifteen to twenty growth rings in an inch. Modern commodity redwood is frequently five or six. Dense, tight-ringed timber with a high proportion of heartwood is dramatically more durable than fast-grown plantation stock, which is a large part of why the original windows have outlasted the 1990s replacements next door.

• Slow-grown European redwood. The like-for-like option. Low natural durability, so anything exposed externally should be preservative-treated, ideally double vacuum treated, before it goes in.
• Accoya. Acetylated softwood, rated Durability Class 1 to EN 350 and carrying the manufacturer's 50-year above-ground warranty, with swelling and shrinkage reduced by 70 to 80 per cent on the manufacturer's own figures. Excellent for cills and other high-exposure elements, and it holds paint unusually well. It costs more per metre and it needs stainless fixings, A2 minimum and A4 near the coast, because residual acetic acid corrodes galvanised and zinc-plated steel. More on where it earns its keep in our note on Accoya for sash windows.
• Hardwoods such as sapele or oak. Durable, but they move differently from the surrounding softwood, and oak's tannins corrode ferrous metal and stain paint. Use only where the original was hardwood.

Match the species where you can. Two timbers with different movement characteristics glued along a long joint will eventually telegraph the joint line through the paint.

Cost, and where it stops making sense

Rough London ranges, and they move a long way with access, storey and condition.

Anything above ground floor needs access, and a tower or scaffold can add more than the joinery does. Ask whether the price is per window or per elevation, and whether access is inside it.

The honest threshold: where more than roughly a third of a sash's section is decayed and the decay has reached both the rails and the stiles at the joints, a new sash made to match is usually better value than a run of splices. New sashes into the original box keep the pulleys, the weights and the external sightlines, which is what the elevation is actually made of. Replacing the whole window, box included, is a much bigger and rarer job, and it reopens the planning question if you are in a conservation area.

Repair sits outside the Building Regulations regime for replacement windows. Splicing, re-puttying and overhauling are maintenance, not the installation of a controlled fitting, which is why neither the Part L U-value standard nor a FENSA or Certass registration comes into a repair at all. If the building is listed, genuine like-for-like repair does not normally need listed building consent, but your local authority is the only body that can confirm that for your building, and a call to the conservation officer costs nothing.

Making the repair last

The splice is only as good as what goes over it. A spliced cill that is never repainted will be soft again in eight years.

• Re-putty properly where the putty has failed, and let it skin before painting. The paint must lap about 2mm onto the glass to seal the putty edge; that detail is what keeps water out of the rebate.
• Paint the cill and bottom rail on a real cycle. South and west elevations in London take the weather and generally want attention every five to seven years; sheltered north elevations stretch further. Order of work matters, and our guide to painting sash windows properly has the sequence.
• Rake out the drip groove along the underside of the cill. Bridged with paint, it walks water back to the wall face instead of letting it fall clear.
• Every autumn, check the glazing putty has not cracked away from the glass. Cracked putty is the cheapest thing on this list to fix and the most expensive to ignore. The choice between linseed and acrylic is in our guide to glazing putty and how long it lasts.

Two questions separate a firm that has surveyed your windows from one that has glanced at them. Ask for the price broken down by element: this cill, that bottom rail, those two stile feet. A quote written that way shows how much of the window has genuinely gone, and a single figure per window shows nothing. Then ask what is being done about the water, because a splice into a frame that is still getting wet has an expiry date on it. On most houses the answer comes back the same way: two or three windows want joinery, and the rest want paint and a clear drip groove.