Timber Window Repair: What Can Be Saved, What Can't

"Beyond economical repair" is the phrase, and it gets used about London timber windows a good deal more often than the timber deserves. The question worth asking is not whether your sashes have decay in them, because almost anything over eighty years old has some somewhere. It is which members have gone, and whether those members are ones a joiner can renew.

A box sash is not a single object. It is a dozen separate pieces of timber doing different jobs: two sashes of rails and stiles, a boxed frame with pulley stiles and linings, a cill, and the beads holding it all together. Each fails in its own way and at its own price. Work out which have failed and you have most of your answer before anybody quotes.

The verdict, member by member

That is what a joiner is doing when he walks round with a bradawl: sorting the members into piles.

Two things follow. Most of what rots on a sash window is a component, and a component can be renewed without touching anything around it. The box frame is the exception, because it is the part genuinely built into your wall.

The SPAB takes the same view and is blunter than most: historic windows should almost always be retained rather than replaced, and replacement is a last resort. The reason is practical as well as conservation. Original London sashes were slow-grown Baltic redwood, tight-ringed and full of heartwood, and timber of that quality is hard to buy now at any price.

Why it is always the bottom

Read back down the table and almost every failure is at the bottom of something. Three mechanisms account for that.

End grain. Cut across the grain and you open thousands of tubes. A sawn end takes up water many times faster than a planed face, and every stile on a sash window points its end grain straight down at a wet horizontal surface.

Standing water. The bottom rail is horizontal and the putty line above it cracks first, because that is where the paint film is thinnest and the movement greatest. Once water is behind the putty it sits in the glazing rebate with nowhere to drain, and condensation running down the glass on winter mornings arrives at the same rail from the other side.

Capillary action at the cill. Where the box sits down onto the cill there is a joint that never really dries, and water is drawn up into it against gravity. A cill whose weathering fall has been flattened by fifteen coats of paint, or whose drip groove is bridged with gloss, sends water back towards the wall instead of dropping clear. The cill repays understanding on its own.

None of it happens in dry timber. Decay fungi cannot establish themselves below roughly 20 per cent moisture content, which is why a well painted sash in a sheltered street runs two centuries while a neglected one on a west elevation goes soft in fifteen years.

Nearly all of it is wet rot. Dry rot, Serpula lacrymans, wants humid, still, unventilated conditions, so in a window it turns up inside the box or behind shutters screwed closed for forty years, and it leaves timber dull brown and cracked into cubes. Find that in a pulley box and take advice before cutting into it, because dry rot in a window usually means dry rot elsewhere in the building. Telling the two apart is worth ten minutes.

Beetle matters less than people fear. Common furniture beetle leaves 1mm to 2mm flight holes and prefers damp wood, so it arrives alongside rot rather than instead of it.

Three tests that settle most arguments

The probe line

Take a bradawl or a slim screwdriver and work along the bottom rail at roughly 150mm intervals, then down the bottom 150mm of each stile, then along the cill and under its front edge, pushing in at about 45 degrees. Sound softwood bites and stops the point in the first 2mm or 3mm. Timber that swallows the point to 10mm and beyond on hand pressure has gone. Mark every soft spot in pencil.

The pattern matters more than the count. Soft spots strung along one exposed face are a splice. Soft spots either side of a shoulder line, where the rail meets the stile, mean the decay has found the joint, and that is a different and dearer job.

The rack test

Grip the meeting rail with one hand and the bottom rail with the other and try to push the sash out of square. Sound joints do not move. If the corners visibly shift, or a black line opens along the shoulder as you push, the mortice and tenon joints have let go. This test decides more repair-or-replace arguments than any amount of probing, because a sash is only four joints holding a frame square around a sheet of glass.

The dry-week reading

If you can borrow a pin-type moisture meter, read the bottom rail and the cill after three or four dry days. Mid-teens is normal. Anything still over 20 per cent in dry weather is actively wet and something is feeding it. Do not let anyone repair timber in that state: adhesives, resins and primers all assume a reasonably dry substrate, and sealing moisture in behind fresh paint is the commonest reason a sound repair fails inside two winters.

What a repair can and cannot do

Once the water is stopped, the method depends on how much sound timber is left and what the member does structurally.

Two-pack epoxy systems are proper trade materials, nothing like a tub of car body filler. Repair Care's DRY FLEX 4 goes on over the maker's own consolidant, is laid at 5mm to 20mm, gives 20 to 25 minutes of working time at 20°C, and can be sanded and overpainted after about four hours on a 10mm repair. Cured, it stays elastic and moves with the timber instead of cracking away at the perimeter, which is where a rigid filler fails.

What resin cannot do is be structure. Bridging a joint, rebuilding the end of a member that carries load, or filling much past 20mm depth is using a compound as a substitute for joinery, and it fails, usually by opening a hairline crack round the edge and letting water in behind. The tell on a bad previous repair is a hard grey plug the size of a fist in the end of a bottom rail.

The alternative is a splice: decay cut out well past the last stained fibre, a long sloping joint that sheds water outwards rather than holding it, matching timber let in and worked down to the original profile, and non-ferrous mechanical fixings as well as adhesive. The detail of how that joint is cut matters more than most people expect.

So does the timber that goes into it.

• Match the species. Two different timbers glued along a long joint move at different rates and telegraph the line through the paint within a couple of seasons.
• Match the grain density. Victorian joinery redwood often shows ten or more growth rings to the inch. A length off a modern merchant's rack may show three or four, with wide bands of soft earlywood, and it will not last in the same position.
• Use heartwood and reject sapwood, which has no natural durability at all.
• Get the moisture content close to the timber it is joining, or the joint opens in the first wet autumn.

Accoya, the acetylated softwood, is worth considering for cills and bottom rails where wetting and movement are worst, though it costs more and needs stainless fixings. One useful exception to matching the species: where only the thin outer tongue of a glazing bar has gone, most joiners will let in a hardwood tongue on a softwood bar, the section being too slender to do anything else with.

When a joiner should tell you to stop

Four situations, and an honest firm will name them without being asked.

The joints have gone. Splicing a new foot onto a stile whose mortice has turned to fibre is decorating, not joinery. There is nothing left to repair into. This is what the rack test is for.

More than about half the member would be new. You are then making a new rail with an awkward joint in the middle of it. Making the piece complete is stronger, quicker and generally cheaper.

The repairs to one sash come to more than a new sash. A new sash made to match, single glazed, glass reused where it can be, hung on your existing weights in your existing box, is roughly £400 to £800 plus VAT in London. A sash wanting a new bottom rail, both stile feet and three glazing bars is at that figure already, and you would still have old timber everywhere else in it. Specify slim double glazed units at the same time and the arithmetic changes, because the sections must be heavier and the glass is expensive: that is £820 to £1,980 a window, and it belongs in the replacement price guide rather than here.

The box frame has gone. If the cill, the outer linings and both pulley stiles are decayed at the bottom, new sashes are money spent on a frame failing behind them. A new box frame with new sashes, softwood and double glazed, runs roughly £2,080 to £3,380 plus VAT a window in London, and it is a far bigger disruption because the plaster reveals and architraves come off with it.

What does not justify replacement, whatever is said on the doorstep, is a window that rattles, draughts, sticks or will not stay up. Those are cords, beads, seals and paint, and they say nothing about the condition of the timber. Keeping the frame has a quiet advantage too: replacing the sashes or the glass within an existing frame is not providing a controlled fitting under the Building Regulations, so no Part L U-value target applies and no FENSA or Certass certificate is involved. Replace the whole unit, frame included, and both arrive with it.

What each of those jobs costs

London ranges, excluding VAT, and every one of them moves with access and condition:

• Localised epoxy repair to one area: £80 to £200
• Splice to a sash rail or a stile: £160 to £300 per member
• Cill splice, piecing new timber into a sound cill: £180 to £300
• Complete new cill: £250 to £450
• Bottom rail renewed with new tenons: £250 to £600
• New sash made to match, single glazed, in your existing box: £400 to £800
• Full overhaul: cords, weights, beads, draught seals and timber repairs: £950 to £1,950

That last line is not an alternative to the one above it. A new sash is one component. An overhaul is the whole window put right: both sashes, all four cords, the weights rebalanced, new beads, seals and decoration. Our job-by-job breakdown of repair prices goes into why the same work varies so widely between quotes.

Access moves these numbers more than the joinery does. A third-floor front window on a red route may need a tower or a scaffold, a highway licence applied for by the scaffolder, and possibly a suspended parking bay. That can cost more than the timber work underneath it.

One thing to settle before anyone starts sanding: assume the lower paint coats on any London window decorated before the 1970s contain lead. Dry sanding and burning off release lead dust and fume, so wet abrasion, chemical stripper or an infrared plate are the sensible routes, and the right order for redecorating afterwards is worth following.

Stop the water before you spend anything

Whatever the repair turns out to be, it fails again if the water is still arriving. Nine times out of ten the source is one of four things: a cracked or missing putty line, a drip groove bridged with paint, soft pointing above the opening, or a gutter overflowing onto that elevation. Deal with it first, even if the joinery waits a month for the timber to dry.

Then take your pencil map to whoever is quoting and ask each of them the same question: which members are you replacing, and which are you splicing? A quote reading "repair windows, £2,400" tells you nothing. A quote reading "new bottom rail with tenons to lower sash, splice both stile feet, new cill, re-cord and rebalance" is one you can compare, argue with, and hold somebody to.

If you would rather someone else did the probing, our sash window restoration work starts by going over every member with a bradawl and telling you plainly which have gone, and the wider sash window repair service covers every London borough.